Definition Spurs Debate
            Despite this being the thirteenth installment, the term “decision fatigue” does not refer to any difficulty that may be experienced engaging with the “Making Decisions” series in its entirety.  An enduring goal of this series is to limit the effects of fatigue by providing tools that facilitate decision-making by maintaining structure and rationality.  This installment also provides additional tips for minimizing the influence of fatigue on decisions.
            To be more accurate than the section title, the lack of a widely-agreed definition of decision fatigue contributes to disagreements about its causes, effects, and even validity of the concept.  Many discussions contain descriptions of decision fatigue that do not rise to the level of “definition.”  Some refer to causes, some to effects, some are circular, and some include terms that require further explanation (i.e. definition) to be helpful.  The summary table provided by Pignatiello, et al. (Exhibit 1) reflects the dispersion of the term’s meaning among researchers.

            Decision fatigue is a term used generically to refer to one’s decline in, or impairment of, decision-making ability as additional decisions are made.  The implication is that the more successive decisions are made, the lower their quality becomes as a consequence of the cognitive load endured.  This generic formulation neglects other influences; specifically, other types of fatigue that effect decision-making.
            The lack of a well-formed, inclusive definition leads some to conclude that decision fatigue “is not real.”  If a narrow definition that neglects important influences is used, their arguments may be persuasive.  However, such debates provide little to no value to those seeking better outcomes from their professional and personal decision-making processes.
            To maintain focus on practical application of available information, in lieu of academic debate, readers are encouraged to make a cognitive transformation.  When the term “decision fatigue” is encountered, replace it with “fatigue’s effects on decision-making,” or similar phrase, in the mental model formed.  This simple transformation separates the relevant subject matter from the fruitless debates about the term’s definition, facilitating practical application with minimal confusion.  Central to this transformation’s value is its broader inclusion of fatigue types and other influences on decision quality.

Contributors
            A variety of conditions can contribute to fatigue that impairs decision-making capabilities.  Among them, physiological, emotional, cognitive, and situational factors combine to establish one’s overall state of fatigue.  The complexity of the human psyche ensures that these factors are also interrelated, where the onset of one may exacerbate others.
            Physiological factors affecting a decision-maker are typically most salient to others.  Physical exhaustion, due to exertion or sleep deprivation, is often the only type of fatigue recognized, as many are unaware of the others’ significance.  Illness and injury add to fatigue by diverting the body’s resources, as well as decision-maker’s attention, from cognitive tasks to recovery.  Other physical conditions, such as low blood sugar, can also contribute to physiological fatigue.
            Emotional fatigue can also leave one feeling drained of energy, despite the absence of physical exertion.  Contentious relationships are common sources of emotional fatigue; competition with a coworker, unresolved disagreements with family members, and disputes with neighbors or unsatisfactory service providers are typical examples.  Concern for another, such as a friend’s or family member’s illness, injury, self-destructive behavior, or death, can also contribute substantially to emotional distress and fatigue.
            Decision fatigue, as often defined, is one form of cognitive fatigue.  That is, making several decisions in succession may be an underlying cause of one’s cognitive fatigue, but it is not necessarily the only, or even the dominant one.  Decision-makers are typically responsible for a wide range of cognitive tasks, of which decision-making is only a subset.  Writing reports, solving logistical challenges, administering subordinates’ performance reviews, and so many more activities, often unrelated to a pending decision, engage cognitive resources and deplete energy reserves.
            The most proximate source of cognitive fatigue, and perhaps the most relevant to decision-making, is the effort required to collect, organize, analyze, and understand all of the information needed to render a rational decision.  The more complex the decision environment, the more difficult this task becomes.
            Situational factors are easily overlooked, or dismissed, in many circumstances because they are deemed “normal” and, therefore, unworthy of special consideration.  Though they often are “normal,” they may be far from ideal; team members should remain cognizant of their impacts on decision-making processes.
            The number, rate, and sources of inquiries regarding the status of a decision can influence a decision-maker’s process.  Likewise, people hovering while they await a decision, particularly if they are high-ranking individuals, can also exert influence.  This is true whether or not their presence is intended to apply pressure for an expedited decision or to achieve a specific result.
            The time of day at which a decision is rendered may seem like an extraneous detail; however, it often plays a significant role in decision-making processes.  An important reason for this is its relation to decision-makers’ level of fatigue, accumulated throughout the day.  Another is its relation to a deadline.  For example, a decision deadline of “close of business” on a given day is common.  If all required information is available before lunch, the afternoon may be sufficient to render a quality decision.  However, scrambling to locate necessary data at 4:00 PM is likely to result in a very different decision process and lower quality.

            The preceding discussion has alluded to the interrelationship of fatigue contributors; reciprocal influences exist among all of the contributors presented.  For example, excessive emotional fatigue can lead to difficulty sleeping or unhealthy lifestyle choices.  Subsequent physiological fatigue reduces performance in a variety of contexts.  Recognition of impaired performance can cause new stresses, initiating a deleterious cycle.  Preoccupation with an external situation or distraction, with myriad potential causes, creates a situational factor that could have a significant effect on the final decision and the process used to reach it.  Similarly, situational factors can exacerbate an emotional contribution to fatigue.

Consequences
            There are myriad, often stealthful, ways for fatigue to rear its ugly head.  Issues encountered in a decision-making process, whose root cause is fatigue, can easily be misdiagnosed as incompetence, lack of fortitude, or other uncomplimentary affliction.  Difficulties in execution and dissatisfaction with results of a decision are subject to similar misdiagnosis.
            Fatigue can render a compilation of decision-relevant information nearly incomprehensible.  Additional time required to review information reduces the efficiency of a decision-making process and further fatigues the decision-maker.  Quality suffers when decisions are based on erroneously-interpreted data or when a lack of comprehension causes important information to be discounted or disregarded.
            Difficulty processing decision-relevant information can increase its perceived complexity.  This can drive a decision-maker to overanalyze a situation or overestimate a decision’s significance to broader objectives.  If others perceive it to be a straightforward, or “simple,” decision, delays may cause them to lose confidence in the decision-maker’s fitness for the position s/he holds.  In an often counterproductive attempt to maintain the confidence of peers and leaders, a decision-maker may be tempted to take a variety of shortcuts.  These may be prompted by a genuine desire to contribute to team success, or simply face-saving measures.  Either way, shortcuts are risky and often misguided.
            One potential shortcut is to satisfice, reducing decision-making effort by accepting the first alternative, or course of action, that appears to satisfy minimum requirements.  A lower-quality decision results, but it can be rendered quickly and decisively.
            Two shortcuts that represent “opposite sides of the same coin” are conservatism and impulsivity.  Increasingly-conservative decisions tend to result from a focus on risks and de-emphasis of upside potential; that is, a decision-maker becomes more risk averse.  This is often a preference for the status quo to a difficult, detailed analysis that could reveal deficiencies in decision-makers’ comprehension or ability to execute.
            Conversely, impulsive, or risk seeking, decisions often reflect decision-makers’ overconfidence or short-term perspective.  Such bravado can also be used to disguise an inability to sufficiently analyze and comprehend relevant information.  Impulsivity can be more dangerous than satisficing, as even minimum performance may not be assured with an impulsive decision.  The sad irony is that this approach exacerbates the situation; ostensibly, a decision is reached efficiently, but progress is actually hindered.  Constant “firefighting” and failed initiatives leave little opportunity to improve decision-making processes, develop analysis skills, or recover from fatigue.
            Difficulty processing information can lead to procrastination and avoidance of a decision.  Lack of participation in low-value decision-making may be inconsequential, or appropriate prioritization of duties.  However, these decisions also require little effort and may cause more stress when allowed to linger than if made poorly.  In contrast, putting off an important decision, such as a response to a competitor’s new product launch or promotional campaign, could be catastrophic.
            It is useful to compare avoidance to selection of the “Do Nothing” option.  Choosing to “Do Nothing” is an active decision, requiring analysis and consideration of alternatives.  The decision is often accompanied by stated contingencies or related parameters, such as “revisit in 6 months” or “reconsider if stock price falls below $80/share.”  The potential modifications to a “Do Nothing” decision are innumerable, including financial, competitive, geopolitical, climatic, resource availability, technological, and other considerations.
            Decision avoidance is passive; analysis is incomplete and alternatives are not vetted.  The situation may change, for better or worse, but the dynamics are not monitored and, therefore, cannot be exploited or the consequences mitigated.  A decision-maker defines what is done in a specific circumstance, while avoidance allows the circumstances to define what is done to the decision-maker or his/her organization.

            A fatigued decision-maker may rely on biases and heuristics to a greater extent than normal, often unaware of the shift.  Heuristics can be useful, particularly for low-risk decisions.  They can also be used to simplify portions of a larger analysis; however, the more complex or consequential a decision, the greater risk is inherent in the use of heuristics.
            Biases come in many varieties and are rarely, if ever, helpful.  Positive outcomes derived from decisions influenced by bias are likely coincidental and suboptimal.  The topic of bias is too expansive to adequately present here; therefore only a brief introduction to a few biases prevalent in decision-making are discussed.
            Confirmation bias leads a decision-maker to place greater emphasis on information that supports a desired conclusion.  When a decision is based on a superficial review of alternatives, confirmation bias is a powerful tool of justification used to reinforce one’s self-image as a competent decision-maker without expending the effort required for thorough analysis.  Confirmation bias also causes contradictory information to be discounted as inaccurate or irrelevant without proper consideration of its implications.
            Recency and availability biases cause decision-makers to assign more “weight” to highly-salient information, though it may not be the most critical or accurate information available.  This is the “squeaky wheel gets the grease” style of decision-making; if a decision-maker is bombarded with details of risks, it is unsurprising when his/her decisions become more conservative.
            Social biases can also unjustifiably impact decisions.  A decision-maker may be influenced by other stakeholders’ political, religious, or family affiliations, attendance of a rival university, residence in a particular city or state, or other group status.  While the other biases mentioned prompt inappropriate prioritization, the information involved is usually still valid.  Social biases, however, cause decision-makers to dwell on extraneous details in lieu of proper consideration of the merits of alternatives and are, therefore, especially pernicious.
            In general, fatigue can be expected to reduce decision quality, consistency, and efficiency.  Fortunately, there is a number of techniques that can be employed to combat the negative impacts of fatigue on decision-making.

Defenses
            Employing the tools and techniques outlined in previous installments of this series can reduce decision-making effort and fatigue in a cyclical manner.  To maximize the potential benefit, begin by developing a decision-making standard, as presented in “Vol. IV:  Fundamentals of Group Decision-Making” [20May2020] in the section titled “How should a decision-making standard be structured?”  Although presented in the context of group decision-making, the portions of the process not directly related to group dynamics are fully applicable to individual decision-makers.
            The purpose of a decision-making standard is two-fold:  it serves to maintain consistency in decision-making, increasing efficiency, while ensuring thorough analyses are conducted by appropriate means.  When brainstorming, participants are encouraged to “think outside the box.”  Decision-making, however, is more reliable when conducted “in a box;” an established standard is the “box” in which effective decision-making is done.
            An established process can minimize procrastination and avoidance behaviors by being transparent to stakeholders and other observers.  Repeated execution, with subsequent refinement, builds confidence and reduces stress, further reducing tendencies for procrastination.
            Within an individual’s or group’s decision-making standard, third-party-developed guidelines may also be utilized.  Financial decisions are facilitated by guidelines that incorporate savings, investment, and retirement objectives.  Diagnosis of psychological disorders is expedited by referencing the Diagnostic and Statistics Manual (DSM).  Equipment failures are analyzed by following a troubleshooting guide.  Code compliance is evaluated with a regulatory checklist.  Such examples are numerous; guidelines can be as general or specific as needed to support the decisions undertaken.
            As mentioned previously, there are some situations in which the use of heuristics is advisable, though they must be customized and carefully reviewed to ensure they are appropriate.  Like any other aid, heuristics should be refined by experience, including the scope of applicability, to prevent transition from valuable tool to detrimental shortcut.
            Generalized heuristics are uncommon due to the vast expanse of potential decision environments.  However, some do exist, including the common examples of wardrobe and meal planning.  Wearing only one type or color of garment and eating a specific meal on a given day of the week simplify shopping and laundry.  Eliminating these decisions, proponents claim, frees cognitive resources to be expended on more-important decisions.  While the logic of eliminating unnecessary decisions is valid, these examples are unsatisfying, as they should not be terribly stress-inducing.
            A better example is an automobile lease or purchase.  Colors and features that provide the highest safety and reliability ratings can guide all future vehicle acquisitions.  In addition to eliminating seemingly simple decisions, the heuristic also provides value in the form of lower insurance and repair costs and reduced suffering from accidents.
            Decisions should be prioritized and scheduled.  This ensures that the most-important issues are addressed first and that decision-making isn’t relegated to a “spare time” activity.  As “spare time” doesn’t exist, unscheduled decisions are often made haphazardly at the last possible moment – the perfect formula for low-quality decisions and disappointing outcomes.
            Noncritical, low-value, or long-time-frame decisions can be delegated.  The lower they appear on one’s prioritized list of decisions to be made, the better candidates they are for delegation.  This, too, must be done carefully, however; review “Of Delegating and Dumping” [27Mar2019] for tips on effective delegation.
            Advice that readers may be familiar with is to “sleep on it.”  Many times, this expression is used quite literally; a decision is postponed until the following morning when the decision-maker, presumably well-rested and clear-thinking, is best-equipped to render an optimal decision.  Taken less literally, a decision can be postponed for a longer period of time; the key is to schedule the final decision to be rendered at a time when the decision-maker is not excessively fatigued, otherwise impaired, or under undue influence.  That is, a time when the decision-maker expects to reach the best decision.  Obviously, this is not an option for time-critical “firefighting” situations, but can be advantageous for intermediate time-frame objectives.
            Detailed data should be summarized and simplified to aid comprehension and retention of relevant facts.  An automobile purchase, again, provides a representative example.  A build sheet that specifies low-rolling-resistance tires, a 4-cylinder engine, vinyl interior, and manual (crank) windows can be summarized as “economical.”  One specifying Z-rated tires, V8 engine, and a 1000 W stereo is described as “fun.”  Exotic leather seats, extra sound insulation, automatic climate control, and voice-activated entertainment and navigation systems earns a label of “luxurious.”  If approving a company car for a Senior Vice President, the luxurious model may be acceptable, but the delivery driver is probably getting the economical one.  If it’s for yourself, choose the fun car.  Obviously.
            Distilling a set of data, features, etc. down to an accurate, comprehensible, and retainable summary may require a decision-maker to rely on the expertise of others.  This is not delegation; it is translation of information from its raw form to one more useful to the decision-maker.  If the decision-maker possesses the requisite expertise to perform the translation, assignment to another may still be done to meet time constraints.
            Break complex decisions into pieces; that is, replace one “big decision” with a series of smaller, simpler ones.  Doing so may facilitate later decisions, as optimization of early decisions clarifies objectives or eliminates alternatives, reducing analysis effort.  This is not always a viable option, but when it is, decisions can seem less daunting, allowing them to be made with greater efficiency and less stress.
            As an example, consider a rack stereo system; a prepackaged system can be purchased or each component specified individually.  Comparing features and specifications of prepackaged systems may be difficult and the sound quality unpredictable.  It is also possible that every available system lacks an important feature and would, therefore, be unsatisfactory.
            Selecting the desired turntable (vinyl is making a comeback), receiver, subwoofer, and speakers limits the selection of amplifiers with sufficient inputs and output power to drive the system.  If a home theater system is to be integrated, the selection of capable amps is further reduced.  By deciding on constituent components individually, even a hardcore audiophile is assured satisfaction with the completed system.  Also, far fewer tradeoffs or concessions are needed, eliminating the associated analysis.
            The framing of a decision is the way it is presented to decision-makers and stakeholders.  Its urgency, scope of influence, potential implications, and relationship to higher-level objectives are all part of a decision’s framing.
            To properly frame a decision, ask a series of question pairs to identify information that is relevant to the decision and that which is not.  “What is/is not at stake?” elucidates the scope of the decision.  Similar information may be gleaned by asking “what is gained by making the ‘right’ decision?” and “what is lost by making the ‘wrong’ one?”
            “What is the deadline for this decision?” and “what is the earliest that all required information will be available?” establishes a timeframe in which decision-makers must operate.  It also identifies decisions that must be based on incomplete information (i.e. deadline occurs prior to information availability) so that appropriate strategies can be devised to limit the impact.  “What information is/is not available?” serves to inform such strategy development.
            To identify how a decision fits in the grand scheme of objectives, ask “what lower-level objectives directly influence this decision or its outcomes?” and “to what higher-level objectives does this decision pertain?”  “Can this decision be divided to improve optimization?” and “should this decision be combined with any other to ensure consistency in organizational stewardship?” further define the relationship of the decision to the desired direction of the organization.
            The example question pairs cited are generalized and near-universal.  Additional questions, general or specific to a decision environment, can be developed to guide decision framing.
            Framing a decision appropriately helps decision-makers in several ways.  It provides information needed to prioritize it among all other responsibilities competing for attention.  This, in turn, assures that necessary resources are assigned to it.  Maintaining an appropriate sense of urgency allows efficient decision-making processes to be followed without undue stress.  If all decisions are treated as existential threats, stress levels will be high, but decision quality will be low.
            Choosing the proper framing constitutes an early stage of research.  Answering framing questions identifies information, expertise, time, and other resources needed to execute an efficient decision-making process.  Much of the “behind the scenes” activity needed to support analysis and rendering of a final decision is defined by framing.
            Proper framing of a decision also serves to verify that potential outcomes are aligned with the organization’s objectives.  If they are not, it is best to discover as early as possible so that adjustments can be made or the decision abandoned to avoid squandering resources.
            Underlying all of the defenses to fatigue is self-care.  Physical and mental health, socialization, intellectual stimulation, safety, and security are all important components of well-being.  Each contributes to the various types of fatigue and influence decision-making individually, collectively, and synergistically.
 
 
            Awareness of the types of fatigue and their potential impacts is the basis for improvement of decision-making processes and, subsequently, outcomes.  Recognizing fatigue, without misdiagnosing it as a character flaw, relies on widespread understanding.  Learning how to defend oneself and an organization against the effects of fatigue begins the improvement process in earnest.  Learning how to limit fatigue is a great leap forward.  Incorporating this knowledge in daily practices and organizational policies is the final component of an efficient decision-making process.

            For additional guidance or assistance with decision-making or other Operations challenges, feel free to leave a comment, contact JayWink Solutions, or schedule an appointment.

            For a directory of “Making Decisions” volumes on “The Third Degree,” see Vol. I:  Introduction and Terminology (8Apr2020).

References
[Link] “Why do we make worse decisions at the end of the day?”  Lauren Braithwaite.  The Decision Lab.
[Link] “The Morning Morality Effect.”  Maryam Kouchaki and Isaac H. Smith.  Psychological Science; January 2014.
[Link] “Decision fatigue: A conceptual analysis.”  Grant A Pignatiello, Richard J Martin, and Ronald L Hickman Jr.  Journal of Health Psychology; January 2020.
[Link] “What is decision fatigue?”  Jon Johnson.  Medical News Today; July 7, 2020.
[Link] “Decision Fatigue Is Real. Here’s How to Beat It This Year.”  Rachel Feintzeig.  The Wall Street Journal; January 3, 2022.
[Link] “Is Decision Fatigue Real?”  Eva M. Krockow.  Psychology Today; September 28, 2023.
[Link] “6 Tactics To Reduce Decision Fatigue.”  Svetlana Dimovski.  Forbes; April 1, 2024.
[Link] “Identifying the Causes and Effects of Decision Fatigue through a Systematic Review (Extended Abstract).”   Nurul Ahad Choudhury and Pratima Saravanan.   Proceedings of the Human Factors and Ergonomics Society Annual Meeting; September 2024.
[Link] “How To Avoid Decision Fatigue: 10 Tips and Examples.”  Indeed Editorial Team.  Indeed.com; July 24, 2025.
[Link] “No evidence for decision fatigue using large-scale field data from healthcare.”  David Andersson, Malou Lindberg, Gustav Tinghög, and Emil Persson.  Communications Psychology; February 26, 2025.

Jody W. Phelps, MSc, PMP®, MBA
Principal Consultant
JayWink Solutions, LLC
jody@jaywink.com

Preference Functions
            The foundation of PROMETHEE is the use of various preference functions to reflect the true nature of criterion assessments.  While any number of preference functions can be defined for a decision environment, the six types summarized in Exhibit 1 are widely used; this set is deemed sufficient to support the vast majority of decisions.

            Each preference function is defined according to the type of criterion.  That is, whether it is a cost to be minimized or a benefit to be maximized.  Qualitative criteria are converted to quantitative measures for use with preference functions.
            Predefined parameters are used to determine function values.  A threshold of indifference (q) is the difference in criterion values between alternatives at which a preference emerges.  The difference in criterion values at which a preference becomes complete is the threshold of preference (p).  For Type II criteria, p = q; therefore, only q is defined.  For Types IV and V, a criterion difference that exceeds the threshold of indifference (i.e. d > q or d < -q) creates a partial preference.
            Type VI criteria require σ, the standard deviation, to be defined.  This function is represented by an inverted “bell curve,” or normal distribution; hence the Gaussian label.
            To demonstrate use of PROMETHEE methods, the hypothetical machinery purchase of previous installments is revisited.  The performance matrix, expanded to include preference function and parameter information for each criterion, is shown in Exhibit 2.  Once again, labels J, W, and A are used to refer to the Jones, Wiley, and Acme alternatives, respectively.  Likewise, labels C, P, and S are used to reference cost, productivity, and service life criteria, respectively.

            To determine preference function values, criterion value differences for each alternative pair must be computed.  Generically, this is written dj (a,b) = gj (a) - gj (b), where dj is the value difference on the jth criterion, a and b are the two alternatives compared, and gj is the performance level of an alternative on the jth criterion.
            For our example, cost differences are calculated as follows:
dC (J,W) = C(J) - C(W) = 1.2 - 1.4 = -0.2
dC (W,J) = C(W) - C(J) = 1.4 - 1.2 = 0.2
dC (J,A) = C(J) - C(A) = 1.2 - 1.8 = -0.6
dC (A,J) = C(A) - C(J) = 1.8 - 1.2 = 0.6
dC (W,A) = C(W) - C(A) = 1.4 - 1.8 = -0.4
dC (A,W) = C(A) - C(W) = 1.8 - 1.4 = 0.4
Though seemingly redundant, the bidirectional difference calculations are needed in subsequent stages of analysis.  Repeating this step for the productivity and service life criteria yields the results summarized in the tables of Exhibit 3.  In each table, the row represents the first alternative in the comparison (i.e. a), while the column represents the second (i.e. b).  Each entry can, therefore, be read as “row minus column.”  Units are omitted for simplicity.

            Preference function values are then determined for each alternative pair by using the criterion differences in the preference function assigned to each criterion, as tabulated in Exhibit 2.
            Cost is a V-shaped criterion with a minimization objective; therefore, the second preference function in the Type III row of Exhibit 1 is used.  Only the (Jones, Wiley) comparison yields an intermediate value, calculated as fC (J,W) = -(-0.2/0.25) = 0.8.  The other two criteria are maximization objectives; therefore, the first preference function in the corresponding row is used for each.
            Productivity is a Level (Type IV) criterion.  The (Wiley, Jones) and (Acme, Wiley) comparisons reside at the intermediate level; only (Acme, Jones) yields a complete preference.  Identical service life projections for Wiley and Acme provide both a partial preference to Jones as a Linear (Type V) criterion.  For both, this value is (d-q)/(p-q) = (2-1)/(4-1) = 0.3.  All preference function values for the example are tabulated in Exhibit 4; values are always in the range 0 to 1, inclusive [0 ≤ f(a,b) ≤ 1].

Preference Indices
            An alternative’s preference index (π) is a cumulative measure that incorporates all criteria.  It takes the form

where π(a,b) is the preference index of alternative a with respect to alternative b, n is the number of criteria considered, fj (a,b) is the preference function value for the (a,b) comparison with respect to the jth criterion, and wj is the relative weight of the jth criterion.  If criterion weights have not been assigned, wj is replaced with (1/n) to reflect equal weighting.  For our example, the normalized criterion weights found in the AHP (Vol. III) and tabulated in Exhibit 2 are used.  Doing so maintains consistency and comparability across installments of the series.  Other methods of assigning criterion weights could be used, however.
            The preference index calculations for our example are as follows:
π(J,W) = (0.064)(0.8) + (0.669)(0) + (0.267)(0) = 0.051
π(W,J) = (0.064)(0) + (0.669)(0.5) + (0.267)(0.3) = 0.424
π(J,A) = (0.064)(1) + (0.669)(0) + (0.267)(0) =0.064
π(A,J) = (0.064)(0) + (0.669)(1) + (0.267)(0.3) = 0.758
π(W,A) = (0.064)(1) + (0.669)(0) + (0.267)(0) = 0.064
π(A,W) = (0.064)(0) + (0.669)(0.5) + (0.267)(0) = 0.335
Note that values are always in the range 0 to 1, inclusive [0 ≤ π(a,b) ≤ 1]; the sum of complementary indices also lie in this range [0 ≤ π(a,b) + π(b,a) ≤ 1].

Outranking Flows
            An alternative’s outranking flows compare it to all other alternatives under consideration.  An alternative’s positive outranking flow, or leaving flow, (φ⁺) reflects its superiority to other alternatives.  This is called its outranking character or power and is expressed as

where φ+(a) is the positive outranking flow of alternative a and m is the number of alternatives under consideration.  The summation is performed for all competing alternatives; that is, all alternatives to a considered in the decision.
            Negative outranking flow, or entering flow, (φ-) represents an alternative’s inferiority to other alternatives, called its outranked character or weakness.  It is expressed as

As for the leaving flow, the summation is performed for all competing alternatives.
            An alternative’s net flow (φ) is simply the difference between its positive and negative outranking flows.  That is, φ(a) = φ⁺(a) – φ⁻(a).
            The outranking flows for the Jones Machinery option of our example are as follows:
φ⁺(J) = [1/(3-1)] (0.051 + 0.064) = 0.058
φ⁻(J) = [1/(3-1)] (0.424 + 0.758) = 0.591
φ(J) = 0.058 – 0.591 = -0.533
A summary of outranking flow calculations for the example is tabulated in Exhibit 5.

PROMETHEE Outranking Methods
            Outranking is a term used to describe the assignment of preference, indifference, or incomparability to alternatives.  Two alternatives found to be incomparable are defined by criteria values that are too contradictory for the method in use to reach a conclusion.  Other methods may be capable of differentiating and ranking them, however.
            Notation used to represent outranking relationships varies among sources.  For purposes of this presentation, a symbol set is adopted to make the information as intuitive as possible.  This symbol set is summarized in Exhibit 6; alternative representations that may be encountered in cited references and elsewhere, including this page due to limitations of the text editor, are also shown.  A Roman numeral in parentheses or superscript identifies the PROMETHEE method by which the outranking relationship was established.

            PROMETHEE I provides a preorder, or partial ranking, of alternatives by comparing positive and negative outranking flows.  The ranking is established according to the following rules:

            Three reminders are useful here: (1) “iff” is read “if and only if;” both conditions in a statement must be true for the corresponding ranking to be assigned.  (2) The third rule is a mathematical expression of the contradictory nature of decision criteria values, i.e. each alternative outranks the other with respect to a subset of criteria. (3) Outranking notation and mathematical relationships are both used to define outranking rules; these sometimes have opposite meaning (see the note in Exhibit 6).
            To rank alternatives, an exhaustive set of comparisons can be created; however, one-half will be inconclusive!  Instead, identify the alternative with the largest positive outranking flow (φ⁺).  Compare this alternative to each of the others, in turn.  This selection reduces the rule set to three statements where φ⁺(a) > φ⁺(b).  Comparing the negative outranking flows (φ⁻) of the alternatives determines which of the remaining statements and, therefore, the outranking relationship that applies.
            For our example, the largest value of φ⁺ is associated with Acme (0.546).  Comparing Acme’s φ⁻ (0.064) to that of Jones (0.591) and Wiley (0.193) reveals that the first rule statement applies in both cases.  Therefore, A >~(I) J and A >~(I) W.  This process is repeated for the second-highest φ⁺, comparing to remaining alternatives.
            The second-highest φ⁺ in our example is associated with Wiley (0.244).  Comparing Wiley’s φ⁻ (0.193) to that of Jones (0.591), the only remaining alternative, the first rule statement once again applies; W >~(I) J.  This gives the final ranking of A >~(I) W >~(I) J with the minimum number of comparisons.
            A larger decision matrix is analyzed in the same manner, by repeating the steps outlined above, progressing through the list of alternatives in order of decreasing φ⁺.  Where equal values of φ⁺ are encountered, the subset of rule statements considered shifts from the three “>” relations to the two “=” relations.  The rule set shifts back to the “>” relations to compare these alternatives to those remaining with lower φ⁺ values.

            PROMETHEE II provides a complete ranking of alternatives by comparing net outranking flows (φ).  Ranking by this method is established according to the following rules:

• a >~(II) b   iff   φ(a) > φ(b).
• a ~~(II) b   iff   φ(a) = φ(b).

Note that, by comparing only net flows, incomparabilities are eliminated.  While this simplifies its application, this loss of information is also commonly cited in caveats related to the use of PROMETHEE II.
            In our example φ(A) > φ(W) > φ(J) (see Exhibit 5), corresponding to the ranking A >~(II) W >~(II) J.  PROMETHEE I and II yield identical results in our relatively-simple example; however, the loss of incomparability information may cause the results of a larger analysis to differ.

            PROMETHEE III provides a complete ranking of alternatives by comparing intervals.  Like the block distances used in ORESTE (Vol. XI), intervals are in “decision space.”  To determine each alternative’s interval, intermediate values must first be computed.
            An alternative’s mean flow is φ̅(a) = φ(a)/n, where φ is the net outranking flow of alternative a and n is the number of decision criteria.  Its standard deviation can then be calculated:

            Each alternative’s interval [X(a), Y(a)] is determined according to X(a) = φ ̅(a) – ασ(a) and Y(a) = φ ̅(a) + ασ(a), where α is a parameter, the derivation of which is omitted for simplicity, with a typical value of 0.15.  Ranking of alternatives is then established according to the following rules:

• a >~(III) b   iff   X(a) > Y(b).
• a ~~(III) b   iff   X(a) ≤ Y(b) and X(b) ≤ Y(a).

            In our example, Jones’ mean flow is φ ̅(J) = -0.533/3 = -0.178.  Its standard deviation is

Using α = 0.15, Jones’ interval is now found:
X(J) = -0.178 – (0.15)(0.319) = -0.225,
Y(J) = -0.178 + (0.15)(0.319) = -0.130;
[X(J), Y(J)] = [-0.225, -0.130].
A summary of interval calculations for our example is tabulated in Exhibit 7; a visual representation of the intervals is provided in Exhibit 8.

            Comparing interval limits to apply the preference rules, we find X(A) > Y(J) and X(A) > Y(W); therefore, A >~(III) J and A >~(III) W.  Also, X(W) > Y(J); therefore, W >~(III) J.  Thus, the final ranking is A >~(III) W >~(III) J, in alignment with the PROMETHEE I and II results.
            Though the final ranking obtained matches those of the previous methods, the addition of a visual element enables decision-makers to rapidly gain insights regarding the alternatives’ relative performance.  Observing Jones’ position on the far left of the interval diagram, its entire range less than zero, and Acme’s interval on the far right makes the outranking character of the Acme alternative much more salient than numbers in a table.  This is a significant contribution of PROMETHEE III to decision-makers’ available information.

            PROMETHEE IV extends PROMETHEE II for applications in which alternatives are defined by continuous variables rather than discrete values.  Calculating flows in PROMETHEE IV requires integration of preference indices, while the ranking rules remain the same as in PROMETHEE II.  Details of this method, and the “Brans simplification,” can be found in cited references or other sources.

            Additional techniques have been developed as part of the family of PROMETHEE methods.  Decision under constraints, advanced visual techniques, and sensitivity analysis are among variants of PROMETHEE methods.  These variants employ sophisticated techniques that are beyond the scope of this series, the charter of which is to present tools that are reasonably straightforward and practical for implementation by a broad spectrum of practitioners in a wide range of applications.


            PROMETHEE methods are valuable additions to one’s decision-making toolbox; they provide perspectives that other tools do not offer.  All of these tools become more valuable when used synergistically, exploring the nuance provided by each.  Similar to other tools discussed, the PROMETHEE methods presented are amenable to development of spreadsheet templates or purpose-built software that minimizes the effort required for each new analysis, further enhancing their value.


            For additional guidance or assistance with decision-making or other Operations challenges, feel free to leave a comment, contact JayWink Solutions, or schedule an appointment.

            For a directory of “Making Decisions” volumes on “The Third Degree,” see Vol. I:  Introduction and Terminology (8Apr2020).

References
[Link] Multiple Attribute Decision Making:  Methods and Applications.  Gwo-Hshiung Tzeng and Jih-Jeng Huang.  CRC Press; 2011.
[Link] Multiple Criteria Decision Analysis:  State of the Art Surveys, 2ed.  Salvatore Greco, Matthias Ehrgott, and José Rui Figueira (eds).  Springer; 2016.
[Link] New Methods and Applications in Multiple Attribute Decision Making (MADM).  Alireza Alinezhad and Javad Khalili.  Springer; 2019.
[Link] “Multicriteria Decision-Making Methods.”  M. Pavan and R Todeschini.  In Comprehensive Chemometrics, 2ed ; S. Brown, R. Tauler, and B. Walczak (eds).  Elsevier; 2020.

Jody W. Phelps, MSc, PMP®, MBA
Principal Consultant
JayWink Solutions, LLC
jody@jaywink.com

The ORESTE Method
            The example used to demonstrate the ORESTE method is the same as that presented in Vol. III:  Analytic Hierarchy Process and Vol. X: Fuller Triangle.  For convenience, the performance matrix is reproduced in Exhibit 1.  In this installment, criteria and alternatives are identified by the first letter of each:  C, P, S for criteria and J, W, A for alternatives.  Presentation in this fashion reflects readers’ familiarity with the example, gained in previous installments, while continuing to provide greater clarity than arbitrary alphanumeric identifiers.  This may not be feasible in a larger analysis; however, experience gained prior to such an undertaking should obviate this practice.
            As in previous analyses, productivity and service life are deemed more important than cost, and productivity is also deemed more important than service life.  This hierarchy is presented as PROD > SVC > COST.  In terms of preference rankings, this gives rP = 1, rS = 2, and rC = 3.
            The alternatives are ranked in order of preference with respect to each criterion.  Replacing the values in the performance matrix with the rank order values yields the position matrix, P, shown in Exhibit 2.  Note that Wiley and Acme offer identical service life, resulting in equal rank, each being the average.

            Next, the block distance of each position is calculated.  In two-dimensional Cartesian space, the block distance between two points is |x1 – x2| + |y1 – y2|.  Also called “city block distance,” the distance traversed between two locations, when only travel parallel to the axes is permitted, is determined.  However, ORESTE is not conducted in Cartesian space, but in “decision space.”
            The ORESTE decision space has an arbitrary origin, 0.  Block distances of each position from the origin, d(0, Aij), are determined as

where dij is the distance from the origin of the position of the ith alternative with respect to the jth criterion, pij is the position of the ith alternative with respect to the jth criterion, rcj is the preference rank of the jth criterion, and r is a scaling factor.  The derivation of r is beyond the scope of this presentation; suffice to say that r = 3 is typically used, as it is herein.  Substitution of these values yields the distance matrix, D, shown in Exhibit 3.

            The distance of each position is ranked from least to greatest; ties (i.e. equal distances) are assigned average rankings.  The ordinal rankings replace the cardinal distances for each position in the matrix to determine the final alternative ranking.  The ranking matrix, R, shown in Exhibit 4, includes a summation column for this purpose.  Each alternative ranking score equals the sum of its criterion position rankings:  Ra = ∑ raj, where Ra is the total ranking score of alternative a and raj is the rank of alternative a with respect to the jth criterion.  The lowest total ranking score corresponds to the preferred alternative.
            It is interesting that the preference ranks obtained by the ORESTE method do not align with those from AHP and FT.  Two key influences are responsible for this deviation.  First, the degree to which an alternative is preferred with respect to any criterion (the magnitude of preferences) is not fully accounted for in scores based on ordinal rankings alone.  Second, whereas the other methods tend to resolve ties at earlier stages of analysis, those in the final ranking matrix in the ORESTE method allow small changes in scoring totals to shift the results.

Speaking of Ties…
            The ORESTE method is often demonstrated with no ties in the position matrix.  However, equal performance of alternatives on one or more metric is not an unusual occurrence.  The significance of a tie depends on it persisting to the final ranking and the rank at which it occurs.  When only one alternative is to be pursued, a tie that occurs below the highest rank requires no further attention.  A tie for the most-preferred alternative, however, requires further analysis.  A few options are discussed below, though it must be noted that these should be used with caution.
            The most obvious option is to abandon the ORESTE method when ties are discovered.  If it is one of several MCA tools in use, this may be advisable.  Doing so eliminates the additional computation and justifications required to rank alternatives.  The final results may still be less clarifying than desired, further incentivizing decision-makers to forego ORESTE analysis for a particular decision.
            Consider the equal service life of the Wiley and Acme machines; with an alternative method of calculating block distances, this tie persists to the final ranking where purchase of either machine is deemed equally desirable and preferred to a purchase from Jones.  This provides little insight beyond that gained from reviewing the performance matrix prior to analysis; no definitive conclusion is reached.
            One approach to breaking such a tie is to intentionally introduce bias into the analysis.  As examples, let’s say that decision-makers perceived one of the suppliers to be more responsive than the other during the proposal process or they have successful project history with one, but the other is a new supplier.  Instead of assigning both an average position of 1.5 on the SVC metric, the favored supplier would be rated 1 and the other 2.
            If Acme is given the advantage in the tie-breaker of our example, the analysis changes to that shown in the composite matrix of Exhibit 5.  In this scenario, the results align with those of AHP and FT, where Acme is favored over Wiley, and Jones is the least-desirable supplier.  In a small analysis like this, the impact such a bias introduction will have on final results is predictable.  This leaves the analysis susceptible to manipulation with weak justifications for modifications to the position matrix.  Thus, the practice should be employed only in larger analyses, where the influence of such a tie-breaker is less obvious, and then only with open acknowledgement that the results are less definitive than other analyses may provide.

            The “intentional bias” approach reduces the reliability of the ORESTE method.  A more-accurate representation of the importance of a relationship with a supplier, or any attribute of an alternative, is achieved by adding it to the performance matrix.  Whether initially overlooked, or deemed too inconsequential to warrant additional computations, the potential impact of any relevant criterion should not be discounted.  The reversal of preferences in our example demonstrates the potential impact of shortcuts in analysis.
            To extend the example, let’s say a fourth criterion represents decision-makers’ confidence in the supplier’s ability to successfully deliver the project as specified.  We’ll call it the “X-factor” to reflect its qualitative nature.  An advantage of ORESTE is that qualitative assessments need not be converted to quantitative metrics.  On this assessment, Jones, Wiley, and Acme are rated very high, high, and moderate, respectively.  Given that “X-factor” was previously used to break a tie with respect to service life, it is now ranked above SVC in importance.  Thus, PROD > X > SVC > COST; rP = 1, rX = 2, rS = 3, rC = 4.
            The expanded analysis is summarized in the composite matrix shown in Exhibit 6.  That Acme is the preferred supplier in the final ranking is no surprise, as the expanded analysis merely formalized a previously revealed preference.  What may be surprising, however, is that Jones is now ranked second.  Very high confidence in successful project delivery substantially narrowed the gap between Jones and the ultimate “winner,” but it could not overcome Acme’s superior machine performance.
            Results of the expanded analysis are not directly comparable to those previously obtained by AHP and Fuller Triangle methods.  For such comparisons to be valid, those analyses must also be expanded to include the “X-factor” evaluations.  A Fuller Triangle analysis with X-factor yields a tie between Wiley and Acme in the final ranking, reminding us that an important decision may require iterative analysis or multiple tools to develop sufficient confidence in the final selection.

ORESTE II
            The second phase of the ORESTE method, sometimes called ORESTE II, uses threshold values to determine a “complete” ranking.  These thresholds differentiate between preference and indifference (β), between indifference and incomparability (C*), and between preference and incomparability (γ).  Its purpose is to refine “weak order” rankings found in the first phase by accounting for strengths of preferences.
            The second phase is often omitted, as is done here.  It does not clarify the decision in the present example and could create unnecessary confusion.  Therefore, a detailed discussion of ORESTE II is postponed until it can be done in a context to which it adds value.

A Final Tip
            In our example, the order of presentation of criteria does not coincide with their relative priorities.  The performance matrix could be rearranged such that the priority ranking of each criterion equals its subscript when using Cj notation to identify them.  This is only a matter of convenience to reduce the potential of errors, particularly in larger matrices.  It is an additional step, performed only if it serves analysts’ or decision-makers’ preferred style of information organization and presentation.


            For additional guidance or assistance with decision-making or other Operations challenges, feel free to leave a comment, contact JayWink Solutions, or schedule an appointment.

            For a directory of “Making Decisions” volumes on “The Third Degree,” see Vol. I:  Introduction and Terminology (8Apr2020).

References
[Link] “Preference relations on actions and criteria in multicriteria decision making.”  Marc Roubens.  European Journal of Operational Research; May 1982.
[Link] Multiple Criteria Decision Analysis:  State of the Art Surveys, 2ed.  Salvatore Greco, Matthias Ehrgott, and José Rui Figueira (eds).  Springer; 2016.
[Link] New Methods and Applications in Multiple Attribute Decision Making (MADM).  Alireza Alinezhad and Javad Khalili.  Springer; 2019.
[Link] “The Issue of Multicriteria Decision-Making:  Model Example of Business Partner Risk Assessment.”  Jaromir Vrbka and Vojtech Stehel.  Proceedings of the 13th International Management Conference; January 2020.
[Link] “Multicriteria Decision-Making Methods.”  M. Pavan and R Todeschini.  In Comprehensive Chemometrics, 2ed ; S. Brown, R. Tauler, and B. Walczak (eds).  Elsevier, 2020.
[Link] “Solution proposal for completed preference structure in ORESTE method.”  Mehmet Akif Yerlikaya, Kürşat Yildiz, and Büşra Nur Keskin.  Scientific Reports; March 2023.

Jody W. Phelps, MSc, PMP®, MBA
Principal Consultant
JayWink Solutions, LLC
jody@jaywink.com

            To facilitate comparison of methods and the results obtained, the Fuller Triangle (FT) method is presented with the same hypothetical machinery purchase decision used in Vol. III:  Analytic Hierarchy Process.  Reviewing AHP alongside FT is recommended to clarify when either method may be more appropriate than the other.

The Fuller Triangle Method
            A Fuller Triangle serves as a visualization of a series of pairwise comparisons.  An abstract example is shown in Exhibit 1, where criteria A, B, C, and D are considered.  Each row represents comparisons of the subject criterion to those to which it has not been compared in rows above.  Only half of the rectangle, above the diagonal, is used, leaving blank row/column intersections representing duplicate comparisons (e.g. row B/column A duplicates row A/column B, each comparing A and B).  Thus, a triangle results.

            For the visualization, the criterion of greater importance in each comparison is circled.  If the two are of equal import, a rectangle is drawn around both.  To score the comparisons, one point is assigned to a criterion for each time it is circled in the triangle.  For each rectangle drawn around a criterion in the triangle, one half (1/2) point is assigned.  Note that, in Exhibit 1, a rectangle surrounds the criterion identifiers each time a criterion is compared to itself.  These are greyed out because “self-comparisons” are not normally presented in the Fuller Triangle.  They are included here for explanatory purposes; their relevance becomes clear in the presentation, below, of the entire FT process.

            Returning to the hypothetical machine purchase, the summary of criteria and source alternatives, called the performance matrix in AHP, is reproduced in Exhibit 2.  The information contained in this table is used throughout the FT example presentation.

            The number of comparisons, p, to be performed is found as p = n(n-1)/2, where n is the number of criteria considered.  In our example, n = 3; the number of comparisons needed is 3(3-1)/2 = 3.  The small number of criteria yields a compact Fuller Triangle, shown in Exhibit 3.  The criteria are identified as COST, PROD (productivity), and SVC (service life).  Larger analyses often use single letters or numbers, rather than names or abbreviations, to identify criteria.  When this is done, a legend should accompany the comparison triangle.
            Following the logic of the original example, both productivity and service life are considered more important than cost and productivity is more important than service life, as shown in Exhibit 3.  This results in criteria preference scores (pc) of pCOST = 0, pPROD = 2, and pSVC = 1.
            Though cost is deemed less important than the other criteria, using a preference score of zero could yield misleading results.  To prevent this, the “self-comparison” point is added to each criteria preference score, ensuring none are zero.  This results in the modified preference score, pm.  The total modified preference score is pm = n(n-1)/2 + n, or pm = p + n; in this example, pm = 3 + 3 = 6.  A summary table of criteria comparison scoring is provided in Exhibit 4.

            Criteria weights are now calculated using the preference scores obtained above.  Each criterion weight is determined as wc = pmc/pm.  Therefore, wCOST = 1/6 = 0.167, wPROD = 3/6 = 0.500, and wSVC = 2/6 = 0.333.

            Next, the alternatives are considered, with respect to each criterion, following the same process as that used for evaluation of criteria.  Alternative comparison triangles and scoring summaries are shown in Exhibits 5, 6, and 7.  The self-comparison entries have been eliminated and the comparison triangles and scoring tables are combined, providing a more-conventional presentation of the analysis.  The lengthy subscripts make evident the benefit of shortened identifiers and an accompanying legend to a large analysis.  The small number of comparisons and the explanatory value warrant use of abbreviations here.
            The number of comparisons needed, relative to each criterion, is, as before, p = n(n-1)/2 = 3; pa is the alternative preference score and pma is the modified alternative preference score, pma = p + n = 6.  Our example uses the same number of criteria and alternatives, but this need not be the case and is not typical.

            The score for each alternative, relative to each criterion, is the product of the alternative’s modified preference score on that criterion and the criterion’s weight:  rac = pmac⋅wc, where rac is the ranking score of alternative a with respect to criterion c, pmac is the modified preference score of alternative a with respect to criterion c, and wc is the criterion weight of criterion c.  The total ranking score of each alternative, ra, is the sum of the alternative’s criterion ranking scores:  ra = ∑ rac.
            A summary of the alternative ranking calculations is shown in Exhibit 8.  The rightmost column of the table shows the final ranking of preferences, where the alternative with the highest total ranking score is the preferred option.  The order of preferences obtained – (1) Acme, (2) Wiley, (3) Jones – coincides with the results obtained for this hypothetical example using AHP.

FT vs. AHP
            The consistency of the result obtained using the Fuller Triangle with that of AHP is expected.  The evaluations of criteria were found to be highly consistent in the AHP analysis; therefore, following the same logic with a similar process can be expected to yield similar results.  If FT were to be used in lieu of AHP for a real decision, however, the reverse cannot be assured.  That is, FT lacks the tests of evaluation consistency that ensure valid results in AHP.
            The magnitudes of preferences are not accounted for in FT.  This may also cause results to differ from those obtained by AHP.  While sensitivity analysis could be performed by adjusting criteria weights, interpreting the results is very difficult.  This is because potential causes of criteria weight changes are not clearly defined; each criterion weight is based on a binary choice.  In contrast, AHP allows an adjustment from e.g. “strongly” to “somewhat” preferred.  It is this nuance that is explored in sensitivity analysis and that is lacking in FT.
                The reduced rigor of FT make it both easier to use and less reliable as the size (i.e. number of criteria and alternatives) and criticality of analysis grows.  When conducting a simple analysis, such as that in our example, it is easy to maintain consistency.  Therefore, the additional rigor of AHP may not be warranted in an actual decision of similar complexity.  The threshold at which the additional rigor of AHP is justified is reduced by the use of spreadsheet templates.  Performing calculations in software significantly reduces the workload involved in a large, complex analysis.  The selection of a decision-making aid requires consideration of the implications of the decision, stakeholders’ acceptance of the method, and other variables.


            Use of one MCA tool does not exclude the use of others.  Multiple tools can be employed to increase confidence in a decision or reveal discrepancies in information that must be resolved to maintain decision quality.  Methods may even be intertwined; for example, criteria weights determined using FT could be inserted in AHP or other analysis as a “shortcut,” a form of sensitivity analysis, or simply to provide an alternative perspective on the decision scenario.  Deeper understanding of the tools and methods and rapid computation provided by a computer expands opportunities to explore possible scenarios and potential outcomes.


            For additional guidance or assistance with decision-making or other Operations challenges, feel free to leave a comment, contact JayWink Solutions, or schedule an appointment.

            For a directory of “Making Decisions” volumes on “The Third Degree,” see Vol. I:  Introduction and Terminology (8Apr2020).

References
[Link] “An Approach to Multi-criteria Environmental Evaluation with Multiple Weight Assignment.”  Boris Agarski, Igor Budak, Borut Kosec, and Janko Hodolic.  Environmental Modeling & Assessment; June 2012.
[Link] “The Issue of Multicriteria Decision-Making:  Model Example of Business Partner Risk Assessment.”  Jaromir Vrbka and Vojtech Stehel.  Proceedings of the 13th International Management Conference; January 2020.
[Link] “Application of the Chosen Multi-Criteria Decision-Making Methods to Identify the Autonomous Train System Supplier.”  Ondrej Stopka, Mária Stopkova, Vladimír Ľuptak, and Srećko Krile.  Transport Problems; June 2020.

Jody W. Phelps, MSc, PMP®, MBA
Principal Consultant
JayWink Solutions, LLC
jody@jaywink.com

Measurements
            A formalized program of in-situ illuminance measurements is called a lighting survey.  A lighting survey should be conducted on a “stable” lighting system.  For this purpose, stable is defined as having at least 100 hours of run time and in thermal equilibrium.  Measurements are less reliable if taken too soon after system installation or immediately after switching on the lights.  A color- and cosine-corrected illuminance meter should be used to duplicate, as closely as possible, human perception of light.
            The number and location of measurement points are defined by establishing a survey grid.  One method of determining an appropriate survey grid involves calculation of the room index (RI):

where l and w are the length and width of the area (m), respectively and h is the mounting height (m) of luminaires above the workplane (commonly accepted as 0.85 m above the floor).
            From room index, the parameter x is derived by rounding RI up to the next integer value, with a maximum value of 4.  The minimum number of measurement points needed is found as (x + 2)^2.  Given that x is always an integer value in the range 1 – 4, the minimum number of measurement points is always 9, 16, 25, or 36.  The measurement points are distributed throughout the space with consistent spacing in each dimension.  It is also advantageous to use similar spacing in both dimensions.
            An alternate method of grid determination involves calculation of maximum measurement point spacing:  p = 0.2 ⋅ 5^log l, where p is the maximum distance between measurement points (m) and l is the length (i.e. larger dimension) of the survey space (m).  Dividing l by p and rounding the result up to the next integer value gives the number of measurement points needed along the length of the space.  Repeating this for the width yields similar spacing in both dimensions.
            It is customary to place measurement points near the perimeter at one-half (1/2) the spacing distance from the edge of the survey area.  A 0.5 m-wide area along walls that border a survey space can be excluded from the survey area if no part of a task area resides within it.  Irregularly-shaped spaces can be divided into rectangular areas and remainders that approximate rectangles, each with its own survey grid.
 
            For each survey, descriptive information should be recorded to establish the context of the assessment, including:

• Motivation for conducting the survey (e.g. complaint, changes in facility layout or task requirements, etc.)
• Name of person(s) conducting the survey, recording data, etc.
• Definition of the survey area.  For small, simple survey areas, a verbal description may suffice; however, a diagram locating the survey area within the greater facility is advantageous.
• Age of luminaires, lamps, ballasts, control gear, and any other relevant system components.
• Condition of luminaires, lamps, etc., such as physical damage, cleanliness, and material degradation (e.g. hazing of diffusers).
• Measurement equipment information, including model and calibration data.
• Notes pertaining to the assessment, including relevant observations (e.g. abnormal air quality, flicker, unplanned task lighting, etc.) and details of deviations from the survey plan.
• Input from occupants of the space, including subjective assessments of lighting quality and visual performance.

            At the time of each measurement, additional information should be recorded, including:

• Date and time of measurement.
• Identification of measurement point.  An alphanumeric row-column identification scheme displayed on a survey grid diagram simplifies measurement point referencing.  A diagram also provides a convenient format on which to display survey results, while an alphanumeric scheme clearly identifies tabulated data.
• Deviations from planned grid position or measurement height required by obstructions (e.g. machinery).
• Illuminance measured by meter.
• Ambient temperature; temperature of lamp or luminaire is also useful, if it can be obtained.
• Voltage applied to luminaires and quality of electrical supply, if appropriate resources are available to safely acquire this information.

            For a daylit space, additional steps are required to determine the contributions of daylight and artificial light to measured illuminance.  Using the survey grid defined above, the procedure is as follows:

1. Measure the daylight illuminance in an exterior location unaffected by artificial light.
2. With artificial lighting switched off, measure illuminance at each survey grid point.
   
3. Calculate the daylight factor (DF) for each survey grid point:
   

  4.  Switch on artificial light and allow it to stabilize (minimum 15 minutes).
  5.  Measure total illuminance at each survey grid point and daylight (exterior) illuminance.
  6. Calculate the contributions of daylight and artificial light to interior illuminances as follows:
        a.  Multiply the exterior illuminance (step 5) by the daylight factor (step 3) to obtain the contribution of daylight to interior illuminance at each survey grid point.
        b.  Subtract the daylight contribution (step 6a) from total interior illuminance (step 5) to obtain the contribution of artificial lighting to interior illuminance at each survey grid point.     Note that the interior and exterior measurements must be “approximately simultaneous;” that is, separated by only a very short period of time (e.g. a few minutes).  Highly variable sky conditions, such as intermittent cloud cover, can also invalidate these measurements.
            If the survey is concerned only with the lighting system’s ability to provide adequate illumination in the worst case, i.e. with no assistance from daylight, measurements can be taken at night.  This eliminates the daylight contribution calculations and expedites completion of the survey.  It does not, however, provide a thorough assessment of lighting system performance, including daylight glare concerns, in combined lighting conditions.
            If an average daylight factor (see Part 13) has been determined for the survey space, it can serve as a shortcut.  DFavg can be substituted for individually determined daylight factors, eliminating the simultaneous interior and exterior measurements.  The illuminance values obtained may provide an indication of the lighting system’s sufficiency, but should not be used for specific lighting decisions.  Visual performance and comfort at any workstation can only be assured by location-specific and time-coordinated measurements; that is, a proper lighting survey.

            Multiple surveys may be conducted throughout the lifecycle of a lighting system.  Conducting surveys at different times of day and year provides insight into the variability of daylight contributions and its impact on visual performance, energy consumption, and other metrics.  Surveys can also be used to evaluate the effectiveness of maintenance procedures and to reveal performance trends that improve predictions of maintenance needs or identify potential control strategy improvements.  Cleaning and relamping schedules can be adjusted accordingly to reduce unjustified cost or improve system performance.
            Subsequent installations may also benefit from improved estimates of maintenance and utilization factors.  Survey results should validate design assumptions or be used to revise the models used for lighting system design.  Changes need not be limited to lighting hardware; materials and surface treatments used in a space can also be modified to improve lighting conditions.

            Discussions of lighting and surveys are often dominated by concerns of illuminance.  However, luminances in the workspace are also important and can be directly measured.  Doing so ensures appropriate luminance contrasts are maintained to support visual task performance.  These measurements can reveal the need for maintenance, such as cleaning or painting of surfaces.  Changes in materials or physical layout may also be identified that would improve a space’s lighting conditions, including the occurrence of glare.

Calculations
            Many values pertaining to lighting can be calculated, either as predictions or as determinations of system performance.  Efficacy, contrast, and uniformity and diversity ratios (see Part 14) have been discussed in previous installments of this series.  Calculations required to design effective daylighting were discussed in Part 13.  Many others may also be needed, depending on the type of source, application, and objectives.
            Exposures to UV, IR, and laser emissions and associated risk factors can be calculated.  As these are somewhat limited in application, exposure calculations are not presented here.  ACGIH provides guidance on these calculations and exposure limits in its “TLVs for Physical Agents;” this or other relevant regulatory documentation should be consulted when managing applicable operations.
            A sample of other calculations is provided as an introduction to the array of available assessment tools.  Control of glare is a critical function of lighting system design.  Effectiveness of glare-control efforts in indoor applications can be evaluated using the Visual Comfort Probability (VCP) or Unified Glare Rating (UGR).

Visual Comfort Probability (VCP)
            VCP predicts the proportion of an observer population that will deem a lighting installation acceptable, comfortable, or unobjectionable.  To facilitate reliable comparisons, luminaires are evaluated under standard conditions:

• 1000 lx (100 fc) initial illuminance.
• 80% ceiling cavity, 50% wall, and 20% floor cavity reflectances (see “Zonal Cavity Method” below).
• Luminaire mounting heights of 2.6 m (8.5 ft), 3 m (10 ft), 4 m (13 ft), and 4.9 m (16 ft) above the floor.
• Square, long-narrow, and short-wide room configurations.
• Uniform distribution of luminaires in the survey space.
• Observation point located 1.2 m (4 ft) in front of the center of the rear wall of the space and 1.2 m (4 ft) above the floor.
• Horizontal line of sight, directly forward.
• Field of view limited to 53° above and directly forward of the observation point.

            To determine VCP, a series of intermediate calculations is required for each luminaire in the survey space.  First, the position index (P) is calculated with the formula
P = exp[(35.2 – 0.31889 α – 1.22 e^(-2α/9))⋅10^-3 ⋅ β + (21 + 0.26667 α – 0.0029663 α^2)⋅10^-5 ⋅ β^2], where α is the angle (degrees) from vertical of a plane containing the source (luminaire) and the line of sight and β is the angle (degrees) between the line of sight and a line connecting the observer and the source.  These angles are depicted in Exhibit 1.

            Next, the average luminance of the field of view, LFOV, is calculated:
LFOV = [Lw ⋅ ωw + Lf ⋅ ωf + Lc ⋅ ωc + ∑( Ls ⋅ ωs)]/5, where L is the average luminance (cd/m2), ω is the solid angle (sr) subtended at the observer, and the subscripts refer to walls (w), floor (f), ceiling (c), and source (s).  This approximation assumes that the entire field of view subtends 5 sr in standard conditions; hence the division by five to obtain the average.
            The next step is to calculate the function Q for each luminaire:
Q = 20.4 ωs + 1.52 ωs^0.2 – 0.075.  Using the values defined above, an index of sensation, M, is calculated:

The index of sensation is then used to calculate a discomfort glare rating (DGR) for the entire field of view: 

where n is the number of sources in the field of view.
            Finally, VCP is calculated:

Fortunately, VCP can also be estimated using the plot of DGR vs VCP shown in Exhibit 2.  VCP data for luminaires tested in standard conditions are often tabulated as shown in Exhibit 3 for rapid reference by system designers.  VCP ≥ 70 is a typical threshold of acceptability.

            The system of VCP calculations has only been validated with lensed direct fluorescent luminaires.  Its limited scope of validation requires an alternative assessment for many applications.  Enter the Unified Glare Rating (UGR).

Unified Glare Rating (UGR)
            CIE developed the Unified Glare Rating (UGR) to provide consistent evaluations across luminaire types, locations, and other installation parameters.  The basic UGR formula is

where Lb is the luminance (cd/m^2) of the field of view less that of the luminaire, L is the luminance (cd/m^2) of the luminaire in the observer’s direction, ω is the solid angle (sr) subtended by the luminaire, and P is the position index of the luminaire, calculated as in the VCP system.  Large luminaires can be subdivided into areas treatable as point sources, as discussed in Part 12, to improve calculation accuracy.
            While UGR is broadly applicable, it, too, has limitations; it is to be used only in situations where 0.0003 ≤ ω ≤ 0.1.  The method can also be modified to use installation-specific parameters (“application-based”) in lieu of the standardized arrangement (“tabular method”).
            UGR values are correlated with subjective assessments of lighting conditions.  Values of 10 or less correspond to imperceptible glare, while values of 28 or higher correspond to uncomfortable conditions.  Several intermediate values along a continuum are also described, as shown in Exhibit 4.  If the VCP of an installation is known, its UGR can be estimated using the curve in Exhibit 5.

            CIE has developed a similar system for outdoor lighting applications.  The Glare Rating (GR) is based on observation of an array of points from a single location.  GR is often used to evaluate sports complexes and is highly dependent upon the observation point chosen.  These characteristics limit its utility in the context of workplaces and, therefore, a detailed discussion is foregone.

Zonal Cavity Method
            The zonal cavity method is used to account for differences between workplane height and the floor and between ceiling height and luminaire mounting height in illuminance calculations.  In this method, a space is divided into three cavities, as depicted in Exhibit 6.  The ceiling cavity consists of the space between the ceiling and the plane of luminaires.  The room cavity contains the space between the plane of luminaires and the workplane.  The floor cavity consists of the space between the workplane and the floor.

            The height of these cavities, identified as hCC, hRC, and hFC, respectively, are used to calculate cavity ratios: 

where h is replaced with hCC, hRC, and hFC to calculate the ceiling cavity ratio (CCR), room cavity ratio (RCR), and floor cavity ratio (FCR), respectively.  The length and width are assumed to be the same in each cavity; therefore, l and w are used without subscripts.  The CR formula above assumes a regularly-shaped (i.e. rectangular) space.  For an irregularly-shaped (i.e. non-rectangular) space,

            Calculated cavity ratios and known room surface reflectances are used to determine effective cavity reflectances.  Effective cavity reflectances replace surface, or base, reflectances in determinations of luminance ratios, etc.  Exhibit 7 shows the look-up table for base reflectances of 50 – 90%.  Note that when CR = 0, the effective cavity reflectance equals the base reflectance.  For example, surface-mount or recessed lighting creates a zero-height ceiling cavity and, therefore, CCR = 0.  Thus, the ceiling (i.e. base) reflectance is the cavity reflectance.

Cubic Illuminance
            The use of vector algebra in its calculations places a thorough discussion of cubic illuminance outside the scope of this series.  Nonetheless, an introduction to the concept is useful, as it facilitates visualization of some practical concerns of lighting installations.
            Consider a cube of infinitesimal size, centered at a point of interest, such as the center of a workstation or other task area.  The cube’s surfaces are aligned with the coordinate system used to define the space.  Each face of the cube receives optical radiation according to the lighting conditions of its surroundings, defined by opposing pairs of illuminances.  For simplicity, these can be called the left/right, top/bottom, and front/rear pairs.
            Continuing to consider cubic illuminance only in conceptual terms, each pair of facial illuminances affects visual performance in its own way.  For purposes of this presentation, it is assumed that a person working in the hypothetical task area is positioned on the “front” side of the cube.  In this scenario, the front/back illuminance pair represents the potential for shadowing of the task area by the operator (front illuminance) and for glare (rear illuminance).
            Similarly, the top/bottom pair represents the contrast of the task area.  That is, illuminance from below must be in service of the visual task performed (e.g. intentional backlighting) or be counteracted by illuminance from above to maintain visual performance and comfort.
            If at similar levels, the left/right pair are nearly independent of each other, illuminating the activity of each hand, for example.  If the levels are significantly different, however, shadows on one side and/or glare on the other can limit visual performance and comfort.
            Without the calculation procedure delineated, cubic illuminance provides a useful aid to workspace planning.  The orientation of workspaces and the placement of equipment and materials relative to a lighting system can be effectively guided by this model, even if only qualitatively.

     Numerous calculations are relevant to lighting system design and evaluation.  Any attempt to represent the full range quickly becomes unwieldy in a presentation of this type.  In this series, only a sample can be provided; the references cited, and other resources, provide much more expansive presentations for those interested in diving deeper.

Simulations
            Software becomes ever-more versatile as computing power increases and the cost to obtain it decreases.  Several available tools are capable of accepting application-specific information, including three-dimensional models of planned spaces.  Luminaire information can also be loaded directly from many manufacturers’ databases, ensuring models are based on accurate information.
            Various types of output can be obtained, including 2D (plan view or elevation) illuminance maps and 3D renderings of entire spaces.  Very detailed 3D representations are possible, with high degrees of accuracy, when all contents of a space are included in the model input.  A large amount of data is required for the most accurate renderings; these can be computationally intensive.  However, the confidence gained in the long-term performance expectations of a lighting system can justify the proactive investment.
            Perhaps most impressive is that many of these powerful tools can be utilized free of charge.  Some are created by individual manufacturers and are, therefore, limited to analysis of their own products.  Others are subsidized by large consortia of manufacturers, vastly increasing the options available in a model.   None are named here, lest it be construed as an endorsement; these tools must be evaluated with respect to a project and the abilities of team members to use them effectively.


            For additional guidance or assistance with Safety, Health, and Environmental (SHE) issues, or other Operations challenges, feel free to leave a comment, contact JayWink Solutions, or schedule an appointment.

            For a directory of “Workplace Illumination” volumes on “The Third Degree,” see Part 1:  An Introduction to Lighting (21Aug2024).

References
[Link] “Computing Visual Comfort Ratings For a Specific Interior Lighting Installation.”  Sylvester K. Guth.  Illuminating Engineering; October 1966.
[Link] “Cubic illumination.”  C. Cuttle.  International Journal of Lighting Research and Technology; January 1997.
[Link] Lighting for Health and Safety.  N.A. Smith.  Butterworth-Heinemann; 2000.
[Link] The IESNA Lighting Handbook, 9ed.  Mark S. Rea (ed).  Illuminating Engineering Society of North America; 2000.
[Link] Lighting Engineering:  Applied calculations.  R. H. Simons and A. R. Bean.  Butterworth-Heinemann; 2001.
[Link] Energy efficiency in buildings:  CIBSE Guide F, 3ed.  The Chartered Institution of Building Services Engineers; May 2012.
[Link] Handbook of Human Factors and Ergonomics, 4ed.  Gavriel Salvendy (ed).  John Wiley and Sons; 2012.
[Link] The SLL Code for Lighting.  The Society of Light and Lighting; March 2012.
[Link] “Addressing Glare in Solid‐State Lighting.”  Jeff Shuster.  Cooper Lighting Solutions; January 2014.
[Link]  Digital Transformations for Lighting in the Workplace.  Darina Dupláková and  Ján Duplák.  CRC Press, 2023.
[Link] “Threshold Limit Values for Chemical Substances and Physical Agents.”  American Conference of Governmental Industrial Hygienists (ACGIH); latest edition.

Jody W. Phelps, MSc, PMP®, MBA
Principal Consultant
JayWink Solutions, LLC
jody@jaywink.com

Cost Categories
            Cost estimates become more accurate with increasing detail.  Defining cost categories facilitates the collection of information at an appropriate level of detail.  Categories may be defined differently by organizations or individual practitioners.  If no project-specific guidelines have been established, it is useful to employ a framework based on five (5) phases of a lighting system life cycle; each phase is discussed below.  Examples cited are representative, but not comprehensive; a project team must capitalize on its members’ knowledge of the facility, objectives, and available resources to generate the best possible cost estimate.
            Acquisition (1) costs include the direct cost (i.e. price) of components, materials, and supplies required to construct the lighting system.  Transportation of goods to the installation site also incur acquisition costs; taxes and other fees that may be assessed on materials or transportation should also be included.  Administrative costs, such as those associated with generating and tracking purchase requisitions, coordinating shipping and receiving schedules, and similar tasks can also be accounted for here.  If these costs do not differ significantly among project alternatives, they will not affect comparisons, but are needed to track total project cost.
            Installation (2) costs include labor, equipment rentals (e.g. lifts, temporary lighting), tools, and incidentals.  For large installations, the quantity of packaging material may be substantial; its handling and disposal may warrant tracking as a separate line item.  Work performed during normal business hours may cause a loss of productivity.  The impact of any interruption to operations should be included here.
            The operation (3) phase typically has the greatest impact on system cost.  Electrical energy is the most-salient cost of operation, but it is one of several.  It is influenced by daylighting (see Part 13), control strategies, (see Part 12), selection of luminaires and constituent components (see Part 11), and properties of the space (e.g. size, surface reflectances).  Assessments of each must be realistic for cost estimates to be valid or useful.  For example, an intended control strategy may be overridden by occupants because it is ineffective in supporting the visual tasks performed.  Overly-optimistic estimates may be attractive to planners, but can lead to cost overruns and project failures.
            The effects of daylighting apertures and electric lighting on heating and cooling loads must also be considered.  An HVAC system may operate more frequently, or with longer cycles, to compensate for these effects.  A system that must be resized to accommodate the effects of lighting can incur significant costs for installation and operation that should be attributed to the lighting system design in cost calculations.  In an ideal case, operation of a lighting system would decrease the load on the HVAC system, such as when operating in a cold environment.  For this ideal case, a negative cost (i.e. benefit) should be recorded.
            Maintenance of the lighting system is also a significant contributor to operational costs.  Luminaires require periodic cleaning to maintain lumen output.  Relamping may be done individually, as lamps fail, or in groups, as lumen output declines below acceptable levels.  The effective service life and failure rate of ballasts and other system components should also be considered in maintenance cost estimates.
            Decommissioning (4) a lighting system can be considered the opposite of installation.  Many of the same costs are incurred; similar labor and equipment are needed in both phases.  Packaging materials may be required to prepare the removed system for transport to a recycler or refurbishing facility.
            The final phase, disposal (5), incurs costs of transportation of the removed system to an offsite location and any fees charged for accepting the material.  If the system is expected to have salvage value, whether for scrap or reuse, its residual value is recorded as a negative cost.
            The boundaries of these categories are subject to interpretation.  For example, packaging required for transport of a decommissioned system could be associated with decommissioning or with disposal.  Transportation of goods and materials could also be treated as its own category, rather than a component of others.  Myriad variations are possible; specific boundaries of cost categories are not critical to analysis and this framework is not intended to dictate any.  The collection format is secondary; it need only serve the project team’s efforts to generate thorough and comprehensible cost estimates.

Financial Metrics
Cost of Light
            The cost of light can be used to compare alternative lamps installed in the same luminaires.  This may be done to evaluate a change in lamp technology, such as upgrading from incandescent lamps to compact fluorescent lamps or screw-in LED modules.  The cost of light for each lamp is calculated as:

where:

• Clight is the cost of light ($/Mlm-hr);
• Φlamp is the mean lamp flux (lm);
• Clamp is the cost of one lamp ($);
• Clabor is the cost of labor required to replace one lamp ($);
• Llamp is the average lamp life (hrs);
• W is the mean input power per lamp, including losses (ballast, etc.) (W);
• Cenergy is the cost of electricity, including demand-based charges ($/kWh).

            While the applicability of a cost of light comparison is limited to lamp replacements, it may be a useful element of a broader analysis.  It does not account for the time value of money (TVM) or maintenance factors, but these factors can be addressed elsewhere in the analysis before a final decision is reached.

Basic Metrics
            Simple payback, as its name implies, is one of the easiest terms to calculate:  P = I/A, where P is the simple payback period (years), I is the incremental investment ($), and A is the incremental annual cash flow ($) expected.  The simple payback period is the time required for an investment to “pay for itself” if TVM is ignored.  Smaller values of P are preferred; project acceptance in many organizations is contingent upon P ≤ 3 years.
            An incremental investment (I) is the cost of an alternative relative to another.  If a project investment is compared to the status quo (the “do nothing” option), I is the total project cost.  If two potential project plans are compared, I is the difference in cost of the plans.  Similarly, the incremental annual cash flow (A) is the difference in annualized benefits, less annualized costs, of two options.
            An alternative expression is the simple rate of return, which is the reciprocal of the simple payback period, expressed as a percentage:  ROR = (A/I) x 100%, where ROR is the simple rate of return (%); A and I are as previously defined.  ROR provides the share of the initial investment (I) recouped each year by the annualized cash flow (A).  Higher ROR is preferred; a screening value of 20% is often used for project consideration.
            When TVM is considered, the discounted payback period and discounted rate of return (DROR) are calculated.  Discounting facilitates comparison of alternatives with different project lifespans by converting all cash flows to present value (PV or P).  Subtracting the initial investment (I) from PV yields the net present value (NPV) of a project:  NPV = PV – I .

Discounting
            Annualized costs and benefits are converted to present value with the following calculation:

where PV is the present value, or present worth, of the annualized cost A ($) incurred over a period of y years when the interest rate is i, expressed as a decimal.  This is known as the uniform present-worth factor; A and i are “uniform”, or constant, throughout the period of concern.
            The system’s salvage value is converted to present value by calculating the single present-worth factor:

where PV is the present worth of the project at the end of its useful life (EOL), F is the future worth of the project (i.e. salvage value) at EOL, y is the time to EOL in years, and i is the interest rate, or discount rate.
            Summing the PVs of all annualized costs and benefits attributed to the project and the project salvage value yields the present value of the lighting project.  Subtracting the initial investment, without discounting (occurs at time 0), yields the net present value (NPV) of the lighting system.  Comparing NPVs of alternatives provides a straightforward assessment – that with the highest NPV is the preferred alternative.
            The PV calculations above assume constant interest rate and annual cash flows.  Applying an inflation rate, r, to the annualized cash flows, present value is calculated according to:

If the inflation rate, interest rate, or annualized costs vary, the use of tabulated multipliers is in order.  Rather than a single calculation, the present value of each year’s costs are calculated using the multipliers in Exhibit 1 and summed:

            Present value calculations are summarized in the upper section of Exhibit 2; graphical representations of cash flows are also provided.  The lower section provides corresponding information for calculations to convert present and future values to equivalent annualized cash flows.
            The process of calculating and comparing present values of project alternatives may be referred to as life cycle cost-benefit analysis (LCCBA).  IESNA has provided a sample worksheet, shown in Exhibit 3, to facilitate LCCBA.  Its format is conducive to entry in spreadsheet software to minimize calculation errors and iteration effort.  Doing so also simplifies modification, should it be deemed appropriate, to suit a project’s analysis and presentation needs.
            As the discount rate (i) increases, the present value of an investment decreases.  The value of i at which the NPV of an investment is zero is called the internal rate of return (IRR).  It is used to compare an investment’s returns to a baseline value, known as the hurdle rate, established by a firm’s cost of capital, the level of risk inherent in an investment, and other factors.  For an investment to be considered, its IRR must exceed the hurdle rate.  Investment decisions should not be based exclusively on IRR, however, particularly when comparing alternatives with different life spans.  IRR and NPV are inextricably linked by definition and should be reviewed in conjunction.

Life Cycle Sustainability Assessment
            The categories and calculations described above focus on direct financial costs of lighting systems.  While economic analysis is critical to project success and firm solvency, there are other factors to be considered in lighting system design.  These are addressed in a life cycle sustainability assessment (LCSA).
            Financial considerations are one component of a full analysis, called life cycle costing (LCC) in the LCSA context.  The essentials are described above using the LCCBA moniker typical of general discussions of the topic.
            In addition to the economic implications of a project, LCSA considers its environmental, ecological, and social impacts.  Environmental and ecological impacts are studied in a life cycle assessment (LCA).  Potential ecological effects of lighting were introduced in Part 17, including changes in plant and animal behaviors.  Environmental impact studies consider the broader implications of material and energy use in the construction, transportation, operation, and disposal of lighting system components.
            Social impacts of lighting are also derived from nuisance light (Part 17) and security lighting (Part 16).  LCSA elevates these topics from secondary considerations to primary subjects of analysis.  This analysis comprises the social LCA (SLCA) component of LCSA.
            To the extent that environmental and social impacts can be assigned monetary value, these factors can be included directly in calculations of financial metrics.  The remainder of the analysis requires other methods, including simple judgment and ethical guidelines, to determine proper courses of action.  This is an expansive topic; however, this brief introduction must suffice for the current presentation.

Cost Control
            In each phase of a lighting system life cycle, basic cost control methods can be used to improve the financial viability of a project.  Some are described, in brief, below; a project team’s depth of knowledge supports detailed analyses that optimize cost and performance of systems.
Acquisition:  Use of recycled materials can lower cost and environmental impact of an installation.  Lighter-weight components reduce transportation costs and may require less packaging, further reducing material costs.  Any change in performance resulting from lightweighting efforts must be fully analyzed for compatibility with project objectives.  Adjustments to service life and maintenance requirements must also be made in the cost calculations.
Installation:  Lighter components may reduce the cost of labor and equipment used to install a system.  Less-expensive mounting hardware may also be capable of handling the reduced load.
            For a retrofit application, scheduling of installation activities can have a tremendous influence on project cost.  Coordinating installation to minimize the impact on operations can make a huge contribution.  Ideally, all installation work would occur outside normal hours of operation.  Staggered deliveries of materials can also reduce the impact on operations.  A large stockpile may interfere with normal activity, slowing or preventing some activities or requiring additional handling.  Similar concerns exist in new construction, though the nature of competing activities is quite different.
Operation:  A substantial portion of operational cost control relies on optimization of lighting controls.  Control systems that do not serve occupants well will be overridden and potential savings lost.  Ensure that the benefit of daylight is maximized; excessive glare will cause shades to be drawn and the required illuminance replaced by electric lighting.  The addition of a light shelf or other device may be needed to capitalize on available daylight.
            Similarly, automatic dimming controls or occupancy sensors will be deactivated if they are too aggressive or restrictive.  This returns the system to manual control, suffering the corresponding inefficiencies that the automated controls were installed to avoid.  Reprogramming control systems, with input from occupants, ensures that those occupants are well-served at minimum cost.
            Lighting system maintenance is also a substantial contributor to operational cost and, therefore, to potential savings.  Regular cleaning of luminaires contributes to lumen maintenance, extending relamping cycles.  In dusty environments, air filtration can reduce the frequency of cleaning needed to maintain required illuminances.  Contamination of luminaires can also cause heat buildup that can shorten lamp life, increasing the value of cleaning efforts.  When relamping is required, lamps with low lumen loss should be selected to further extend relamping cycles.
            These are direct inputs in the cost of light calculation presented above.  Development of LEDs has brought great improvements in cost and performance, making them viable alternatives in increasing numbers of applications.  This is also true of many relamping applications, though compatibility with the rest of the lighting system must be confirmed prior to selection.
            Where relamping with LEDs is not possible, system replacement may be warranted, as LED performance seems to have not yet peaked.  Analogous to Moore’s law for computing power, Haitz’s law for LED technology predicted, in 2000, that luminous flux would double every 18 – 24 months.  Whether this has held true is unclear, as measurement techniques differ; however, the cost per lumen has certainly fallen dramatically since this prediction was made.
Decommissioning:  Strategies used to reduce installation cost also apply to decommissioning.  Considering both in the planning stage of a project yields the best results.  During installation, information that could be helpful when decommissioning a system should be documented in the project file to support planning for this phase of the life cycle.
Disposal:  Minimizing cost of disposal requires planning at the outset of a project.  Material selections are key to this effort; recyclable materials typically incur less cost than disposable materials, for example.  A better alternative may also be available, in which the lighting system is sold for reuse.  For example, the lighting requirements may have changed in the facility in which a system was initially installed, though the system remains fully functional.  Installing this system in a facility for which it remains suitable provides cost and environmental benefits for both facilities.
            These are only high-level recommendations that can be used to initiate discussions of impact reduction.  As stated previously, it is the project team’s creativity and deep knowledge of the installation that will optimize lighting system design.


            All of the calculations and potential substitutions discussed in this installment are based on one simple assumption:  performance of alternatives is comparable.  Illuminance, uniformity, glare, spectral content, and other criteria must be within specifications for assessment and comparison to be valid.  An ineffective lighting system encourages workarounds, such as adding unplanned task lighting.  The net result is almost certainly less efficient and more problematic than a system well-designed from its inception.


            For additional guidance or assistance with Safety, Health, and Environmental (SHE) issues, or other Operations challenges, feel free to leave a comment, contact JayWink Solutions, or schedule an appointment.

            For a directory of “Workplace Illumination” volumes on “The Third Degree,” see Part 1:  An Introduction to Lighting (21Aug2024).

References
[Link] The IESNA Lighting Handbook, 9ed.  Mark S. Rea (ed).  Illuminating Engineering Society of North America; 2000.
[Link] The IESNA Lighting Handbook, 10ed.  David L. DiLaura, Kevin W. Houser, Richard G. Mistrick, Gary R. Steffy (eds).  Illuminating Engineering Society of North America; 2011.
[Link] Energy efficiency in buildings:  CIBSE Guide F, 3ed.  The Chartered Institution of Building Services Engineers; May 2012.
[Link]  “Introduction to energy efficiency.  A companion to CIBSE Guide F:  Energy efficiency in buildings.”  David Cheshire.  The Chartered Institution of Building Services Engineers; May 2012.
[Link] Handbook of Advanced Lighting Technology.  Robert Karlicek, Ching-Cherng Sun, Georges Zissis, Ruiqing Ma (eds).  Springer International Publishing; 2017.

Jody W. Phelps, MSc, PMP®, MBA
Principal Consultant
JayWink Solutions, LLC
jody@jaywink.com

            Sources vary in their use of terminology related to this topic.  Light pollution has been the term of choice for many to refer, collectively, to unwanted effects of artificial light.  However, this term may evoke the notion of a biological toxin, prompting a visceral response.  If not the goal of the term’s coining, this response is useful to activists.  Though biological effects can indeed result from errant light, the term nuisance light is favored in this presentation to recognize its broader impacts and maintain objectivity.

Types of Nuisance Light
            An example of a mast-mounted luminaire installation is depicted in Exhibit 1, with contributions to nuisance light labeled.  Nuisance light is typically described as consisting of three components:  sky glow, light trespass, and glare.

            Sky glow is the luminance of the night sky in excess of that produced by natural sources, such as moonlight or starlight.  Light emitted slightly above the horizontal (90 - 95°) (see “direct upward light” in Exhibit 1) is the greatest contributor to sky glow; light emitted in the 5° bands above and below this are also significant.  Contribution to sky glow of light emitted in various ranges of angle from the vertical are tabulated in Exhibit 2.  Light emitted in the range of 85 - 100° creates the bright “dome” visible at long distances; above 100°, the light is visible only at much shorter distances or from above.
            Sky glow is often associated with large metropolitan areas, where details of the night sky are almost totally obscured.  Small towns and individual sites, however, also generate substantial amounts of nuisance light.  Oil refineries, other processing plants, and dockyards (shipping ports), for example, are notorious for their sky glow.  In addition to direct emission, light can also be reflected upward from the many structures and surfaces present at these sites (akin to groundlight in the daylighting context), intensifying sky glow.
            Environmental factors also influence the intensity of sky glow.  Specifically, water vapor (humidity, clouds) and particulate matter (dust, pollen, pollutants) in the air diffuse light.  In fact, air itself also does so; light is subject to greater diffusion at low altitude due to the higher air density than that at high altitude.  Unlike sound, light is not directly affected by wind; however, a wind-blown dust cloud, for example, could distort the shape of a sky glow dome, “carrying” light further from the site than it might otherwise be visible (i.e. in still air).

            Light trespass occurs when light is incident where it is unwanted.  Not only does this light provide no benefit to the task area (i.e. it is wasted light), but it is a detriment to the irradiated space.  Any of the types of light shown in Exhibit 1 can be obtrusive, or trespass, depending on the characteristics of the task area, lighting system, and surrounding areas.
            Errant light is not necessarily obtrusive; unintended illuminance of an unutilized space could be inconsequential.  A negative impact on the illuminated area or an occupant’s visual field is implicit in the term light trespass.  Reduced visual performance, ecological disruptions, degraded aesthetics, or other issues may result.
            When light trespass occurs on a window of a nearby structure, light intrusion occurs.  A common concern is light intrusion of residential properties that effects circadian timing (see Part 4) and quality of life.  Light intrusion through bedroom windows is particularly egregious, as it can reduce the quantity and quality of sleep, negatively impacting well-being and potentially leading to long-term health effects.
The Institution of Lighting Professionals (ILP) has deemed light trespass a disfavored term, preferring spill light be used.  Spill light is that outside the intended target of illuminance; both direct and reflected light may contribute.  This is an unfortunate change in position by the ILP for a few reasons.
            First, overuse of similar terms can confuse would-be practitioners.  Spill light is the consequence of a light spill, the emission and/or reflection of light into unintended spaces.  The definition of light spill is conspicuously similar to that of light trespass, with one important distinction:   a light spill is the source of trespass, but its occurrence does not guarantee a trespass, as explained previously.
            Second, the stated logic for the change is flawed:  “…trespass is to physically encroach on land and light can’t do that…”  Both the premise and the conclusion are erroneous.  Trespass can refer to any type of transgression, not only physical acts.  This misguided premise is mooted, however, because light (i.e. radiation) is a physical phenomenon.  Therefore, the conclusion that “light can’t do that” is patently false, even if the premise is accepted.
            Finally, a change in terminology should provide a clearly articulable benefit to discussions of the subject matter; clarity, accuracy, and differentiation are common, laudable objectives.  The title of this installment provides a relevant example.  As explained in the introduction, supplanting light pollution with nuisance light provides an intuitive entry point to discussion of the topic and tags the phenomenon with appropriate stigma.
            The value of the terminology shift adopted by ILP is dubious at best.  It does not provide clarity, it reduces it.  Even those who are not lighting professionals intuitively understand light trespass is “light where it doesn’t belong;” for this reason, it remains an accurate, useful term.

            The effects of glare on visual performance and comfort are discussed in other installments of this series.  Particularly relevant to this discussion is the “Lighting Conditions” section of Part 9, briefly revisited here to link glare to nuisance light.
            Glare is quintessential nuisance; at its least severe, the consequence may be only a minor distraction.  Discomfort and disability glare are significantly more bothersome, decreasing visual performance and increasing risk for both personal injury and task failure.  The potential for glare corresponding to the angle of emission from a luminaire is tabulated in Exhibit 2.  Shades, other forms of shielding, and specialized eyewear may be used to minimize the impact of glare.  However, doing so is also a burden, often borne by those subject to, but not responsible for, adverse lighting conditions.  Its detrimental effects and that it is often avoidable undoubtedly qualifies glare to be labeled nuisance light.

Principles of Responsible Lighting
            Like any precious resource, light should be delivered only in the quantity required, in the limited area required, and during the period of time required.  The desire for energy conservation has provided the simplest and most-salient support of responsible lighting, reminding us to turn off lights any time they are not needed.  This is the “when” of lighting, as in “only when needed.”  Visual task performance, safety, security, and emergency situations dictate when lighting is required.  Other concepts in responsible lighting may be less intuitive, but it is easy to understand that light that is not emitted cannot contribute to nuisance.
            The “where” of lighting seems equally straightforward until one considers the challenges of achieving illuminance “only where needed.”  One objective is to eliminate spill light and, thus, light trespass.  Effectively directing light to a task area also minimizes glare and contributions to sky glow.
            The type of luminaire used and its mounting configuration are critical to effective light direction.  The beam angle should be 70° or less, as shown in Exhibit 3, to minimize glare.  Mounting luminaires at greater height facilitates achieving low beam angles.  As shown in Exhibit 4, this also reduces spill light and shortens shadows.

            Luminaires should be shielded or constructed with reflectors that achieve a beam angle below 70° and cutoff within the task area; an example of each approach is depicted in Exhibit 5.  Many styles of luminaire are now available with shielded designs to minimize nuisance light.  Externally, they are often indistinguishable from their unshielded counterparts, allowing lighting performance to be upgraded without affecting aesthetics.
            Shielding can also be achieved externally when necessary.  The prevention of nuisance light by both internal and external shielding of vertical illuminance is depicted in Exhibit 6.  Building facades and billboards are typically lighted this way.  Vertical and horizontal illuminances are managed by the same methods.
            Providing an appropriate quantity of light is achieved through source selection and control strategies.  A light source must be capable of providing the maximum illuminance required, while controls reduce output when lower illuminance suffices.  Daylighting, dimming, occupancy sensors, and other controls can be combined to minimize nuisance light.
            The spectral makeup of light also influences its contribution to nuisance.  Artificial light affects circadian rhythms in humans, photosynthesis in plants, and navigation, feeding, and other behaviors in animals.  Surface reflectances are also spectrally dependent.  For example, asphalt and concrete surfaces reflect more long-wavelength light than short-wavelength.  Thus, a parking lot illuminated by sodium lamps may contribute more to nuisance light than if it were illuminated by LEDs, for example.  Despite this, warmer colors (i.e lower CCT) are generally recommended for their lesser ecological impact.  Filtering can be used to modify a lamp’s output, though it may impact its efficacy.
            The International Dark-Sky Association (IDA) and the Illuminating Engineering Society (IES) provide a useful summary of considerations to minimize nuisance light in their “Five Principles for Responsible Outdoor Lighting,” shown in Exhibit 7.  A design process can be defined that adheres to these principles.  Descriptions of this process may differ among organizations, but the fundamental basis and objectives served remain the same.

Regulation Contemplation
            Lighting restrictions, like those on noise, can be imposed by various levels of government and administrative bodies.  A number of states have passed legislation to curb energy use, reduce sky glow, and protect natural habitats.  Many of the existing statutes are quite similar, suggesting that advocacy for the use of regulatory templates has been successful.  One such template is the IES/IDA Model Lighting Ordinance, which provides guidelines for limiting obtrusive light.
            Common features of state legislation include the limitation of scope to the prohibition of use of state funds to install or replace components of a lighting system that do not meet standards for nuisance light.  Unless local ordinances are also adopted, privately-owned and -operated installations remain unregulated.  A generic “exceptions may apply” provision leaves attainment of objectives subject to lobbying, interpretations, and other influences.
            Recommendations for the control of light, based on characteristics of an area, have been developed by various organizations.  The IDA, ILP, and CIE have developed systems of zones in which the recommendations for light management are defined.  A summary of some aspects of these overlapping systems is provided in Exhibit 8.

            Zone identification systems can be referenced directly, facilitating adoption of a model ordinance or crafting of unique legislation.  The recommendations for upward light ratio (ULR) and vertical illuminance (Ev), cited in Exhibit 8, come from ILP.  The recommended luminance limits for electronic message centers (EMCs) were developed by IDA.  Sky Quality Meter (SQM) values are also cited in zone descriptions; SQM measures the brightness of the night sky.  Values range from 16.0 to 22.0, where lower numbers indicate higher luminances.  Visit the IDA website for more information on this measurement and other assessment methods.
            Established curfews reduce energy consumption and wasted light by requiring that artificial light be dimmed or extinguished during periods of disuse.  Curfew periods vary by jurisdiction and area characteristics, but usually fall within the hours of 11 PM to 7 AM.  Most lighting is unnecessary after the curfew periods ends.  However, control of EMC luminance remains important in early-morning hours.  Likewise, waning daylight prior to the curfew period warrants a gradual reduction in luminance.

Final Thoughts
            The infographics in Exhibit 9 put the scale of the nuisance light problem in perspective.  According to IDA, an astonishing 99% of outdoor light is wasted.  To visualize this, consider the light pollution map in Exhibit 10; a color-coded SQM scale is overlaid on a map of the continental United States.  Major cities across the country appear as “hot spots,” where the sky glow is many, many times brighter than the natural sky.  Light that can be seen from space provides no benefit there, nor here on earth, yet consumes resources and ruins natural views.

            For additional guidance or assistance with Safety, Health, and Environmental (SHE) issues, or other Operations challenges, feel free to leave a comment, contact JayWink Solutions, or schedule an appointment.

            For a directory of “Workplace Illumination” volumes on “The Third Degree,” see Part 1:  An Introduction to Lighting (21Aug2024).

References
[Link] Lighting for Health and Safety.  N.A. Smith.  Butterworth-Heinemann; 2000.
[Link] The IESNA Lighting Handbook, 9ed.  Mark S. Rea (ed).  Illuminating Engineering Society of North America; 2000.
[Link] “Model Lighting Ordinance (MLO).”  Illuminating Engineering Society and International Dark-Sky Association; June 15, 2011.
[Link] Human Factors in Lighting, 3ed.  Peter R. Boyce.  CRC Press; 2014.
[Link] Handbook of Advanced Lighting Technology.  Robert Karlicek, Ching-Cherng Sun, Georges Zissis, Ruiqing Ma (eds).  Springer International Publishing; 2017.
[Link] “Guidance for Electronic Message Centers (EMCs).”  The International Dark-Sky Association; May 10, 2019.
[Link] “The Reduction of Obtrusive Light.”  Guidance Note GN01/21.  Institution of Lighting Professionals; 2021.
[Link] “States Shut Out Light Pollution.”  Jennifer Schultz.  National Conference of State Legislatures; March 25, 2022.

Jody W. Phelps, MSc, PMP®, MBA
Principal Consultant
JayWink Solutions, LLC
jody@jaywink.com

Safety Lighting
            Safety is a relative term, as it is impossible to eliminate all risk from any activity.  This distinction is integral to the definition of safety lighting:  a system of lamps, luminaires, controls, and material properties used to mitigate risks associated with an activity or inherent in a space.  Risks can be mitigated, but not eliminated, with effective lighting; undesirable or unpredictable behavior, individual capacities, and other uncontrollable factors (e.g. acts of nature) ensure that some risk remains.
            Safety lighting is one aspect of an organization’s safety culture.  Done well, it contributes to a low occurrence rate of incidents.  However, this correlation must be reinforced in organizational communication to prevent complacency and risk-tolerant behavior from negating the benefits that effective safety lighting provides.  See “Safety First!  Or is it?” [16Dec2020] for a discussion of safety culture and vocabulary.
            The defining purpose of safety lighting is the protection of people; it is required to function any time a space is occupied.  Most often, it is associated with “normal operations” in indoor spaces; however, it is not limited to this scenario.  Periods of reduced activity and outdoor spaces may also require safety lighting, though the operating parameters may be significantly different.
            Common workplace accidents include slips, trips, and falls.  Surface conditions must be monitored to remedy temporary hazards, such as liquid spills, as quickly as possible, while lighting provides awareness of the hazard in the interim.  Lighting is often the primary mitigation technique for permanent hazards, such as an uneven surface or non-standard step height, while signs and special markings increase awareness.  Such conditions may exist inside a facility or on a sidewalk, parking area, etc. external to it.
            Collisions with stationary objects or mobile equipment are also common.  The risk increases with the number of convergent paths and number of travelers.  Safety lighting improves visibility at intersections and increases accuracy of speed and distance judgments.  Collisions are often associated with material-handling equipment (e.g. forklifts, cranes), but can also occur with other vehicles, pedestrian traffic, automated equipment, etc.  The potential for collisions of the head, specifically, in cramped workspaces or during procedures performed outside normal parameters (e.g. equipment repair) should also be evaluated when considering lighting as a preventive measure.
            The contributions of lighting to accidents include the usual suspects.  Insufficient illuminance and low contrast limit visibility of hazards.  Direct and reflected glare, as well as deep shadows, can also obscure hazardous details.  Any combination of conditions that accelerates visual fatigue or eyestrain contributes to accident occurrence.
            Flicker increases the risk associated with rotating or oscillating equipment by distorting observers’ perception of its movement.  Incomplete adaptation, due to moving between spaces with significantly different illuminances, also increases risk of accidents.  Pausing to restore visual capability in new conditions before engaging in demanding visual tasks substantially reduces risk.
            Where possible, this transition time, spent idle, may be supplanted by a transition space, in which low-risk activities are conducted while visual system adaptation occurs.  This is the technique used in many building lobbies; visitors transition between interior and exterior conditions in two or more stages.  The adaptation process becomes less noticeable and the transition less visually taxing.
            In brief, to be effective, safety lighting must not only be present, but appropriate for the space, tasks, and individuals.  First and foremost, the lighting system must not create a hazard.  Avoiding the pitfalls mentioned above are key to this objective in any environment.  In some environments, however, additional precautions must be taken.
            In addition to temperature and humidity considerations, the presence of flammable gas, combustible dust, or similarly-dangerous airborne substance requires the specification of luminaires suitable to such an environment.  A summary of area classifications and luminaire types appropriate for each is shown in Exhibit 1; see “NFPA 70” for detailed information.  See Part 11 for basic luminaire information.

            Illuminance recommendations for various combinations of activity level in a space and the degree to which the space is hazardous are given in Exhibit 2.  Descriptions of the activity levels used in these recommendations are provided in Exhibit 3.  The low illuminances recommended, with no adjustment for observers’ age, suggests that maintaining a minimum level of safety is not visually demanding.  As is the case with other lighting recommendations, specific attributes of a space or its occupants may warrant deviation from these values.  Satisfaction of regulatory requirements must always be verified before finalizing a lighting system, whether deviating from published recommendations or strictly adhering to them.

Security Lighting
            Security is the extension of safety from the physical to the psychological realm.  Psychological safety is achieved by mitigating concerns of potential hazards, particularly criminal activity.  Thus, security lighting is a system of lamps, luminaires, controls, and material properties used to deter crime and unauthorized access to a site or space and increase the probability of detection, identification, and apprehension of perpetrators, thereby enhancing the perception of safety of persons and property.
            Safety lighting creates favorable conditions for all occupants of a space.  Security lighting creates advantageous conditions for authorized occupants by simultaneously creating difficult visual conditions for unauthorized occupants.  This method is often associated with exterior applications, but can also be used effectively to protect interior spaces.  It is active during normal operations; when a site is unoccupied, its mode of operation may be modified.
            A common application of security lighting is the site-entry gatehouse; an example is depicted in Exhibit 4.  Sites with stricter access-control requirements, such as nuclear processing facilities and military installations, use multiple techniques to increase the visual advantage provided to security personnel.  Requirements vary according to the type of traffic being controlled.  Pedestrian traffic requires sufficient illumination at face-level to identify each person and color-rendering capability sufficient to accurately describe attire and appearances (CRI > 80 is typically deemed acceptable).

            Vehicular traffic poses additional challenges.  In addition to identification and description of each passenger, it may be necessary to scan the entire vehicle interior for unauthorized objects or materials prior to entry and for contraband before exiting the site.  Inspection of the vehicle undercarriage may also be necessary, requiring high illuminances at ground- and near-ground levels.
            Large vehicles, such as delivery vans and freight trucks, further complicate inspection of the vehicle interior.  When the passenger compartment is above guards’ sightline, an elevated platform may be necessary to interact with the driver effectively and to scan the interior.  A potential offset may exist when undercarriage inspection is facilitated by greater ground clearance.
            In addition to intentional glare, gatehouse windows may be covered with a material with a high-reflectance outer surface and low-reflectance inner surface.  Windows may also be angled to minimize reflections from interior lighting, as shown in Exhibit 5.

            Interior conditions of a gatehouse can also be controlled for maximum security.  Minimum illuminance, provided by targeted lighting (i.e. shielded or directional luminaires), limit visibility of the positions and activities of personnel from outside the gatehouse.  Use of dark colors and low-reflectance materials further inhibits monitoring of personnel.  Positioning and brightness of self-luminous displays (SLDs) must also be carefully planned to prevent highlighting faces or hindering outward visibility.
            Lighting of the property barrier, extending from the gatehouse to surround the site, must also be considered; areas most vulnerable to intrusion should be best protected.  If a solid barrier (i.e. wall) is in place, lighting it from above minimizes shadowed areas on both sides in which intruders can hide.  Lighting of building facades serves a similar purpose, depriving a potential intruder the cover of darkness in which to gain unauthorized access to the structure.  This approach may be used in the absence of a site barrier or as an additional deterrent.
            Chain-link fence is more common than perimeter walls; its installation prompts additional lighting system design decisions that must be made to ensure the effectiveness of security lighting.
            A key decision to be made is whether the fence should be used to obscure details of the site or if high visibility is to be maintained.  For example, patrols conducted outside the perimeter fence are able to monitor activity along the fenceline and on the site grounds if visibility through the fence is maintained.  Low-reflectance materials aid visibility through a fence, while high-reflectance materials and reduced mesh size increase obscurity.  Like the windows of a gatehouse, these properties can be applied selectively to bias visibility as desired.
            The methods of intrusion anticipated influence the placement and aiming of luminaires.  For example, an intruder may climb over a fence, tunnel underneath it, or cut a hole in it.  The probability of each method of intrusion informs the specification of perimeter lighting.
            Parking facilities inside a secure site benefit from controlled access and routine monitoring; safety lighting may be sufficient to provide a sense of security among its users.  This is not the case in unsecured parking areas; users are much more exposed to potential threats, increasing the importance of proper security lighting.
            IESNA has recommended open parking areas be provided a minimum horizontal illuminance at ground level of 30 lx (3.0 fc) with an average-to-minimum uniformity ratio of 4:1 and vertical illuminance at 1.5 m (5 ft) above ground level of 3 lx (0.3 fc).  Sidewalks and other adjacent areas should be provided horizontal and vertical illuminances of 6 lx (0.6 fc) to allow identification of other persons in the vicinity.
            Enclosed parking structures pose an increased threat to users due to relative isolation and limited visibility.  A commensurate increase in illuminance is recommended to 60 lx (6.0 fc) minimum horizontal at ground level and vertical at 1.5 m (5 ft) above, with average-to-minimum uniformity ratio of 4:1.  With little daylight penetration typical of these structures, electric lighting is relied on to maintain the recommended illuminance levels at all times.

            Interior security lighting can function similarly to that on the exterior in some applications.  Examples include screening areas, such as those at entrances to municipal courthouses and airports.  Lighting the walls in public spaces with uncontrolled access, such as transportation platforms, can eliminate miscreants’ hiding places.  These examples are for normal activity in occupied spaces; security lighting is also used to protect unoccupied spaces.
            The absence of occupants shifts the function of security lighting exclusively to the protection of property.  A prominent example is the lighting of retail spaces that are closed for business.  A darkened exterior and glass façade make the presence of a person in the lighted interior easy to detect by a passerby or police patrol.  An alternative to fully lighting the space is to strategically place luminaires such that an intruder’s silhouette is projected to the façade for detection.  Motion-activated lighting also makes an intruder’s presence obvious.

            When video surveillance is in use, it must be integrated with the lighting system to ensure its effectiveness.  Haphazardly-placed cameras could be rendered ineffective if glare from security lighting “blinds” them by oversaturating its image or similar interference.  This is equally true for interior and exterior applications, though the installation challenges differ.

            Security lighting requires customization for the types of threats anticipated to both persons and property; these define the visual tasks it must support.  Illuminance, uniformity, surface reflectances, background luminance, color properties, and a newfound appreciation of glare are important factors that influence design decisions and system performance.

Emergency Lighting
            The primary purpose of emergency lighting is the protection of people, though some applications may also include a property-protection component.  It consists of escape lighting and standby lighting.
            Standby lighting is often associated with facilities that cannot be immediately evacuated in an emergency.  Hospitals and some processing plants are common examples where immediate evacuation creates additional hazards or causes loss of life.  The need to continue life-supporting treatments in a hospital is obvious; the need for standby lighting and auxiliary power in an industrial facility may be less so.  However, some industrial processes can become very dangerous if a controlled shutdown process is not followed.  When the greatest risk is of equipment damage or excessive material loss, the consequences are much less severe, but standby systems may be in place to allow these procedures to be followed when safety of personnel can be maintained while doing so.
            Of greater import to this presentation, escape lighting provides visual stimuli to aid evacuation of a space in an emergency situation.  It may also be called egress lighting, though this term seems more appropriate for use in normal conditions, when exit markings guide a visitor through a space, but the consequences of a wrong turn are miniscule.  Every escape lighting application contains sufficient nuance, equipment options, and regulatory variability to force this presentation to be only an introduction to the topic.
            Emergency lighting is a system of lamps, luminaires, auxiliary power sources, and self-luminous signs used to aid evacuation of a facility and/or execution of critical response procedures when normal operations cannot be sustained and/or dangerous conditions exist.  The most common emergency situations are power outages and fires.  The matrix in Exhibit 6 summarizes typical occurrences of these, both independent and simultaneous.  Standby lighting activation is indicated in the no-power scenarios; however, this is only true for installations, such as the examples cited above, in which standby systems have been integrated.  Escape lighting is required in all facilities; standby lighting is not.
            It should also be noted that a power outage does not always prompt an immediate evacuation; a “shelter in place” order may be the norm.  However, if power is not restored in a timely manner, evacuation becomes necessary; emergency lighting is intended only for short-term use.  The facility must be evacuated before escape lighting begins to falter.

            Emergency lighting can be powered by a generator or battery backup.  Either type of system must activate automatically when the primary power source is lost.  A sophisticated system may utilize both types, initially using battery power for rapid activation upon power loss.  Once a generator is operating, lights are switched to this source, conserving the batteries, to ensure that extended use is uninterrupted.
            The majority of escape lighting is located inside a facility.  However, evacuation may require lighting paths and assembly areas at a safe distance from the structure.  Existing safety lighting may be powered by an emergency backup source for this purpose or a separate system may be installed.
            An escape lighting installation can include a combination of wall- and ceiling-mounted luminaires and self-luminous exit signs and path markings.  The following points provide a brief description of an emergency lighting design process, highlighting important aspects for consideration.

• Identify all exits and emergency exits.
• Plan escape routes from every area within the facility, including exterior paths and assembly areas.  Plan alternate routes to ensure unforeseen circumstances do not leave occupants trapped or bottlenecked in merging escape routes.
• Define all areas and processes requiring standby lighting.
• Identify all points on all escape routes that require a direction change, surface condition warning, or other message to be visible in emergency situations.
• Identify all emergency equipment stations that require lighting.
• Identify all isolated areas, such as restrooms, that require emergency lighting to prevent disorientation, trips, falls, etc.
• Consider additional lighting and signage that may be needed in areas occupied by persons unfamiliar with the building, its escape routes, or emergency procedures.
• Consider the effect of smoke on visibility of escape route markings; it may be necessary to install floor-level signs or relocate luminaires that could obscure exit signs or other path markings.
• Specify the number of each type of emergency lighting luminaire, self-luminous sign, path marking, etc. required for all escape routes, isolated rooms, stairwells, escalators, elevators, fire equipment stations, and any other location necessary.
• Design emergency backup circuits for all luminaires that are not self-contained (i.e. standalone battery backup units), including any standby lighting specified above.
• Define test and maintenance procedures and schedules for all emergency lighting components.
• Verify visibility of all escape route markings and efficiency of evacuation routes prior to occupancy and any time that layouts or other conditions change.

            Consult the latest version of all regulations to which a site is subject to ensure that all emergency lighting requirements are met.  Illuminances, sign color, textual vs. graphical signage, and other requirements are subject to change.  A mix-and-match approach is not desirable and is often prohibited; ensuring compatibility of all components at the outset saves time, money, and frustration for the entire project team.
            The concepts presented here serve only to introduce readers to the many aspects of safety, security, and emergency lighting that must be considered to ensure the protection of persons and property in all foreseeable situations.  The references cited below are excellent resources for additional information on related research, recommendations, and regulations.


            For additional guidance or assistance with Safety, Health, and Environmental (SHE) issues, or other Operations challenges, feel free to leave a comment, contact JayWink Solutions, or schedule an appointment.

            For a directory of “Workplace Illumination” volumes on “The Third Degree,” see Part 1:  An Introduction to Lighting (21Aug2024).

References
[Link] Lighting for Health and Safety.  N.A. Smith.  Butterworth-Heinemann; 2000.
[Link] The IESNA Lighting Handbook, 9ed.  Mark S. Rea (ed).  Illuminating Engineering Society of North America; 2000.
[Link] The IESNA Lighting Handbook, 10ed.  David L. DiLaura, Kevin W. Houser, Richard G. Mistrick, Gary R. Steffy (eds).  Illuminating Engineering Society of North America; 2011.
[Link] Human Factors in Lighting, 3ed.  Peter R. Boyce.  CRC Press; 2014.
[Link] “NFPA 70:  National Electrical Code.”  National Fire Protection Association.
[Link] “NFPA 101:  Life Safety Code.”  National Fire Protection Association.
[Link] “ANSI/IES RP-7-21 Recommended Practice: Lighting Industrial Facilities.”  ANSI.

Jody W. Phelps, MSc, PMP®, MBA
Principal Consultant
JayWink Solutions, LLC
jody@jaywink.com

The “Office”
            The first step in designing effective office lighting is to understand what constitutes “office space.”  The definition used here is more inclusive than is typical to maximize the benefit to be gained from the discussion.  For purposes of this presentation, a space accurately described by any of the following three statements is treated as an office:

1. The primary task performed requires viewing a self-luminous display (SLD) (see below for more information on SLDs).  Examples include computer-aided design and drafting (CADD) and air-traffic control.
2. A significant portion of task performance relies on viewing an SLD, while the remainder requires switching between the SLD and other visual tasks, such as reading papers on a desktop.  Examples include engineering and production management.
3. Task performance requires frequent or critical interaction with readouts, gauges, and control mechanisms, including digital LCD displays, analog gauges, switches, knobs, and levers.  Examples include various types of control centers, such as those in power-generation facilities or aircraft cockpits.

            A self-luminous display (SLD) is one in which the presentation of information also provides the light required for its visibility.  Most often, this refers to computer monitors and similar screens, alternatively called visual display units (VDU), video display terminals (VDT), or similar term.  For purposes of this presentation, backlit displays and lighted gauges are also considered SLDs to ensure they receive proper attention.
            Information can be displayed with negative or positive polarity.  Negative polarity describes the display of bright text on a dark background.  Early “monochrome” computer monitors and digital alarm clocks (i.e. red numbers) are examples of negative polarity displays.  Positive polarity describes the display of dark text on a light background.  LCD screens common on calculators and digital watches are examples of positive polarity displays.  Though polarity has been defined using its common application, text, this definition should also be expanded to include the appearance of an indicator needle relative to a gauge face, graphical displays, and other non-text information.  Exhibit 1 provides examples of graphical displays in positive and negative polarity.

Effects of Office Lighting
Non-Visual Effects
            Much of the discussion of workplace lighting is centered on its stimulation of the visual system.  However, its effect on the non-visual system must also be considered to ensure effective lighting.  The nature of “office work” can make the non-visual impacts more relevant than in other types of activities.  The non-visual system, introduced in Part 4, is revisited in the context of office lighting before returning to visual system-relevant topics.
            One goal of office lighting is to generate positive affect in the space’s occupants.  A generally-favorable opinion of a space improves mood, collaboration, and overall morale with a subsequent improvement in productivity.  A popular method of increasing positive affect is to incorporate daylight and a view of the outdoors in the office plan.
            Studies and surveys have shown that the retained connection to the outside world provided by windows is highly desirable.  It can reduce the occurrence and severity of seasonal affective disorder (SAD) and can mitigate feelings of isolation that sometimes occur in “cube farms” or similarly partitioned workplaces.
Windows can also provide a physical benefit in the form of eyestrain relief.  The opportunity to focus on a distant object allows eye muscles to relax, providing a momentary reprieve from close-up tasks and intense concentration typical of a workday.
            It should also be noted that deviation from recommended lighting parameters, e.g. illuminance and luminance contrast, are typically deemed less objectionable or more acceptable when caused by daylighting than when the result of an electric lighting mishap.  Occupants of these spaces value daylight so highly that they are willing to overlook lighting conditions that would otherwise be unacceptable.  This is a testament to daylighting’s contribution to positive affect.
            The spectral content of light has a significant impact on occupants’ perception of a space.  Curves representing subjective assessments of a space’s illuminance at various lighting color temperatures are plotted in Exhibit 2.  In general, high-color temperature lighting at low illuminances is considered cold and drab, while low-color temperature lighting at high illuminances is deemed unnatural and “overly colorful.”  The area between the curves represents preferred combinations of color temperature and illuminance.  Not surprisingly, the “preferred” area expands rapidly in the region of high color temperature and high illuminance, a combination commonly associated with daylight.

            The scales of color temperature and illuminance in Exhibit 2 seem to suggest well-defined relationships between these parameters and occupant assessments.  However, these curves do not fully account for several variables, including light distribution characteristics, color gamut, task requirements, age and visual capabilities of occupants, and state of visual system adaptation.  Thus, the value of the plot lies in its conceptual framework, described above, rather than the values indicated on its axes.
            In addition to the aesthetic impact of light’s spectral content, it can also influence observers’ circadian timing (see Part 4).  High illuminance, short-wavelength light contributes to melatonin suppression, enhancing alertness.  Given that many office environments require long periods of sedentary activity that may also be repetitive, this is an important aspect of office lighting to consider.  Achieving balance among lighting objectives, including both subjective and objective measures, is the key to maximizing productivity and satisfaction in an office space.

Effect of Lighting on Displays
            Light incident on an SLD screen can be detrimental to visual performance in three ways.  First, excess illumination reduces screen contrast and, therefore, visibility of the information displayed.  Depending on the task, eyestrain, increased error rates, and reduced processing speeds may result.
            Second, images reflected in a screen can distract the user from the required task.  Attention is naturally drawn to the brightest object or area in the visual field, requiring frequent refocusing or intense concentration to stay on task.
            Third, the different distances at which the display and reflected images exist cause adjustments in accommodation.  This can cause the task to be out of focus for significant amounts of time, again increasing processing times and error rates.
            In the case of an analog gauge, there is another mode of error introduction.  An observer changing his/her viewing position to eliminate veiling reflections may obtain erroneous readings due to induced parallax.
            The extent to which incident light creates these detrimental effects is strongly influenced by the type of SLD screen in use.  Screens are divided into three categories, as shown in Exhibit 3.  Panel (a) depicts a matte-finish screen (Type I) characterized by diffuse reflection of incident light that minimizes reflected glare.  Type I screens are the most accommodating of various lighting conditions.
            Type II screens have a semi-specular finish, as depicted in panel (b).  This reduces glare by partially diffusing reflected light, but some specular reflection still occurs.  For best results in most lighting conditions, positive polarity is recommended.
            As depicted in panel (c), Type III screens are most susceptible to veiling reflections due to their specular finish.  This is particularly true when used with negative polarity or in high illuminance.  For best results, use in low to moderate illuminances with positive polarity.

Lighting Recommendations
            While those cited in previous installments of this series remain valid and generally applicable, recommendations customized for office lighting applications have also been developed.  Several important design factors are identified in the image in Exhibit 4, including lighting and control strategies.

            To minimize visual system adjustments required when switching between tasks, luminance ratios of the SLD, other (i.e. paper) tasks, and other surfaces within and surrounding a workstation must be controlled.  The recommended maximum luminance ratios for important surfaces in an SLD-centric workspace are shown in Exhibit 5.  These ratios are similar to those given in Part 12 Exhibit 12 for Environmental Classification A (i.e. controlled reflectances).  Recommended reflectances of surfaces throughout office spaces are shown in Exhibit 6.  The key difference between these recommendations and those given in Part 12 Exhibit 11 is the higher ceiling reflectance recommended in office spaces.

            To minimize reflected glare in SLD screens, indirect lighting is highly desirable.  A high-reflectance ceiling increases the efficacy of indirect luminaires by distributing light in the room while preventing direct glare.  To prevent the ceiling from becoming a source of glare or distraction, its luminance should be maintained below 850 cd/m^2 with a uniformity ratio no greater than 8:1.  Uniformity less than 4:1 is preferred; if negative polarity screens are used, 2:1 is the recommended maximum.
            If direct luminaires are used, the installation must be designed carefully to minimize direct and reflected glare.  Direct luminaires intended for office spaces limit the luminance above a defined critical angle, as shown in Exhibit 7.  A critical angle of 65° is often used; for applications known to be susceptible to lighting issues (e.g. reflective screens), a 55° critical angle may be substituted.  When SLD use is only incidental, luminaires with a 75° critical angle may be installed to achieve other lighting objectives.

            The table in Exhibit 8 provides light distribution guidelines for direct luminaires used in office spaces.  As can be seen in Exhibits 7 and 8, recommendations vary by source; it is imperative that requirements for a specific site be verified before a lighting system design is finalized.
            If both direct and indirect lighting is used, guidelines for each must be followed simultaneously.  A highly-effective lighting system can result from the combination, but an iterative design process may be required to achieve desired results.
            An excerpt of the IESNA table of recommended illuminance targets for offices is shown in Exhibit 9.  In the table, office tasks are broken down in more detail, with lighting recommendations defined for each.  This provides additional information to facilitate selection of a lighting strategy and design of a corresponding lighting system.  Additional explanations of the table’s contents are provided in the footnotes shown in Exhibit 10.

Visual Task Characteristics
            Visual performance is influenced by several factors.  In general, these factors can be controlled through job design; however, the nature of a task may limit control of one more factors.  Where control of any factor is limited, the creation of compensatory conditions should be pursued.  Likewise, compensation for any visual deficiency that may occur in those performing a task should also be pursued to the extent possible within the limits of control.
            The luminance, or brightness, of a visual task target is a critical factor; if it is insufficient to activate the eye’s photoreceptors, the target is imperceptible.  There also must be sufficient contrast between the target and its background for an observer to differentiate them.  Differentiability is usually achieved via luminance contrast; recommended maximum luminance ratios are given in Part 12 Exhibit 12.  Very low luminance contrast increases the utility of chromatic contrast (see Part 6) and it may become critical to visual performance.
            A target’s size, relative to the visual field, is another key visual performance factor.  The greater acuity required to accurately interpret a visual scene (see Part 6), the longer it takes an observer to reach a conclusion about the correct interpretation.  Thus, the duration of a target’s presentation (i.e. time) is another critical factor.
            Other factors also impact visual performance.  Rapid motion of a target may make it difficult to discern details.  If motion itself is the visual stimulus, rapidity may not be detrimental and may even reduce response time and increase accuracy.
            While objective measures of a target’s visibility (e.g. size, luminance contrast) may predict high performance on a visual task, visually “busy” or otherwise distracting surroundings can negatively impact task performance.  Conversely, monotony can cause vigilance to falter, increasing response times and error rates.  Consistency in presentation is helpful to the extent it ensures recognition of a target; beyond that, it provides diminishing returns.  If presentations are too consistent, an observer may begin to respond to a learned temporal pattern, rather than the visual stimuli presented.

Visual Comfort
            The objective of task lighting is to sustain high levels of visual performance in a small area or workstation.  Key to meeting this objective is maintaining visual comfort or, alternatively, avoiding visual discomfort.  There are several potential causes of visual discomfort; the more that exist simultaneously, the greater the compound effect.
            Glare is a common cause of visual discomfort; the greater its severity, the more detrimental it is likely to be to visual performance and physical well-being.  Light sources must be carefully placed relative to observers to minimize glare.  Light directed onto a workplane over the shoulder of an observer prevents direct glare; however, if the observer is not stationary, the movement could create shadows on the task.  Variation in positioning and/or physical stature of multiple users of a workstation could cause similar issues.
            Light sources forward of an observer should be placed outside the “forbidden zone” depicted in Exhibit 1.  The lower position depicted represents task lighting; the upper position can represent task or area lighting.  Task lighting is best placed below eye level or above 45° from the horizontal.  Area lighting should also be placed above the 45° line, unless at sufficient distance or sufficiently shielded to prevent glare at the observer’s position.

            Beyond this basic guideline, other features of the task and workstation should also be considered.  The layout of tools, materials, displays, etc. on the workplane may impose limitations on luminaire placement; for example, two light sources may be needed when only one would normally be used.  Luminaire placement may also be dictated by a need to highlight specific physical features or potential defects of a component or assembly.
            The need to highlight a part feature is an example of advantageous application of nonuniform illumination.  Without careful planning and execution, however, inconsistency or nonuniformity in lighting conditions can lead to discomfort and reduced performance.  Both spatial and temporal inconsistencies can be detrimental to response times and error rates.
            Temporal nonuniformity can distort a target’s appearance and cause eyestrain.  Potential issues associated with flicker are presented in Part 8, including the stroboscopic effect.  Exceeding the critical flicker frequency (CFF) at all times is critical to task lighting effectiveness.
            Spatial nonuniformity can lead to perceptual confusion (see Part 9), or misinterpretation of a visual scene.  The potential for this to occur can be assessed by the uniformity ratio and the diversity ratio, as follows:

• Uniformity ratio = minimum illuminance/mean illuminance.
• Diversity ratio = maximum illuminance/minimum illuminance.

In general, lighting is to be designed such that these ratios are as near to 1.0 as is practicable.  However, the practical limits of each ratio will vary with the tasks performed in a space, the individuals performing those tasks, and the size of the space evaluated.  The ratios may provide the greatest utility when comparing potential lighting strategies in a planned workspace, rather than as absolute measures.
 
Visual Task Classification
            Planning for task lighting can be facilitated by categorizing visual tasks according to physical properties of the targets and their backgrounds.  IESNA has published, and ANSI has adopted, recommendations for various combinations of target and background attributes.  Recommendations for flat surfaces of varying transmittance are given in Exhibit 2.  Recommendations for lighting three-dimensional (3D) objects are given in Exhibit 3, again arranged by level of material transparency.
            In the “luminaire location” columns of Exhibits 2 and 3, there are several references to “Fig. 19-15,” shown here as Exhibit 4.  In it, examples of luminaire types and installation positions relative to workplane and observer are shown for various task classifications.

            Common physical structures of luminaire types referenced in Exhibits 2 and 3 are shown at the top of Exhibit 4.  A brief description of each type and its key characteristics follows:

• Type S-I (Directional):  concentrating luminaires using collimating lenses or reflectors.
• Type S-II (Spread, High-Luminance):  small-area light sources with incandescent or HID lamps.
• Type S-III (Spread, Moderate-Luminance):  fluorescent luminaires with luminance variance across the light-emitting surface greater than 2:1.
• Type S-IV (Uniform-Luminance):  luminaires with variance across the light-emitting surface less than 2:1, often achieved via a diffuser.
• Type S-V (Uniform-Luminance with Pattern):  Type S-IV luminaires, sans diffuser, configured to project precise, high-contrast lines on the workplane.

            No set of tabulated recommendations can perfectly address every potential lighting scenario; they are best used as the first iteration in the design process.  Though imperfect, the recommendations are well-considered and can significantly reduce the effort required to plan a work area’s task lighting.  Evaluating deviations from recommendations can often be completed more rapidly than a clean-sheet design process.

Lighting Techniques
            Positioning luminaires above or forward of an observer requires consideration of viewing angles and reflectances of all items in the workspace; this challenge is depicted in Exhibit 5.  Lighting a workspace with a single luminaire may require a compromise mounting location, leading to veiling reflections and visual discomfort.  Often, it is advantageous to use multiple luminaires, each of lower luminance, to achieve the total illuminance required on a workplane.  This approach allows strategic placement of luminaires to achieve proper object modelling, maximizing visual comfort and performance.

            An alternative light source placement position exists below the work surface.  Backlighting a workplane, as depicted in Exhibit 6, can provide high contrast at relatively-low luminance.  The combination of high luminance contrast and minimum visual discomfort maximizes visual performance.  Backlighting is particularly helpful when working with small parts for which adequate modelling is difficult to achieve by other means.  Vertical surfaces can also be backlit to highlight features of larger objects.
            Color can be used to enhance visual performance; if neglected, poor color characteristics of a task environment can be detrimental to performance and comfort.  Contrasting colors can be used to maintain high visibility of tools, to quickly locate needed materials in storage bins, and similar process-aiding functions; see Part 10 Exhibit 1 for high-contrast color-pairing recommendations.  Like most things, color should be used in moderation; too many colors or color transitions, or too vibrant an environment can cause distraction or perceptual confusion.
            When the colors of materials cannot be modified, such as components of an assembly, modifying the spectral content of workspace illumination may be a feasible alternative.  To accentuate a color, a light source with output biased toward its wavelength can be used.  Alternatively, viewing objects through a filtering medium can achieve a similar effect without changing the overall quality of lighting.  Review the “Spectral Distributions” section of Part 7 for more information on how manipulation of light source power distribution, surface reflectances, and media transmittances can create visual stimuli that enhance task performance.  Though it does not provide a precise definition of a light source’s spectral power distribution (SPD), its correlated color temperature (CCT) may be a useful metric in early iterations of lighting system design.
            The use of color discussed thus far is for the purpose of maximizing visibility or differentiability of objects.  However, an object’s color, itself, may be the most important characteristic of the visual task.  In this case, the color rendering index (CRI) may be a useful metric.  However, the more critical is color identification, the more detail is needed to make appropriate lighting decisions.  For example, the size and shape of the color gamut (see Part 7) provides much more information about the appearance of colors under a considered light source.

The Design Guide Revisited
            The IESNA Lighting Design Guide is introduced in Part 12 (see Exhibits 6 and 7) as a helpful reference for planning facility lighting systems.  When local lighting is in use, area lighting is typically employed only for tasks with descriptors such as “coarse” or “simple,” while task lighting is crucial to those described as “fine,” “difficult,” or “exacting.”
            Planning for workspace lighting system design can be significantly accelerated by applying the Design Guide methodology.  Assessments, categories, and recommendations provided in the Guide are valid for any lighting strategy pursued.  A summary of the design process is provided in the “Design for Illuminance” section of Part 12.

Physical Well-Being
            Throughout this installment, task lighting decisions have been discussed with regards to their impact on visual comfort.  However, subsequent effects on physical comfort and well-being should not be overlooked.  The potential for visual discomfort to cause eyestrain and other physiological detriments is discussed in Part 9.
            Avoiding visual discomfort can cause physical discomfort, potentially escalating to aches, strains, and chronic conditions.  Exhibit 7 depicts the potential influence of task lighting on a person’s seated posture.  Upright posture, encouraged or facilitated by proper task illumination, reduces neck and back pain.  Unhealthy posture could also result from efforts to reduce glare and eliminate veiling reflections.  That is, an individual may shift body position to one not ideal for the task to compensate for poor lighting conditions.

            If an evaluation of the workspace does not reveal significant issues with viewing angles, illuminance, or other parameters, it may be appropriate to refer the individual for an eye exam.  It is often surprising how much our vision deteriorates before we realize it is happening.  The negative impacts of attempts to compensate for visual deficiencies provide warning signs that should be heeded.  Restoring an individual’s visual capabilities provides immeasurable benefits to both the individual and those around him/her.


            For additional guidance or assistance with Safety, Health, and Environmental (SHE) issues, or other Operations challenges, feel free to leave a comment, contact JayWink Solutions, or schedule an appointment.

            For a directory of “Workplace Illumination” volumes on “The Third Degree,” see Part 1:  An Introduction to Lighting (21Aug2024).

References
[Link] Fundamentals of Industrial Ergonomics, 2ed.  B. Mustafa Pulat.  Waveland Press; 1997.
[Link] Lighting for Health and Safety.  N.A. Smith.  Butterworth-Heinemann; 2000.
[Link] The IESNA Lighting Handbook, 9ed.  Mark S. Rea (ed).  Illuminating Engineering Society of North America; 2000.
[Link] Lighting Engineering:  Applied calculations.  R. H. Simons and A. R. Bean.  Butterworth-Heinemann; 2001.
[Link] “Lighting.”  Hugh King in Plant Engineer’s Reference Book, 2ed (Chapter 25).  Dennis A. Snow (ed).  Butterworth-Heinemann; 2002.
[Link] Kodak's Ergonomic Design for People at Work.  The Eastman Kodak Company (ed).  John Wiley & Sons, Inc., 2004.
[Link] The IESNA Lighting Handbook, 10ed.  David L. DiLaura, Kevin W. Houser, Richard G. Mistrick, Gary R. Steffy (eds).  Illuminating Engineering Society of North America; 2011.
[Link] Handbook of Human Factors and Ergonomics, 4ed.  Gavriel Salvendy (ed).  John Wiley and Sons; 2012.
[Link] Human Factors in Lighting, 3ed.  Peter R. Boyce.  CRC Press; 2014.
[Link] “ANSI/IES RP-7-21 Recommended Practice: Lighting Industrial Facilities.”  ANSI.

Jody W. Phelps, MSc, PMP®, MBA
Principal Consultant
JayWink Solutions, LLC
jody@jaywink.com

            Some fundamental terms of daylighting are at risk of confusion with colloquial use of these or similar words.  The meaning of daylight, of course, is central to this discussion.  Daylight is comprised of three components:  sunlight, skylight, and groundlight.
            Sunlight is the optical radiation received directly from the sun without reflection from another body.  Skylight is the optical radiation from the sun received after scattering in the atmosphere.  Groundlight is the optical radiation from the sun received after reflection from terrestrial surfaces, such as terrain, vegetation, and structures.
            The definition of daylight presented here is more-inclusive than may be provided by other sources, in which groundlight is often omitted.  However, the necessity of its consideration in daylighting calculations and the nature of its origin and existence render the inclusion of groundlight as a component of daylight logical and defensible.  Also, optical radiation is used, rather than visible light, to identify the relevant flux received from the sun.  Ultraviolet (UV) and infrared (IR) radiation also require consideration in daylit spaces; thus their inclusion here, serving as a reminder of such, is also logical.
            Moonlight is the optical radiation from the sun received after reflection from the moon.  Moonlight also contributes to groundlight; however, neither is sufficient for practical application to workplace illumination.  The illuminance provided by moonlight is insignificant; the fraction reflected as groundlight is negligible.
            Natural light is comprised of daylight and moonlight.  Because moonlight provides such low illuminance, its existence is often ignored and the terms natural light and daylight used interchangeably.  As colloquial uses of terms go, this one, in this context, is low-risk; moonlight will not be discussed in significant detail.
            A summary of these terms is provided in Exhibit 1.  Typical illuminance and luminance values, demonstrating the significance of groundlight and the insignificance of moonlight, are presented in Exhibit 2.

Daylighting Design Sequence
            The design sequence summarized in Exhibit 3 can be used to generate an initial concept for a daylit structure.  It is based on rules of thumb and approximations, providing a simplified assessment of each space’s feasibility or potential for daylighting.
            Use of this preliminary design sequence is also based on several assumptions, including:

• Overcast sky conditions provide uniform, diffuse light.  This assumption removes the effects of direct sunlight from the analysis; thus, orientation of the structure’s façade, relative to the sunpath, is not considered in the preliminary design phase.  Any direct sunlight that will be incident on a space must be addressed in the final design; some approaches are discussed later, in the context of design tradeoffs and daylighting methods.
• Spaces are sidelit.  Sidelighting is achieved with openings in the vertical walls of a space (i.e. conventional windows) adjacent to the workplane.  In contrast, toplighting, discussed later, is achieved with openings above the workplane, such as roof monitors.
• The approximate layout of furniture and equipment, or the intended primary use of the space is known.  The more detail that is known about the anticipated configuration of the space, the earlier effects on daylighting can be addressed (e.g. tall cubicles or numerous machines).  If no information is available, the preliminary analysis can be based on an empty room; however, this limitation should be clearly noted so that the proper adjustments can be made as information becomes available.
• Reflectances of room surfaces and anticipated contents are known.  An estimate of average room reflectance, based on experience with similar spaces, may suffice for a preliminary design.

Given the uncertainty introduced by these conditions, final design parameters should not be specified until a rigorous analysis or simulation has been conducted.

Step (1) Site conditions and programming needs
            The planned use(s) of a structure, and any number of spaces within it, provides first-stage differentiation of potentially daylit spaces.  “[C]omparable programmatic needs” may include similar contents, layouts, and occupants (e.g. children or elderly) of rooms, location on the same floor of a multistory building, and purpose or activities taking place (e.g. retail, industrial, healthcare, recreational).
            The sky angle (θ) is the angle between any horizontal obstruction of daylight (e.g. neighboring building) and the vertical obstruction created by any overhang of the window (e.g. recess in the façade), measured at the center of the window.  Shown pictorially in Exhibit 4, θ represents the vertical range of visibility of the sky from the window.  In a multistory building, the sky angle increases on each higher floor as the extent of obstruction declines.  If no obstruction is present, θ is measured from the horizontal.

            Daylight factor compares the illuminance of a point in a daylit space to the maximum possible in the given sky condition.  Formally, daylight factor (DF) is the ratio of the illuminance due to daylight at a point within an interior space to that on a horizontal plane with an unobstructed (i.e. hemispherical) view of the sky expressed as a percentage.  It is comprised of three components:

• sky component (SC), related to the sky angle;
• externally-reflected component (ERC), i.e. groundlight; and
• internally-reflected component (IRC), related to room surface reflectances.

DF = SC + ERC + IRC; Exhibit 5 depicts each component contributing to a space’s daylight factor.
            In the preliminary stages, for which this sequence is intended, the average daylight factor is both more useful and easier to estimate.  It is found using the Lynes formula:

where:

• DFavg is the average daylight factor of a rectangular daylit space (%);
  
• Aglazing is the net glazing area of daylighting apertures (i.e. area of windows less mullions and frames) (m^2);
  
• τvis is the visible transmittance of the glazed area (dimensionless);
  
• θ is the sky angle (degrees);
• Atotal is the combined area of all interior surfaces (m^2); and
  
• Rmean is the area-weighted mean reflectance of all interior surfaces, including windows (dimensionless).
  

            A modified Lynes formula can also be used:

While the Illuminating Engineering Society of North America (IESNA) has endorsed the modified Lynes formula, others cite the value of a safety margin provided by the original.  The marginal computation cost is vanishingly low, making it worthwhile to calculate both values; the additional “data point” may be edifying.
            IESNA has also provided a guideline to facilitate selection of daylight factor targets:  a space with DFavg ≥ 5% will appear well-lighted, while one with DFavg < 2% will appear dimly-lighted.  While this guideline provides insight into common perceptions of daylit spaces, it in no way limits the target values that can be pursued.  However, spaces with lower daylight factors, particularly those below 2%, will require more-frequent use of supplementary lighting if visually-demanding tasks are to be performed.

Step (2) Daylight feasibility test
            The glazing transmittance, τvis, is typically 0.80 – 0.85 for clear glass and 0.70 – 0.75 for double glazing.  If additional glazing treatments are anticipated, this value should be adjusted accordingly.  An allowance can also be made to represent the real-life performance of dirty windows.
            The feasibility of designing effective daylighting in a space is assessed by its window-to-wall ratio (WWR), the proportion of the wall area that must be glazed to achieve the desired daylight factor.  A space’s minimum window-to-wall ratio for effective daylighting is estimated as

where DFavg is the target average daylight factor in the space; τvis and θ are as previously defined.  Spaces with WWR > 80% are not considered good candidates for daylighting; adjustments should be considered to increase a space’s daylighting potential.  For example, selecting glazings with higher transmittance can reduce the area required; a refined local lighting plan could lower the target daylight factor.
            For any space that fails to pass the feasibility test, project stakeholders must choose a path on which to proceed.  Options include:

• Continue to seek adjustments, as described above, to specify attainable objectives.
• Accept a suboptimal daylighting design and adapt the electric lighting plan to compensate.
• Abandon daylighting as a significant component of the structure’s lighting strategy.

Spaces with borderline feasibility assessments (i.e. WWR ≈ 80%) can be pursued further, refining the analysis to achieve definitive results, or abandoned.  This decision is influenced by several factors, including the strength of stakeholders’ desire for daylighting, the balance between aesthetic and utilitarian values of the space, project budget, and time constraints.

Step (3) Room proportions
            To continue the preliminary daylighting design process, select values for the following parameters of the space to be daylit:

• Rmean:  mean reflectance of room surfaces, as defined previously (dimensionless).
  
• w:  width of the interior daylit space (m).
• hwindow-head-height or hw-h:  height of window head (i.e. top of window) from the floor (m).
  

            The depth of daylit area (Ddaylit-area) represents the extent to which daylight can be expected to penetrate into an interior space; it is the minimum of three calculated values determined by the parameters chosen above.
            The first value is the Lynes limiting room depth (DLynes or Dlrd), calculated as

It is used to limit excessive illuminance contrast ratios caused by excessively-high illuminances near the windows.  A steep illuminance gradient can result in a deceptively-high average daylight factor.
            The second value is the no skyline depth, calculated as Dno-skyline = (hw-h – hwp) tan θ, where hwp is the workplane height (m).  The no skyline depth defines the distance from the window beyond which there is no view of the sky on the workplane, as depicted in Exhibit 6.

            The third value, the daylight penetration depth, is dependent upon the need for shading devices, such as venetian blinds.  It is determined as follows:

Thus,

            If the depth of daylit area does not satisfy design objectives, room parameters can be adjusted and Ddaylit-area recalculated.  Several iterations can be completed quickly at very low computational cost.
 
Step (4) Required glazing area
            Setting room depth equal to Ddaylit-area (from Step 3), calculate the total surface area, Atotal, of the space.  Then calculate the minimum glazing area, Aglazing, required to daylight the space:

Verify that the Aglazing value obtained is consistent with the room dimensions selected and the WWR from Step 2.  Address any discrepancies and iterate as necessary.  Further analysis of design tradeoffs is required to ensure an optimal daylighting solution is developed.

Design Tradeoffs
            Effective daylighting requires a number of tradeoffs to be addressed in the design.  Some of the most salient tradeoffs are financial in nature; construction, operation, and maintenance expenses must be balanced to ensure a project’s viability.  There are many design choices to be made that affect each cost category, the possible combinations of which are limitless.  Therefore, a detailed discussion of all possible interactions is impossible.  Instead, some of the most-ubiquitous approaches to cost-effective daylighting are discussed.  Each presents advantages and challenges; designers must weigh these to choose appropriate solutions, as each project is unique.
            A key challenge for any daylighting project is to optimize energy consumption by balancing lighting requirements with those of heating and cooling the space.  In cold climates, solar heat gain afforded by large glazed areas may be advantageous.  However, large glazings can also cause substantial heat losses; the net effect must be determined to ensure the design specifications allow the project to meet energy efficiency targets.
            Likewise, solar heat gain must be controlled in warmer climates to maintain comfort.  In either case, the heating and cooling systems must be of sufficient capacity to accommodate the needs of the daylit space.  Reduced heat gain from electric lighting supplanted by daylight is a benefit in warmer climates, but the heating system must compensate for it in lower temperatures.

            The selection of glazings strongly influences the balance between lighting and heat load.  Glazing choices are differentiated by a number of characteristics.  Visible transmittance, τvis, is used in the determination of glazing area needed, as described above.  The solar heat gain coefficient (SHGC) quantifies the proportion of incident UV and IR radiation, as well as visible light, that is transferred to the interior by any means (i.e. transmission, reradiation, conduction, convection).  SHGC ranges from 0.0 to 1.0, though the end values are more theoretical than practical.  An SHGC of 0.0 implies the existence of an opaque perfect insulator and an SHGC of 1.0 implies the absence of any material (i.e. no glazing present).
            The ratio of visible transmittance to solar heat gain coefficient (τvis/SHGC) is called the light-to-solar-gain ratio (LSG).  It can be used to quickly evaluate a glazing’s compatibility with a project’s climate.  High-LSG glazings reduce cooling requirements, while low-LSG glazings are preferable in cold climates, as they reduce heating costs by exploiting solar heat gain.
            A glazing’s U-factor (U) is a measure of its heat conductivity (W/h/m^2/K); it is the inverse of R-value, a measure of insulating performance.  U-factor is influenced by the glazing material, thickness, and treatments, the number and spacing of layers, and the contents of the gaps.  Catalog values typically refer to heat conduction at the center of a pane.  However, this does not represent the total heat transfer by conduction through a window assembly, as frames and mullions can create thermal bridges.
            Low-emissivity (low-E) coatings can be applied to glazing surfaces to reduce reradiation, usually on an internal surface of a multilayer assembly.  Which direction the coating should face depends on the heat-gain or heat-rejection properties desired.
            Other treatments can also be applied to glazings, such as tints or spectrally-selective coatings.  A spectrally-selective coating can be particularly helpful in hot climates.  Heat gain is primarily associated with infrared radiation, which comprises nearly half of the solar radiation received.  While UV radiation comprises a comparatively small portion of solar radiation (~7%), it is notoriously detrimental to many materials, particularly synthetics.  Reflection of incident radiation outside the visible spectrum maximizes illuminance while minimizing detrimental effects of extraneous radiation.
            Typical transmittance and U-factor values for several glazing types are compared in Exhibit 7.  Using these values, preliminary design variables can be specified rapidly and refined as the project progresses.

            Another glazing-selection criterion to consider is the extent of image preservation or diffusion required.  A single layer of clear, untinted glass provides the greatest image preservation.  However, there are cases in which image preservation is a lower priority than other objectives and may even be undesirable.  Spaces in which privacy is paramount can still take advantage of daylight by using diffuse glazings that transmit light while obscuring vision.
            Intermediate conditions can be achieved with multilayer glazings, tints, or other treatments, such as fritting.  Fritted glass preserves the view through the glazing while diffusing some of the incident light.  At sufficient distance, fritting becomes unnoticeable; however, that used on the periphery of automotive windshields is quite familiar to many.
            Image preservation is relevant to the psychological benefits sought by many daylighting projects.  The ability to see the outside world can profoundly influence morale, positive affect, and overall job satisfaction.  Daylit spaces can also reduce the occurrence or severity of seasonal affective disorder (see Part 4).  In some cases, however, the outside world can encroach on concentration and productivity; for example, nearby activity can be distracting.

Daylighting Methods
            The preliminary design sequence presented was developed for sidelighting applications; this may be considered “traditional” construction practice.  Similar procedures can be followed for toplighting and hybrid applications, which are often incorporated into contemporary designs.
            Toplighting can be achieved in various ways.  A skylight is a glazed area built into a flat or pitched roof.  Some are domed to capture more low-angle sunlight (i.e. early morning and/or late evening).  Efficiency of a skylight well is heavily influenced by its size and shape, as shown in Exhibit 8.  In addition to improving transmission efficiency relative to a straight-walled well, a splayed skylight well can also improve the light distribution, reducing the luminance contrast of the well and surrounding area.

            A light pipe, or “tubular skylight,” can also be used to transmit light from a rooftop to an interior space.  A significant advantage of light pipes over conventional skylights or other glazed apertures is their flexibility.  The path from the external collector to the internal light-distribution device (e.g. diffuser) need not be a straight line, nor are complex structures required to direct light into the interior space.  Exhibit 9 depicts how a light pipe accommodates varying external and internal geometries of a structure.

            A roof monitor is a section of a structure raised to accommodate daylighting apertures above the surrounding roof.  Examples of a standard roof monitor and a sawtooth roof configuration are depicted in Exhibit 10.  A roof monitor directs light deep into the interior space with greater uniformity than can be achieved with sidelights.

            Clerestories are usually considered sidelights; however, they blur the line between toplights and sidelights.  Any sidelight located above eye-level can be called a clerestory, though the term commonly refers to windows placed immediately below the roof.  As these are often installed in tall spaces, the effect is more akin to toplighting.  This is particularly true when additional design elements, such as sloped ceilings, are incorporated to extend daylight penetration and improve light distribution uniformity.

            Techniques for controlling glare are not daylighting methods per se, but without them, daylighting would be much less effective and appealing.  The worst-case glare scenario occurs when the sun is directly visible in the workspace.  Shades, blinds, or other window coverings can be used to block the sun during these periods.  They are adjustable from the interior of the space and under the control of its occupants.
            An awning installed above a window is not adjustable.  Therefore, its benefit during periods of direct glare may be offset by undesirable illuminance reductions at other times.  In some applications, a light shelf provides a “best of both worlds” solution.
            A light shelf can be mounted on the exterior or interior of a daylit space.  It is a passive device (i.e. not adjustable) that shades the lower section of a window or adjacent area while reflecting daylight onto the ceiling through the upper section.  This increases daylight penetration and improves light distribution throughout the space.
            Baffles can be installed in skylight wells or roof monitors to protect against direct glare while minimizing the overall impact to illuminance.  Likely lines of sight to clerestories can be similarly baffled.
            These techniques are in addition to any glazing characteristics specified to aid glare control.  The techniques described are common, but not exclusive.  Designers should be encouraged to develop alternative solutions when needed to ensure an optimal daylighting solution.

Daylighting in Brief
            This installment is intended to be only an introduction to daylighting, the challenges it presents, and the benefits it offers.  For it to be manageable, the scope has to be very limited, leaving much for the interested reader to explore.
            Key topics for daylighting that could not be adequately addressed here include the use of sunpath and weather data to predict levels of solar radiation throughout the year.  The myriad building codes and other regulations in effect across various jurisdictions renders meaningful treatment of the topic impossible.  Let its mention here serve as a reminder to thoroughly research all requirements applicable to a specific site as early as possible in a project.
            Discussion of financial considerations could also be expanded.  Beyond the generalized expense categories mentioned, there may be energy and tax credits or other incentives available to improve a project’s financial viability.  These also vary by location and time and are, therefore, best left to project- and site-specific research and planning.
            Though psychological effects of daylighting were discussed only briefly, they can have an outsized impact on a project’s perceived success.  So important are these factors to productivity and morale that a view of the outdoors may be maintained even when it is contraindicated by other, usually financial, metrics.


            For additional guidance or assistance with Safety, Health, and Environmental (SHE) issues, or other Operations challenges, feel free to leave a comment, contact JayWink Solutions, or schedule an appointment.

            For a directory of “Workplace Illumination” volumes on “The Third Degree,” see Part 1:  An Introduction to Lighting (21Aug2024).

References
[Link] Lighting for Health and Safety.  N.A. Smith.  Butterworth-Heinemann; 2000.
[Link] The IESNA Lighting Handbook, 9ed.  Mark S. Rea (ed).  Illuminating Engineering Society of North America; 2000.
[Link] Lighting Engineering:  Applied calculations.  R. H. Simons and A. R. Bean.  Butterworth-Heinemann; 2001.
[Link] Kodak's Ergonomic Design for People at Work.  The Eastman Kodak Company (ed).  John Wiley & Sons, Inc., 2004.
[Link] “Guide for Daylighting Schools.”  Daylight Dividends, 2004.
[Link] “A rules of thumb-based design sequence for diffuse daylight.”  CF Reinhart and VRM LoVerso.  Lighting Research & Technology; March 2010.
[Link] The IESNA Lighting Handbook, 10ed.  David L. DiLaura, Kevin W. Houser, Richard G. Mistrick, Gary R. Steffy (eds).  Illuminating Engineering Society of North America; 2011.
[Link] Handbook of Human Factors and Ergonomics, 4ed.  Gavriel Salvendy (ed).  John Wiley and Sons; 2012.
[Link] Human Factors in Lighting, 3ed.  Peter R. Boyce.  CRC Press; 2014.
[Link] “Architecture Terminology Lighting.”  What Art.  HubPages, March 21, 2019.
[Link] “ANSI/IES RP-7-21 Recommended Practice: Lighting Industrial Facilities.”  ANSI.

Jody W. Phelps, MSc, PMP®, MBA
Principal Consultant
JayWink Solutions, LLC
jody@jaywink.com

            The lighting strategy chosen for a facility drives the selection of luminaires and lamps and their placement.  The three basic strategies available are general, localized, and local lighting.
            Local lighting is used to illuminate a relatively small area in which visual tasks are performed.  Often called task lighting, it requires additional illumination to be provided between workstations to maintain safety.  For this reason, local and general lighting are often used in combination.
            Localized lighting illuminates workstations and adjacent areas.  This type of system allows illuminances in each workspace to be customized for the tasks performed therein, while maintaining safe levels of illumination in the intervening spaces.  This strategy may be employed in a partitioned area of a facility, for example, while general lighting is used elsewhere.
            General lighting, or area lighting, is used to provide a uniform illuminance throughout a facility or an area within it.  However, due to the placement of equipment, building structures, and so on, uniformity can be elusive.  Uniform illuminance can also be highly inefficient, as the entire space is illuminated to the highest illuminance required within it at any time.  Aisles, storage areas, and similar spaces are provided much higher illuminance than necessary, or desired, when this type of system is used exclusively to meet all lighting requirements.
            High illuminances in all areas also increases the risk of discomfort and disability glare.  The most efficient lighting system is often a combination system defined by a split strategy.  A simple example, comparing the three lighting strategies discussed, is depicted in Exhibit 1.  Note that general lighting is shown in use in all cases, albeit with decreasing luminaire density.

            Various control strategies can be applied to any of the lighting strategies described above.  Control strategies can also be used individually or in combination to achieve defined objectives.  Several lighting control strategies are described below:

• always on:  lighting is powered at all times, providing constant illumination.  This strategy may be appropriate for general lighting that provides the minimum illuminance necessary for safe navigation of a facility, for example.
• manual switching:  lighting is powered on or off by a person in the illuminated space.  This strategy can lead to unnecessary energy consumption and increased maintenance requirements.
• dimming:  energy consumption of lamps is reduced, lowering the illuminance of a space, whether done manually or by other control mechanism.
• daylight sensor:  the availability of daylight allows lamps to be dimmed or extinguished, maintaining sufficient illuminance with reduced energy consumption.
• occupancy sensor:  lamps are dimmed or extinguished when the illuminated space is not in use.
• scheduling:  lighting is powered on and off according to a programmed time schedule.  This strategy may be appropriate for spaces to which access is prohibited at certain times, such as after hours in a business, school, or government building.

            The most-sophisticated lighting control systems combine two or more control strategies to maximize efficiency.  Continued development of “smart buildings” supports the proliferation of such multi-strategy and split strategy control systems.
            Specification of lighting controls must be compatible with the lamps they operate.  Energy consumption is a short-term metric that must be balanced with the long-term performance objectives of a lighting system.  For example, the effect of dimming or frequent switching on a lamp’s lumen output and service life should be considered before pairing a lamp and a control mechanism.
            Automated controls must also provide convenient overrides to accommodate atypical situations.  Maintenance tasks performed in off-hours are less intrusive to “normal business” and emergencies can occur at any time.  A spike in workload may require work at unscheduled times.  Proper illumination is required in each of these scenarios; any control strategy in place must not prevent full illumination when it is needed, even at times that the lights would normally be off.

Horizontal and Vertical Illuminances
            Thus far in the series, illuminance has been discussed, somewhat generically, as the luminous flux incident on a surface (see Part 5).  However, the orientation of a task may require differentiation between horizontal and vertical illuminances to ensure satisfactory visual performance.
            Unspecified, or undifferentiated, illuminances are typically assumed to be horizontal illuminance – luminous flux incident on a horizontal surface.  This corresponds to a typical workbench at which a wide variety of tasks can be performed.  Vertical illuminance refers to luminous flux incident on a vertical surface, such as a box on a shelf.  A common example where vertical illuminance is the critical factor is order-picking in a warehouse.
            The example shown in Exhibit 2 illustrates the utility of horizontal and vertical illuminances.  In this scene, horizontal illuminance facilitates identification of objects or irregularities on the ground, such as lost keys or a trip hazard.  In contrast, identification or description of an approaching person is aided by vertical illuminance.

            Of course, not all surfaces are perfectly horizontal or vertical.  Illuminance of an angled surface is defined by the light distribution provided by a luminaire and the angle between the surface and the emitted light rays.  Illuminance of an angled surface is discussed further in the next section.

Illumination Laws
            While some jurisdictions may have laws, or regulations, in place to define illumination requirements for some tasks or situations (e.g. roadway lighting), that is not the focus here.  This section presents physical laws of illumination that are relevant to all types of lamps, luminaires, and applications.  The three laws that are key to effective lighting system and workspace design are the inverse square law, the cosine law, and the cosine-cubed law.
            The inverse square law describes the relationship between illuminance and distance from a point source of light, as depicted in Exhibit 3.  The area illuminated by a point source increases as the square of distance from that source, while the luminous flux emitted remains constant.  Thus, E = I/d^2, where E is the illuminance (lx) of a perpendicular plane at distance d (m) from a point source of light emitting optical radiation of intensity I (cd).

            Note the difference between the inverse square law for light and that for sound [see “Occupational Soundscapes – Part 3:  The Decibel Scale” (23Aug2023), Exhibit 1].  Because sound is transmitted omnidirectionally, its intensity is equal at any point on the surface of a sphere.  Directional transmission of light maintains its intensity as distance increases, though the increasing area of the intersecting plane reduces illuminance.
            It should also be noted that luminaires are generally treated as point sources.  This method provides sufficient accuracy (≲ 1%) for a perpendicular plane at a distance greater than five times (5x) the largest dimension of the light source from the point of view of the plane.  If the plane lies closer to the source, or greater accuracy is required, the emitting area of a large luminaire, such as a multi-tube fluorescent luminaire, can be divided into smaller areas, each treated as a point source.  Summing the illuminance provided by each point source yields the total illuminance on the plane.  Reducing the area of each pseudo-point source increases accuracy at the expense of increased computational effort.

            Determination of a non-perpendicular plane’s illuminance is achieved using the cosine law.  Depicted in Exhibit 4, the cosine law describes how rotating a plane out of perpendicular to a light beam increases the area of the plane intercepting the beam by a factor of 1/cos θ, where θ is the angle of the plane, in degrees, from perpendicular.  An increase in illuminated area corresponds to a decrease in illuminance, giving E = (I/d^2) cos θ.

            The preceding description of the cosine law assumes that the entire light beam is intercepted by an “infinite” plane, or the point of interest is one element of a larger surface.  That is, all emitted light is incident on the area of interest, whether perpendicular or rotated.  This assumption leads to application of the cosine law based on the increase in “effective” area of the plane.
            An alternate interpretation of the cosine law applies to an element or surface that does not intercept the entire light beam; that is, a finite plane.  When a finite plane is rotated out of perpendicular, some of the light intercepted when perpendicular is no longer incident on the surface.  Rotation of the plane causes a decrease in the effective area of the plane, reducing the amount of optical radiation incident upon it and, thus, illuminance.
            Note that the cosine law describes a plane rotated from perpendicular to a light beam about one axis.  To determine the illuminance of a plane rotated about two axes, an additional multiplier is required to calculate the change in the plane’s effective area.
            The cosine-cubed law combines the inverse square and cosine laws in a single calculation.  The physical layout is depicted in Exhibit 5.  A trigonometric derivation (see Smith, 2000) results in the relation EA = (I cos^3 θ)/h^2, where EA is the horizontal illuminance (lx) at Point A, I is the luminous intensity (cd) of the source, θ is the angle (degrees) of the plane from perpendicular, and h is the height (m) of the source above the plane on which Point A lies.

            Note that when h = d, the cosine-cubed law reduces to the inverse square law equation.  Like the cosine law, the cosine-cubed law accounts for rotation of a plane about one axis.  This relation can also be called the combination law.

Design for Illuminance
            A typical process used to determine appropriate illuminance for an interior space can be summarized in four steps, using the IESNA Lighting Design Guide (see The IESNA Lighting Handbook, 9ed).  The first step consists of identifying the task listed in the Design Guide that most-closely resembles the task for which the lighting system is being designed.  A sample of this list is provided in Exhibit 6.  For the representative task selected, convert the illuminance category to a recommended illuminance value using the table in Exhibit 7 (Step 2).  Note that the task plane may be horizontal, vertical, or somewhere between.

            While the first two steps establish a baseline illuminance value, the final two identify adjustments to that value that may be necessary.  Step 3 consists of consulting the Design Guide table (Exhibit 6) once again.  For the representative task selected, consider the impact of the lighting characteristics listed; the significance of each is identified by a shaded block.  It may be helpful to create a ranked-order list of important characteristics to ensure each is given proper priority in design decisions.
            Considering the ranked order of quality characteristics, adjust the design illuminance as needed to optimize the area’s lighting (Step 4).  Adjustments can be made for the degree of similarity of the actual and representative tasks, visual capabilities of the workforce, and any relevant configurational aspects of the task area.  For example, a horizontal workbench may represent the primary task plane, but a backplate containing tools, parts, and other materials may also require illumination.  This “dual task plane” arrangement requires careful consideration of vertical illuminance that is not typically necessary when designing for a generic workbench (i.e. horizontal surface only).
            It is recommended that the magnitude of adjustments not exceed 30% of the baseline illuminance value obtained in Step 2.  Once installed, measurements of illuminance provided by the lighting system should be within 10% of the final design (i.e. adjusted) values.  This tolerance allows for some uncertainty in calculations and variability in lighting hardware.

            To ensure sufficient task illumination throughout the entire service life of a lighting system, additional performance factors should be included in calculations of required illuminance.  A common approach is to apply the “lumen method” of lighting design.  The lumen method defines the relationship between illuminance and lighting system performance factors according to the following:  E = (Φ x M x U)/A, where

• E is the illuminance of an area (lx);
• Φ is the luminous flux incident on the area (lm);
• M is the maintenance factor (dimensionless);
• U is the utilization factor (dimensionless); and
• A is the area of the illuminated surface (m^2).

            The maintenance factor (M) accounts for the reduction of a lamp’s output during its service life due to lumen depreciation and contamination.  Lumen depreciation refers to the decline in luminance that occurs as a consequence of a lamp’s use (i.e. “burn time”).  Lumen depreciation describes the amount of output lost, while lumen maintenance describes the amount retained.  The terms are inextricably linked and, in many cases, interchangeable.  This is also called the light loss factor (LLF), but the retained output is the relevant value in the lumen method.
            Contamination occurs when dust, oils, or other substances circulated in the air are deposited on the lamp or luminaire.  Foreign material can reduce a bulb’s transmittance or a luminaire’s reflectance and, thus, useable output of the lamp.
            Using a value of M that represents the lamp’s minimum output – i.e. just prior to scheduled replacement, assuming no premature failure – ensures that minimum required illuminances are provided for a lamp’s entire service life.  For example, if the combined effect of burn time and dust accumulation reduces a lamp’s output by 30%, at the end of its life, M = 0.70.
            The utilization factor (U) accounts for lamp output that is not incident upon the task plane.  Utilization factor can be reduced in several ways.  First, and perhaps most significantly, use of any type other than direct-lighting luminaires (see Part 11) will substantially reduce the proportion of output available on the task plane.  The use of reflectors and diffusers modify light distribution and may affect the utilization factor.  Characteristics of the room also influence the value of U; room surfaces with high reflectances may provide some recovery of task plane illumination lost to other distribution mechanisms.  If 80% of a luminaire’s light output is incident on the task plane, for example, U = 0.80.
            In addition to contamination, material degradation in luminaire components can also cause a reduction in useable output.  For example, aging of a plastic diffuser or plated reflector can cause transmittance and reflectance properties to drift.  If components that degrade are scheduled for replacement at the time of relamping, these effects can be included in the maintenance factor.  If these components are not treated as maintenance items, their degradation should be accounted for in the utilization factor.  Which factor is used to account for material degradation is not so important, as long as the effect on lighting system performance is included somewhere in the calculations.
            The lumen method uses these performance factors, the luminous flux cited in the manufacturer’s specifications for a candidate lamp or luminaire, and the area of the workspace to calculate the illuminance provided by a single luminaire.  Dividing the required illuminance, determined in the 4-step process described above, by the calculated illuminance yields the number of luminaires required in the task area.  Rounding up to the next whole number provides the minimum number of luminaires needed to meet the illuminance requirement.  Additional luminaires may be added to accommodate premature lamp failures, calculation uncertainty, or to arrive at a number that is convenient for an installation array.

            In addition to task illuminance, room surface illuminances and illuminance ratios are also important.  Each lighting strategy achieves these objectives differently; design decisions, such as the type and number of luminaires, must account for these variations.  Exhibit 8 provides room surface illuminance recommendations expressed as percentages of task illuminance.  In large spaces, ceilings and walls may be far from the task area, limiting the impact of these surface illuminances on visual performance.  However, other building infrastructure (e.g. columns, utilities) and machinery may be nearby.  Applying the guidelines to these surfaces, making adjustments for surface property variations, is appropriate.

            An illuminance ratio describes the variability of illuminance within an area or the transition between adjacent areas.  Within an area, the typical ratio of interest is the average illuminance to the minimum illuminance.  Comparing a task area to an adjacent area is typically done with a ratio of the average task illuminance to the average illuminance of the immediate surroundings.  To evaluate lighting uniformity throughout a workspace containing several task areas and intervening spaces, the ratio typically used is that of maximum task illuminance to the minimum illuminance in the workspace.
            A visual representation of the terms used in illuminance ratios is shown in Exhibit 9.  The task proper encompasses the space occupied by the items with which the task requires direct engagement; alternatively, it is the space “where the hands and eyes operate.”  The task area includes the space inhabited by additional required materials, such as tools, documents, etc.; this space is “within reach, but secondary.”  If specific information about workstation layout is not available, it is customary to assume that the task area extends 18 in forward and 9 in to each side (18 in square) of the person performing the task.
            The task margin is the transition area between a task area and other workstations or non-task areas (e.g. aisles).  A width of 18 in surrounding the task area is recommended to minimize distraction.  Recommended illuminance ratios in these areas are given in Exhibit 10.

Reflectance and Luminance Ratios
            Surface reflectances affect the luminance of task objects, work surfaces, and surroundings.  Room surface reflectance recommendations are shown in Exhibit 8.  In large spaces, as mentioned previously, the presence of machinery may be more relevant than walls and ceilings.  ANSI Recommended Practice accounts for this in its guideline by including a line for machines, equipment, etc., as shown in Exhibit 11.
            The reflectance recommendations in Exhibit 11 apply to Environmental Classifications A and B, as defined in Exhibit 12.  Environmental Classifications differentiate between areas according to the level of control designers and occupants can exert over surface properties.  Luminance ratio recommendations for each of three Environmental Classifications are also provided in Exhibit 12.

Highs and Lows
            Area lighting is often divided into two application types – high-bay and low-bay.  A high-bay application is one in which the mounting height of luminaires is at least 7.6 m (25 ft).  High-bay lighting typically requires a ratio of luminaire spacing to mounting height up to 1.0.  Low-bay mounting height is less than 7.6 m (25 ft); luminaire spacing to mounting height ratio typically exceeds 1.0. 
            The definitions of high-bay and low-bay provide a guideline for designing and specifying luminaires, but are not prohibitive.  Circumstances may warrant the installation of luminaires at heights or with spacing that differs from the typical application described by the guidelines.

Uniformity
            Lighting uniformity has been mentioned, somewhat casually, throughout this installment.  The significance of uniformity differs with the context of discussion and may be expressed in different ways.  Consider an array of overhead luminaires installed to provide general illumination of a facility.
            General lighting is among the earliest installations in facility construction.  At the time of installation, the height, spacing, output, and distribution mechanisms of luminaires provide nearly-perfect lighting uniformity throughout the structure.  That is, all illuminance ratios are ~1.0.  As additional utilities are installed, pipes, ducts, and support structures begin to crisscross the space.  Depending on the specific configuration, this may or may not significantly impact illuminance ratios at normal working height.
            The installation of machines, shelving units, and other structures adds obstructions to the original light distribution in some or all task areas.  Each also exhibits surface reflectances, modifying the area’s luminance ratios.  This idea can be extended to the placement of tools and supplies in the task area and so on.  Again, the specifics of a situation determine the magnitude and acceptability of the resulting changes in illuminance.
            The preceding is, essentially, an expanded caveat, the crux of which is this:  when discussing lighting uniformity, be sure that the context is clear to all participants.  The area of concern (e.g. task area or entire workspace), the relevant measurements and ratios, and other variables (e.g. average vs. max/min values, timeframe) must be agreed for a conversation to be fruitful.


            For additional guidance or assistance with Safety, Health, and Environmental (SHE) issues, or other Operations challenges, feel free to leave a comment, contact JayWink Solutions, or schedule an appointment.

            For a directory of “Workplace Illumination” volumes on “The Third Degree,” see Part 1:  An Introduction to Lighting (7Aug2024).

References
[Link] Lighting for Health and Safety.  N.A. Smith.  Butterworth-Heinemann; 2000.
[Link] The IESNA Lighting Handbook, 9ed.  Mark S. Rea (ed).  Illuminating Engineering Society of North America; 2000.
[Link] Lighting Engineering:  Applied calculations.  R. H. Simons and A. R. Bean.  Butterworth-Heinemann; 2001.
[Link] “Lighting.”  Hugh King in Plant Engineer’s Reference Book, 2ed (Chapter 25).  Dennis A. Snow (ed).  Butterworth-Heinemann; 2002.
[Link] Kodak's Ergonomic Design for People at Work.  The Eastman Kodak Company (ed).  John Wiley & Sons, Inc., 2004.
[Link] The IESNA Lighting Handbook, 10ed.  David L. DiLaura, Kevin W. Houser, Richard G. Mistrick, Gary R. Steffy (eds).  Illuminating Engineering Society of North America; 2011.
[Link] Handbook of Human Factors and Ergonomics, 4ed.  Gavriel Salvendy (ed).  John Wiley and Sons; 2012.
[Link] Human Factors in Lighting, 3ed.  Peter R. Boyce.  CRC Press; 2014.
[Link] “ANSI/IES RP-7-21 Recommended Practice: Lighting Industrial Facilities.”  ANSI.
[Link] “Pedestrian crossings.”  Schréder Lighting.
 
Jody W. Phelps, MSc, PMP®, MBA
Principal Consultant
JayWink Solutions, LLC
jody@jaywink.com

            When considering the lighting system on which their work depends, many only see a “light,” a “light bulb,” or possibly a “fixture.”  A luminaire is all of these, if “loose” terminology is accepted, and much more.  A very basic definition describes a luminaire as a device employed to produce, control, and distribute light.  A complete definition references additional functions and components used in various combinations to achieve desired effects.
            Rather than create an inclusive, but cumbersome, single-sentence definition, this installment expands the definition of a luminaire into more-palatable presentations of components, organized by the function it performs or supports.  This begins with the most salient function, the production of light.

Light Production
            The key component responsible for producing light or, more accurately, emitting optical radiation, is the lamp.  Lamps are often called “light bulbs,” though this is a colloquialism.  A bulb is actually one component of a lamp; it is the outer glass shell that contains the fill gas required to create an environment in which the emitter operates effectively.
            In an incandescent lamp, the emitter is a filament, usually made of tungsten, heated by electrical current.  Incandescence is the emission of visible light caused by thermal excitation of a material’s atoms.  Characteristic of emitted light, such as apparent color and CCT, are dictated by the filament material and temperature.  Filament temperature is dictated by the electrical current supplied to it.
            Characteristics of observed light output can be altered by treating the bulb.  Coating or etching the interior glass surface is common; this is often done to diffuse the emitted light for a uniform light distribution.  A bulb can also be coated with material to selectively absorb wavelengths of light (i.e. filter), customizing the lamp’s spectral output.  An example of incandescent lamp construction is shown in Exhibit 1.

            In discharge lamps, a multi-step process is required to produce light.  The most common type of discharge lamp is the fluorescent “tube,” available in several sizes.  Energizing an electrode causes the emission of electrons from it.  Collisions of these electrons with atoms of mercury, the primary constituent of the tube’s fill gas, releases ultraviolet (UV) radiation.  Phosphors coating the interior of the glass tube absorb the UV radiation and emit visible light; this process is known as down-conversion.  Characteristics of the visible light emitted are formulated by the combination of phosphors used.  A typical fluorescent tube construction is shown in Exhibit 2.
            Other discharge lamps, such as high-intensity discharge (HID) metal-halide (MH) and high-pressure sodium (HPS) lamps have similar operating characteristics, but differ in key ways.  Fluorescent tubes are double-ended (i.e. cap/connector at each end) with a single-layer glass enclosure.  Compact fluorescent lamps (CFLs) often use single-ended configurations of size comparable to standard incandescent lamps.  A typical CFL construction is depicted in Exhibit 3.

            HID lamps are often single-ended with the discharge tube enclosed in an outer bulb.  Phosphors may be used to modify the output of HID lamps, but are not required to convert the radiation emitted (i.e. emission is in the visible spectrum).  Construction of a typical HPS lamp, a common type of HID lamp, is shown in Exhibit 4.
            A characteristic shared by fluorescent and HID lamps is negative resistance.  Discharge lamps require use of ballasts to limit current and maintain the electrical supply to the lamp.  The ballast is often housed in the luminaire, external to the lamp, but may also be mounted remotely if physical space limitations or temperature extremes warrant.  Some lamps, such as CFLs, are constructed with an integral ballast.  It is its integral ballast that allows a CFL to be used as a drop-in replacement for a less-efficient incandescent lamp.  Electronic ballasts are preferred to magnetic types for the performance improvements they provide.  These include increases in efficacy and lamp life, integrated circuits with advanced control capabilities, reduced weight, and the elimination of flicker (see Part 8).

            Solid-state lighting (SSL) is rapidly eclipsing all other lamp types in many applications.  Their long service life, high efficacy, “programmability,” durability, light weight, flexible form factor, low operating cost, and ever-improving installation cost provide economic and performance advantages in a variety of applications.
            SSL is an umbrella term for various types of LED or light-emitting diode.  LEDs are semiconductor diodes; a semiconductor is a material characterized by resistance between that of a conductor and an insulator that can be modified by the application of an electric field and a diode is a device that allows electrical current to flow in only one direction.  LEDs emit visible light via injection luminescence, where electrons and electron holes are recombined, ejecting photons.  An electron hole is a position in a material’s valence band where an electron could exist, but does not; it is an empty space in the valence.  The wavelength of emitted light is dictated by the diode material.
            Further exploration of the inner workings of LEDs is left to the interested reader.  The complexities of semiconductor physics is far beyond the scope of this series, but the increasing importance of SSL should make good references relatively easy to find.
            Optical radiation emitted by LEDs can be manipulated by methods similar to those used for other light sources.  Phosphors can be used to convert UV radiation to visible light and multiple LEDs can be combined, using additive color mixing (see Part 7) to achieve desired output.
            Like discharge lamps, LEDs require a control device between the electrical power supply and the lamp, called a driver.  Drivers are made in constant-current and constant-voltage configurations to match the construction of the LED package.  The driver converts AC power to DC and conditions it to maintain proper function of the LEDs.
 
            Other lamp types are also available, including combination lamps that use more than one of the types described in a single unit.  Those described above represent the vast majority of relevant installations, as well as a progression of lighting technology over the past century.
 
Light Control
            The control of light, as a function of luminaires, is subject to interpretation.  Some control factors were discussed in the previous section, as they pertain to the production of light.  For example, ballasts and drivers, as well as other components, such as igniters, control light by establishing and maintaining conditions conducive to visible light emission.  These controls are electrical in nature.
            Dimming is another electrical control capability; it may be integral in a discharge lamp ballast or LED driver.  It may also be achieved by an external device, as is the case for incandescent lamps.  The mechanism used to dim a lamp’s output varies with the type of lamp; compatibility of devices and objectives must be verified before integrating them in a lighting system.
            Controls of a chemical nature are also used.  Coating a bulb’s interior with phosphors to modify a lamp’s spectral signature is one example.  The selection of fill and ionizing gasses and electrode and diode materials are also forms of chemical control.
            Controlling the distribution of light is so important that its arguably redundant inclusion in the basic definition of a luminaire is acceptable and justified.  It also warrants its own section in this presentation.

Light Distribution
            The distribution of light is controlled by physical means.  The first step in light distribution control is the selection of direct or indirect lighting, or a combination in specified proportions.  CIE classifies luminaires according to the proportions of direct and indirect light emitted.  The six CIE classifications, summarized in Exhibit 5, are based on light distribution as well as directionality.

            Further control can be achieved with diffusers, reflectors, refractors, and louvers.  As discussed previously, light can be diffused at the source – the lamp – by way of bulb treatments.  A diffuser can also be external to the lamp, such as a translucent glass or plastic covering.
            A reflector can also be integral to a lamp.  In lamps with an internal support structure, a formed and polished metal sheet can be attached.  Alternatively, a portion of the bulb can be aluminized to create a reflective surface that redirects light for a more-advantageous distribution.  It is more common, however, for reflectors to be external to the lamp, eliminating the need for precise orientation of the lamp in its socket.
     Refractors are used to modify a luminaire’s light distribution to create a pattern better-suited to the application.  A refractor could narrow an emitted light beam, increasing the illuminance of a reduced area.  It could also be used in the reverse, increasing the area illuminated, though with reduced illuminance.  These are basic refractors, typically consisting of a glass or plastic cover with a prismatic array formed into one side.  More-complex refractors can be developed, but simpler solutions to the lighting challenges they would solve are typically available.
            Louvers are used, primarily, to reduce glare by limiting the angle at which a lamp can be seen.  Louvers are simply small baffles, often arranged in a grid pattern, that block one’s view of a lamp at certain angles without reducing the direct downward emission of light from the luminaire.  They can also be configured to reflect incident light, modifying the distribution.  The choice of material and finish depends on the purpose of the louvers and aesthetic concerns.
            A luminaire may be said to include a lens; however, this is often use of “loose” terminology.  It is most accurate when referring to a prismatic refractor, but it may also be used to identify a diffuser or transparent cover.  It is best to assume it is being used generically to identify some type of cover for the luminaire’s opening.  If the true nature of the “lens” is important, it should be verified.

Physical Structure
            In order for a luminaire to perform any function reliably, it must have an appropriate physical structure.  The first requirement of the physical structure is to provide a stable and secure mounting mechanism.  It must support the weight of the luminaire, withstand any shock or vibration to which it may be subject, and limit the effects of temperature extremes.
            A luminaire housing provides physical protection for the lamp.  In some applications, such as a medical or manufacturing clean room, the environment may require protection from the lamp in the case of a catastrophic failure.  In such applications, the luminaire must contain glass fragments and any internal components of the lamp that separate, preventing debris from escaping.
            A mechanism may be required to facilitate aiming the luminaire as a first step in creating the desired light distribution.  To effect further control, the structure must include provisions for attachment of a diffuser or other light control device.
            Luminaires may be components of a facility’s heating, ventilation, and air conditioning (HVAC) system as well as its lighting system.  In this type of application, pathways are designed into the structure that allow air to flow through the luminaire to attached HVAC ductwork.  This arrangement may facilitate installation in space-limited applications or in maintaining stable conditions conducive to lamp operation and service life.  It is often used to prevent heat generated by lamps from adding to the heat load of an air-conditioned space.
            Less-stringent applications may employ open luminaires that allow free-flow of air to limit dust buildup.  The opposite extreme is occupied by environments that require sealed luminaires to prevent dust, debris, or liquid from entering.  The extent to which a luminaire can prevent such infiltration is described by the ingress protection (IP) rating system.
            IP ratings are cited in the format “IP##,” where the first “#” represents a device’s level of protection against ingress of solid material and the second of liquid (i.e. water).  A “0” in either position indicates that no protection is provided against infiltration of the corresponding material type, while an “X” indicates that the level of protection is unknown (i.e. untested).  A summary of the basic IP rating system is provided in Exhibit 6.  IP rating systems have been defined in several national and international standards with some variation.  For example, some sources include a water-ingress rating of 9, indicating protection against high-pressure, high-temperature streams.  The summary shown is a useful reference; however, current versions of applicable standards should be consulted for critical applications.

            Additional rating systems are available to identify devices appropriate for use in dangerous environments.  Flameproof and explosion-proof luminaires can be specified for installations in the presence of combustible dust or gas.  Applicable standards and manufacturer data should be consulted before specifying luminaires for such an environment.

Electrical Connection
            A luminaire must provide the means to safely connect to an electrical power supply.  The exact nature of the connection may vary depending on the luminaire’s configuration.  For example, connection must be made to remotely-located control gear, if used.  A luminaire with all required control gear housed internally is connected to supply power directly.
            Provisions for safe connection include properly-sized wires, terminals, and circuits, proper grounding, and protection against physical damage, such as wire chafing or mechanical stress.  Equipment purchased from reputable suppliers can be expected to adhere to high standards of electrical safety; however, installation and maintenance must be performed in a manner that ensures a high level of safety is sustained.
 
            Luminaires can be classified or ranked in various ways.  Several of these relate to characteristics of the lamp contained within.  Typical properties of a number of lamp types are summarized in Exhibit 7.  Among them are CCT and CRI; both were presented in Part 7 of the series and are relevant to applications where color properties are important.

            Luminous efficacy and lamp life are important components of financial analyses.  Luminous efficacy relates the amount of electrical power required to the luminous flux generated; it is a direct input to operating cost estimates.  Lamp life drives maintenance needs and associated costs, including a relamping schedule (timeline of lamp replacement).
            Warm-up time and restrike time of a lamp influence the operation of a lighting system.  Warm-up time refers to the lag between powering on a lamp and it reaching its maximum output of optical radiation.  Whether due to power interruption, thermal overload, or other cause, some lamps require a cool-down period to re-establish conditions conducive to its operation.  Restrike time is the amount of time that must elapse after a lamp is extinguished before it can be restruck, or “re-lit.”
            Other classifications refer to the application for which a luminaire is intended.  These include designations for high-bay and low-bay installations, spotlights, floodlights, area lights, task lights, and so on.  Discussion of these differentiators is best pursued in the context of their applications, lest it be too generic to be useful.  It is the aim of future installments of this series to explore various applications, preferred luminaires, and other aspects of lighting systems.


     For additional guidance or assistance with Safety, Health, and Environmental (SHE) issues, or other Operations challenges, feel free to leave a comment, contact JayWink Solutions, or schedule an appointment.

     For a directory of “Workplace Illumination” volumes on “The Third Degree,” see Part 1:  An Introduction to Lighting (21Aug2024).
 
References
[Link] Lighting for Health and Safety.  N.A. Smith.  Butterworth-Heinemann; 2000.
[Link] The IESNA Lighting Handbook, 9ed.  Mark S. Rea (ed).  Illuminating Engineering Society of North America; 2000.
[Link] “Lighting.”  Hugh King in Plant Engineer’s Reference Book, 2ed (Chapter 25).  Dennis A. Snow (ed).  Butterworth-Heinemann; 2002.
[Link] Kodak's Ergonomic Design for People at Work.  The Eastman Kodak Company (ed).  John Wiley & Sons, Inc., 2004.
[Link] The IESNA Lighting Handbook, 10ed.  David L. DiLaura, Kevin W. Houser, Richard G. Mistrick, Gary R. Steffy (eds).  Illuminating Engineering Society of North America; 2011.
[Link] Handbook of Human Factors and Ergonomics, 4ed.  Gavriel Salvendy (ed).  John Wiley and Sons; 2012.
[Link] Human Factors in Lighting, 3ed.  Peter R. Boyce.  CRC Press; 2014.
[Link] “ANSI/IES RP-7-21 Recommended Practice: Lighting Industrial Facilities.”  ANSI.
[Link] “CFL Light Bulb.”  Energy Education.
[Link] “IP (Ingress Protection) Ratings.”  Blue Sea Systems.

Jody W. Phelps, MSc, PMP®, MBA
Principal Consultant
JayWink Solutions, LLC
jody@jaywink.com

Visual Communication Methods and Applications
     Light and color are utilized in various ways to achieve effective communication.  Providing sufficient illumination to read a printed page, for example, is a basic component of visual communication; however, its scope is much broader than the choice of light sources based on luminance, CCT, CRI, and other physical parameters.  In many applications, it is the light that is viewed, rather than used to view other objects.
     Transportation vehicles provide examples of the use of light as both an aid to vision and as a visual target.  On the exterior, headlamps provide light to increase visibility of nearby objects and road surfaces forward of the vehicle.  Reversing lights serve the same function to the rear of the vehicle.  Exterior vehicle lights used as visual targets include brake lights, turn signals, and various “marker” lights.  Use of red lights is limited to the rear of a vehicle to provide a reliable indication of its direction of travel.  Taillights are maintained at moderate brightness, while brake lights are much brighter to provide a warning that the vehicle is slowing.  Turn signals blink, or flash, to attract attention and to indicate an intended direction change.  Though it employs a simple lighting scheme, a vehicle’s exterior conveys a substantial amount of information about its size, direction of travel, and changes in velocity.
     In a vehicle’s interior, entry lighting, map lights, and lights in storage areas (e.g. glovebox, console) aid passengers’ vision.  The driver’s vision is also aided by backlighting of the instrument panel and important controls.  Many lights are used as targets of a driver’s sight; fortunately, most are not presented simultaneously.  Many warning lights exist in modern vehicles, including the infamous “check engine” light, low tire pressure, and low oil pressure warnings.  Warnings of pending catastrophic failures, such as low oil pressure, are typically displayed in red, while those of lower severity are indicated in amber.  Some may blink to attract attention, such as the “fasten seat belt” admonishment; this example is also accompanied by an audible warning (beep or buzz).  Redundancy of audible and visible signals was introduced in “Occupational Soundscapes – Part 10:  Communication Systems” [6Mar2024] to overcome high-intensity noise; in this example, redundancy is employed to maximize salience in all conditions.
     Aviation, rail, and marine applications also provide familiar examples.  Airfield lights differentiate runways and taxiways for approaching pilots and increase visibility in adverse conditions.  Red- and green-lighted buoys are often used as channel markers, allowing boat captains to identify a safe route into a harbor.  A pair of red lights, flashing alternately, identifies a crossing and warns of an approaching train.
     The various forms of transportation provide a plethora of examples of light used in visual communication, but its applicability extends far beyond this realm.  Other forms of visual communication are also used; broad familiarity with various modes of transport provides a convenient source of examples that will continue to be exploited.

     Signs are also common elements of visual communication.  In addition to text, color, symbols, and other graphic elements are used to increase the efficacy of signs as communication media.
     Traffic control and warning signs are ubiquitous on public highways, varying in size, shape, color, and content to facilitate rapid transfer of information.  Large green signs, mounted above travel lanes, provide route information, primarily with text.  Moderately-sized signs are posted along roadsides to convey various information.  Red signs, such as octagonal stop and triangular yield signs, inform drivers of traffic control requirements.  Yellow signs, often diamond-shaped, convey warnings or recommendations, such as reduced speeds in curves.  A blue and white rectangular sign, with a large “H” as its focal point, guides drivers to a nearby hospital.  White and black signs are used to post speed limits and other traffic-flow information.  Orange signs inform drivers of detours, construction zones, or other special circumstances.  Many signs are standardized, usually at the federal level, to ensure consistent presentation, preventing confusion of travelers.
     The U.S. Coast Guard (USCG) standardizes signs, signals, and procedures used in navigable waterways in the U.S.  It also coordinates with other maritime agencies worldwide to ensure safe transportation in international waters.  The USCG publishes various manuals and visual reference guides to the signs, lights, and sounds used for nautical navigation.

     Standardized symbols are also important components of visual communication.  A demonstration of the power of standardized symbols was provided in “Commercial Cartography – Vol. V:  Hazard Mapping” [12Jan2022].  Consistent presentation allows rapid transfer of vast amounts of information because no additional explanation or clarification of symbols is needed for accurate interpretation of intended messages.  This can be achieved with little to no text, increasing the speed at which the information can be assimilated.

     Many of the examples of lights and signs cited thus far are fixed in location and content of the messages they can convey.  Flags, signboards, and hand signals provide flexibility in messaging; these can be used individually or in combination to create desired messages.
     Hand signals, augmented by self-luminous torches when conditions warrant, guide aircraft pilots in commercial terminals and on the decks of aircraft carriers.  The signaler is mobile at all times, allowing both the location and content of messages to be adapted as circumstances change.
     All three of the methods mentioned can be seen in use in motorsports.  A racer’s crew may display a pit board with a brief message as the racer passes.  Around the course, racers receive messages from marshals via flags.  The color and motion of a flag conveys a specific message about conditions on the race course.  Given that motorsports events are often very loud, hand signals provide a convenient method of communication among marshals.
     Another setting in which hand signals play a critical role in safety and efficient operation is a construction site, or any scenario in which rigging equipment is in use.  Verbal communication is often made impossible by distance and ambient noise created by the equipment in use or other necessary activities taking place nearby.  A standard vocabulary of signals conveys to an equipment operator when it is safe to move forward or back, raise or lower a load, and so on.  Without the coordination enabled by such visual communication, these sites and activities would be much more dangerous and much less productive.

     Nonverbal communication, as typically conceived, includes the facial expressions and body language of a person engaged in an interaction with another.  Though these cues are processed visually, they are not typically considered components of visual communication.  The term is typically used to describe prescribed messages in designed formats, transmitted with clarity and intent.  The undefined, often unintentional, nature of nonverbal communication creates only a peripheral relation to visual communication.

Theories of Visual Communication
     Several theories have been developed to explain the mechanisms and effectiveness of visual communication.  Among them are theories of perception, representation, reception, cognition, and semiotics.  A brief description of each is provided below.
     Perception theory contends that all visual communication has a neurological basis.  Vision occurs within the brain and our emotional responses dominate those driven by reason.  For example, a startling noise triggers an emotional fight-or-flight-type response, inducing physiological preparations, before the “rational brain” can assess the threat.
     Representation theory treats an image as a substitute for a real object.  Viewing an image, such as a photograph, invokes the same response as an encounter with the represented item.
     Reception theory is concerned with how visual elements convey a message, but is unconcerned with what meaning is derived from it.  This can include the use of color, symbols, or cultural references that create meaning in accordance with the message context.
     The theory of visual cognition establishes visual primacy, the dominance of vision or its prioritization over other sensory inputs.  Three evidentiary assertions support the primacy of vision:

• The brain contains regions specializing in visual processing.
• Visual processing occurs at higher speed than that of verbal stimuli.
• When verbal and visual stimuli are incongruent, the verbal input is ignored.

     The theory of visual semiotics is concerned with how the meanings of visual images are interpreted.  Generically, semiotics is “the science of signs” or the study of signs and symbols.  Its scope is broad, encompassing graphical images, physical gestures, and text and their intersections with culture and technology.
     This list of visual communication theories is not comprehensive, nor are the descriptions thorough; there is extensive literature for those interested in gaining a deeper understanding of these, or other, theories.  The objective of this brief introduction is to increase awareness of the various perspectives that can be taken with respect to any visual communication.  Awareness serves to improve the design of messages and media to maximize efficacy of visual communication for its intended audience and in unique contexts.

Recommendations
     The following is a selection of recommendations for implementation of visual communication in the workplace.  These recommendations specifically target improved safety and productivity in commercial operations; they may be less appropriate in other settings and should not be considered universal rules.

• Increase size of visual target to compensate for its distance from viewer.  For example, a person whose tasks require substantial mobility may require a larger target to be visible at the furthest points in the workspace.  Balance this with the message’s priority, ensuring that critical signals are easily detected among less-urgent information.
• Increase luminance contrast to improve visibility of visual targets, especially for workers of advanced age.  Luminance contrast is analogous to S/N for audible signals (see “Occupational Soundscapes – Part 9:  Concepts in Communication” [7Feb2024]), where the visual target is the signal and its background is the noise.
• When using hand signals or gestures, exaggerate movements to increase visibility and clarity.  Gestures made in front of the body are often difficult to see; gesture to the side whenever feasible.  Torches and high-visibility gloves are simple additions that can be used to enhance visibility of hand signals.
• Maximize use of standardized signs, symbols, and light signals.  Limit text, to the extent possible, to special situations (e.g. not covered by standardized information) that can accommodate the additional processing time required to assimilate the information.
• Minimize glare, particularly in areas where safety-critical operations are conducted.  Glare is akin to masking of audible signals (see “Occupational Soundscapes – Part 9”).  While auditory masking is frequency-dependent, the effect of glare is intensity-dependent; however, both create the potential for a signal to become imperceptible as a consequence of competing sensory inputs.
  
• Use redundant (e.g. visual + audible) signals for critical information.  This is particularly helpful when one’s tasks require mobility that removes the visual signal from his/her field of view for any length of time.
• Use intermittent (i.e. flashing) signals to attract attention to an urgent message in a “busy” visual field.
• Use a constant signal when additional information is contained within the signal, such as a symbol or text that must be properly identified.
• Combine intermittent and constant signals to maximize efficacy of communication.  For example, two signals can be used; one flashing to draw attention, one constant to convey the core message.  These functions can also be combined in a single indicator by flashing for a period, then remaining constant for a period.
• Use contrasting colors to maximize visibility of messages.  Exhibit 1 shows the ten highest-contrast color pairings under white light.

• Color-code information to accelerate access.  Materials, process steps, storage locations, and more can be identified this way to reduce errors and increase productivity.
• Test lighting and visual elements of communication to validate design assumptions; adjust accordingly.  For example, light sources with lower CRI or smaller color gamut than anticipated may warrant changes in signage to achieve desired presentation characteristics.

     This installment provides only a brief overview.  There is a great deal more to visual communication than was presented here.  The scope of this discussion is intentionally limited, endeavoring to present aspects that are readily applicable in the context of workplace lighting system design for safety and productivity without excessive exploration of tangential topics.
     The presentation of visual communication theories was kept particularly brief, foregoing detailed discussions of how each can be applied to workplace lighting and message presentation.  Application of these theories is highly context-dependent; it is more fruitful for practitioners to consider them with respect to a specific project than to discuss generalities in this already vastly-simplified presentation.
     The concepts presented here can be applied to overall plant layouts, process flows, hazard maps (see “Commercial Cartography” series), work instructions, data collection, product documentation, and so much more.  Lighting and communication in emergency situations warrants its own discussion and is the topic of a future installment.

     For additional guidance or assistance with Safety, Health, and Environmental (SHE) issues, or other Operations challenges, feel free to leave a comment, contact JayWink Solutions, or schedule an appointment.

     For a directory of “Workplace Illumination” volumes on “The Third Degree,” see Part 1:  An Introduction to Lighting (21Aug2024).
 
References
[Link] Lighting for Health and Safety.  N.A. Smith.  Butterworth-Heinemann; 2000
[Link] Human Factors in Lighting, 3ed.  Peter R. Boyce.  CRC Press; 2014.
[Link] Handbook of Visual Communication, 2ed.  Sheree Josephson, James Kelly, and Ken Smith  (eds). Taylor & Francis Group; 2020.
[Link] “Visual Communication:  Examples, Types, Elements & Importance.”  Aditya Soni.  Clearinfo.
[Link] “ANSI/IES RP-7-21 Recommended Practice: Lighting Industrial Facilities.”  ANSI.
[Link] “Visual communication.”  Wikipedia.

Jody W. Phelps, MSc, PMP®, MBA
Principal Consultant
JayWink Solutions, LLC
jody@jaywink.com

Innate Conditions
     An innate condition is one with which an individual is born.  Although changes in visual capabilities occur throughout one’s lifetime, it is a matter of convenience to consider conditions with which humans are often born as innate.  The effects of these conditions and methods of compensation for them are the same, regardless of when they first occur.
     Refractive error is a common innate visual condition, often taking one of two forms.  Myopia, or “near-sightedness,” is the condition in which distant light is focused in front of the retina.  Its counterpart, hypermetropia, or “far-sightedness,” causes the focal point of distant light to be behind the retina.  The colloquial names of these conditions identify the distance at which less difficulty is experienced, though visual performance suffers at all viewing distances.
     Both types of refractive error result in blurred vision and reduced visual performance.  Fortunately, both are correctable by use of eyeglasses or contact lenses; surgical methods have also come into common use (e.g. LASIK).  Pictorial representations of myopia and hypermetropia, and their correction, are provided in Exhibit 1 and Exhibit 2, respectively.

     Astigmatism is another type of refractive error in which the focal point of light varies with the angle at which it enters the eye.  This can be caused by an irregularity in the curvature of the lens or cornea; proper orientation of a cylindrical lens can correct this condition.  Symptoms of uncorrected astigmatism include the perception of equal-length radial lines as having varying lengths; in general, blurred or distorted vision results.  Exhibit 3 provides pictorial representations of astigmatism and its nonsurgical correction.

     Defective color vision is known, collectively and colloquially, as colorblindness.  However, defects in color vision can take several forms, depending on which cone photoreceptors (see Part 3) are malfunctioning or missing from the eye.  Those with normal color vision are called trichromats in reference to the three types of cone photoreceptors, and their constituent photopigments, that enable color vision.
     An anomalous trichromat exhibits a deviation from normal color vision, rather than an inability to perceive color.  That is, s/he sees color, but sees it differently, due to the spectral sensitivity of one photopigment deviating from normal or the expected curve.  This deviation is sometimes called a “weak” response, or low sensitivity; its occurrence in each type of cone is given a unique name:

• tritanomalous describes the presence of “weak” S-cones (blue sensitivity);
• deuteranomalous describes the presence of “weak” M-cones (green sensitivity);
• protanomalous describes the presence of “weak” L-cones (red sensitivity).

     Those that possess two types of cones, but lack the third, are called dichromats.  Dichromats see a limited range of colors and, within that range, perceive them differently than trichromats.  A person lacking each cone type is identified by a unique name:

• S-cones are absent in tritanopes;
• M-cones are absent in deuteranopes;
• L-cones are absent in protanopes.

     Monochromats experience the greatest deviation from normal color vision.  In cone monochromats, one type of cone photoreceptor, usually S-cones, is present, providing very limited color perception.  Rod monochromats exhibit a total absence of cone photoreceptors, leaving only the ability to perceive differences in brightness, or “grayscale vision.”  Rod monochromats may be called “truly” or “totally” colorblind.
     The range of color vision defects that exist creates a significant challenge for lighting system designers.  Few options exist to aid those with defective color vision.  The techniques that are available, such as close control of the spectral content of provided light or filtering to enhance color differences, are not universally applicable or effective.  Customization to an individual’s circumstances is often necessary in order to be effective; this, in turn, may be detrimental to others, including those with normal vision.
     Whenever possible, conveyance of information should not rely on color alone.  Other cues, such as shape, location within a display, intermittent display (e.g. flashing), or other characteristic should supplement color to improve interpretation accuracy.  Any colors used should also be as easy to differentiate as possible; considering lines of color confusion, shown in Exhibit 4, can prevent use of indistinguishable colors.  For each condition noted, the white lines through the corresponding chromaticity diagram indicate loci of colors that are unlikely to be distinguishable.

Acquired Conditions
     An acquired condition, in this context, is a light-related malady that one is not born with, but develops at some time in his/her life.  Although it is possible to be born with some of these conditions, it is more common that they be caused by some type of exposure, injury, or illness.  For simplicity, differentiating afflictions by the most-likely time of onset continues.
     Diplopia, or “double vision,” occurs when there is a failure of coordination among the muscles that, under normal conditions, maintain the focus of both eyes on the same point in space.  This condition is often associated with acute physical trauma, but may also be a symptom of a more serious, possibly chronic problem.
     An occurrence of diplopia is more than a mere inconvenience; during some activities, it can create an extremely dangerous situation.  The brain attempts to ignore one of the images in order to restore near-normal vision; some cover or close one eye to achieve the same outcome.  Whether physically or cognitively achieved, the removal of the double image can be accompanied by a loss of stereopsis that reduces visual performance.  Again, this may create a significant safety hazard.
     Those that experience double vision should consult an eye-care or medical professional.  Properly-fitted eyeglasses may resolve a minor issue; a more-severe condition may require pharmaceutical or surgical treatment.  In any case, it should be addressed before it becomes a larger problem for the diplopic or those around him/her.

     Night blindness is caused by a Vitamin A deficiency. It occurs at low levels of illumination, where exposure to a bright light bleaches the rhodopsin in the rods rapidly enough to deplete its supply.  The eye is temporarily blinded while the rhodopsin is regenerated; when the photopigment is once again available for bleaching, vision is restored.  Cones are unaffected by Vitamin A deficiency; thus, visual capabilities at high levels of illumination remain unchanged, providing no indication of the condition.
     A commonly-cited occurrence of night blindness is that caused by the glare of an oncoming car’s headlights.  After the car passes, a driver can travel a significant distance before vision is restored.  The inability to detect or effectively monitor road hazards, erratic drivers, traffic signals, and other information during this time greatly increases the risk of driving at night.

     Diabetic retinopathy is one of many possible consequences of long-term uncontrolled diabetes and is a leading cause of blindness worldwide.  It is characterized by progressive destruction of the retina, subsequent to blood vessel damage caused by diabetes.  Early stages may be asymptomatic.  As it progresses, dark or “blank” spots can appear in the visual field, followed by blurred vision and partial or total loss of vision.  Keeping diabetes under control is a critical factor for eye health and many other issues.  Medical professionals should be consulted for further treatment options.

     Pressure within the eye (intraocular pressure) is maintained by generating and expelling fluid at equal rates.  An imbalance in this process can cause pressure to rise, damaging the retina and optic nerve.  This damage results in a narrowing visual field, a condition known as glaucoma.  Increased intraocular pressure continues to damage the retina and shrink the visual field; it also degrades night vision and contrast sensitivity.  Left unchecked, total blindness results.
     The need for medical intervention should be obvious; beyond that, options for aiding glaucoma patients are limited.  In environments under our control, the visual scene can be simplified; minimizing distractions and using objects with high contrast allows those with glaucoma to extend their self-sufficiency.  Illumination levels should also be kept sufficiently high to enable foveal vision, as “on-axis” visual acuity is unaffected until the final stage of glaucoma.

     Darkening, or increasing opacity, of the lens is called cataract.  Cataract can begin in various locations, such as the center (nuclear), edge (cortical), or rear surface (posterior subcapsular), but usually progresses to all areas of the lens.  The change causes increased absorption and scattering of light in the eye, resulting in decreased visual acuity, contrast sensitivity, and color discrimination throughout the visual field.  The additional scattering also increases sensitivity to glare.
     Prevention methods are limited to wearing sunglasses (UV is believed to cause or significantly contribute to cataract formation) and generally-healthy practices, including proper nutrition and refraining from smoking.  Treatment options include several methods of surgery to replace the clouded lens with a synthetic one.  Cataract surgery is usually performed on an outpatient basis, with only local anesthesia, and has a high rate of success in restoring visual capabilities.

     A loss of central vision can be the result of macular degeneration.  The macula is the short-wavelength-absorbing area covering the central region of the retina (see Part 3).  Dry macular degeneration involves thinning of the macula and the growth of protein clumps; pigment abnormalities and cell loss (atrophy) may also occur.  Wet macular degeneration involves abnormal growth of blood vessels under the retina.  Leakage of blood and scarring cause vision loss faster than that which occurs in dry macular degeneration.
     Pharmaceutical treatments are available in some cases.  Laser surgery can also slow wet macular degeneration for some patients.  Like cataract and many other issues, a healthy lifestyle is the best, and possibly the only, preventive measure available.

     A comparison of the visual effects caused by some of the acquired conditions discussed is presented in Exhibit 5.  The images provide representative examples of how each condition might change a person’s perception of a visual scene, but individual experience varies.

Age-Related Conditions
     Several acquired conditions are typically associated with advancing age.  In fact, references to age-related macular degeneration (AMD) are far more common than the neutral description provided in the previous section.
     While other conditions described may or may not develop, presbyopia is unavoidable.  It is a form of far-sightedness whose onset typically occurs in middle age.  It is characterized by a reduced range of accommodation caused by a retreating near point (see Part 6).  Hardening of the lens and weakening of the ciliary muscles that occur with age are negatively synergistic, the net result being a reduced accommodation range.  The curves in Exhibit 6 show how the near point and range of accommodation change with age.

     Changes in the lens (e.g. yellowing) also result in increased absorption and scattering.  Exhibit 7 shows the decline in lens transmittance as a function of age and wavelength.  Increased absorption can be compensated with higher luminance; however, this also introduces additional risk.  Increased scattering exacerbates glare, requiring that higher levels of illumination be provided very carefully, lest one limiting characteristic of the environment simply be replaced by another.  Overcoming reduced contrast sensitivity, another age-related condition, requires a similar balancing act.

     For a given luminance, pupil diameter also decreases with age.  Less light entering the eye exacerbates the challenge of balancing illumination level levels with glare risks.  The reduction in maximum pupil diameter, for different adaptation luminances, is shown in Exhibit 8.  A smaller pupil also narrows the visual field, limiting peripheral vision.  This may contribute to the increasing time required for dark adaptation that occurs with age.

     The curves in Exhibit 9 compare the spectral absorption occurring in the lens at various ages.  The plot makes clear that short wavelengths are absorbed most at all ages; more importantly, absorption of short wavelengths increases most with age.  This sharp increase explains, at least in part, why the decline in color discrimination that occurs with age is particularly pronounced in the blue region of the visible spectrum.

     Changes in circadian rhythms also occur with advancing age.  Given the influence of blue light on melatonin secretion and related functions, a change in circadian timing seems a logical byproduct of its increased absorption.  Phase shifts (advances), shortened periods, and reduced amplitudes of circadian rhythms accompany advanced age.  Distorted periods of high and low alertness or sleepiness makes it more difficult to function effectively or maintain a routine.

     Age-related conditions can develop in combination with other acquired conditions and innate conditions.  The variety of potential combinations, ranges of severities, and individual circumstances (e.g. task requirements) pose great challenges in maintaining the visual capabilities of an aging population.  For particularly demanding tasks, such as those that require discrimination of fine detail or subtle color differences, or are performed in rapidly-changing conditions, it is especially difficult to maintain consistent visual performance.

Lighting Conditions
     The influence of illumination, in photometric and perceptual terms, on visual performance also extends to visual comfort.  What is a comfort factor for those with normal vision may escalate to become a performance factor for those with one or more of the conditions described.  A comfort factor may also be escalated to a performance factor if its magnitude increases significantly.
     A common comfort-cum-performance factor is glare, in any of its many forms.  Discomfort glare occurs when an area of high luminance within the visual field directly causes no change in visual performance, but may induce other responses.  For example, to counteract the distraction caused by a source of discomfort glare, one might focus more intently on a smaller portion of the task area; access to information on the periphery could be lost.
     When a glare source is sufficiently intense to raise the adaptation luminance of the eye, lower-luminance details may become imperceptible.  This condition is called disability glare to reinforce the fact that visual performance is hindered under such conditions.  The effect is equivalent to creating deep shadows within the visual field; in either case, the range of luminances is too great for the eye to resolve both extremes.
     Adaptation glare occurs when one is subjected to a rapid, large increase in luminance of the entire visual field or a substantial portion of it.  This can occur when exiting a building or highway tunnel on a sunny day, or during any transition that creates a similar scenario.  As the name implies, adaptation to the higher luminance resolves the issue; however, until such adaptation occurs, visual performance and comfort are drastically reduced.
     Saturation glare, or dazzle, occurs when a large portion of the visual field is of such high luminance that the eye’s ability to adapt is exceeded.  If protective eyewear is not worn, damage to the eye may result, despite reflexive responses that include shielding the eyes or looking away.  Dazzle is the opposite of sparkle (see Part 8); where sparkle is pleasant, dazzle is painful.  Sparkle involves a very small portion of the visual field (essentially a point), where dazzle consumes much or all of it.  Other forms of glare occupy intermediate positions; though adaptation glare can be painful, it is brief.
     There is overlap evident in the types of glare defined.  Adaptation glare and dazzle are certainly disabling, for example.  Several forms of glare presented here can also be direct or reflected glare (see Part 5); the effects are the same, whether the cause of glare is a light source or a reflective surface.

     Nonuniformity of illuminance, in either spatial or temporal terms, can cause visual discomfort.  Temporal nonuniformity, such as flicker, can cause distraction, reduced visual performance, or worse (see Part 8).  Spatial nonuniformity can lead to perceptual confusion, significantly reducing visual performance.
     Perceptual confusion is caused by a discrepancy between an anticipated visual scene and that created by a nonuniform distribution of illuminance.  Anticipation of a visual scene is derived from knowledge of surface reflectances, with an assumption (i.e. cognitive heuristic) of uniform illuminance.  Nonuniform illuminance creates a pattern of luminances on surfaces in a scene that is inconsistent with their reflectances.  The resulting difficulty in accurately interpreting the scene is called perceptual confusion.

     Prolonged visual discomfort leads to eyestrain, or asthenopia.  Performing demanding visual tasks for extended periods, uncorrected visual impairment (e.g. refractive error), and inadequate or inappropriate lighting can cause discomfort and eyestrain.  Irritation and inflammation of the eyes, blurred vision, headache, and other negative physiological effects can result.  Eyestrain and its deleterious effects can be self-reinforcing, as the stress and frustration of poor visual performance may prompt a redoubling of effort, further fatiguing the ciliary and extraocular muscles.
     Multiple sources of discomfort may exist simultaneously, increasing the risk of eyestrain and subsequent effects.  Some discomfort may be unavoidable; some visual tasks remain difficult, despite optimizing all parameters that can be controlled.  Therefore, effective job design includes scheduling time to rest one’s eyes, a fact that is often overlooked for tasks that are otherwise physically undemanding.


     Physiological changes occur naturally via exposure and aging.  Both the visual and non-visual systems are affected; cognitive abilities, such as processing speed, also tend to decline with age.  Though there is great variability, acquired and age-related conditions are fairly predictable.  Acute physical trauma, on the other hand, can create very unique situations that require specialized responses to any visual or cognitive impairment that may result; thus, meaningful exploration of these conditions is impossible in this forum.
     Diminished visual capability, or vision loss, can be caused by any of a number of conditions, many of which have been described above.  The compounding nature of coexisting conditions complicates understanding of the contribution of each; thus the preceding presentation intentionally excluded references to the effect each condition may have on acuity measurements (see Part 6).  However, measurements of visual acuity are the most-familiar aspect of vision for much of the population; therefore, this installment would be remiss if it were ignored altogether.  The key point to remember is that visual acuity accounts for the influences of all existing conditions in conjunction; they cannot be effectively separated.
     To consistently reference the severity or level of vision loss, the categories established by the World Health Organization (WHO) can be used.  Shown in Exhibit 10, the categories and grades defined should be viewed as a continuum.  The criteria used to differentiate categories are similar to those of many classification systems used for many purposes – they are assigned for convenience and consistency, but are not as definitive as they may appear.  The WHO criteria are visual acuity, after correction of refractive error, of the “better” eye, using a far point of 20 ft and the diameter of the visual field in degrees of angle.  When referencing visual capability categories, the classification scheme in use must be identified to avoid confusion.  Similar or identical terms may be used in multiple schemes, or colloquially, while the criteria and definitions could vary greatly.  Specifying the classification system referenced is the only way to ensure that the intended meaning is conveyed.

     For additional guidance or assistance with Safety, Health, and Environmental (SHE) issues, or other Operations challenges, feel free to leave a comment, contact JayWink Solutions, or schedule an appointment.

     For a directory of “Workplace Illumination” volumes on “The Third Degree,” see Part 1:  An Introduction to Lighting (21Aug2024).

References
[Link] Lighting for Health and Safety.  N.A. Smith.  Butterworth-Heinemann; 2000
[Link] The IESNA Lighting Handbook, 9ed.  Mark S. Rea (ed).  Illuminating Engineering Society of North America; 2000.
[Link] Kodak's Ergonomic Design for People at Work.  The Eastman Kodak Company (ed).  John Wiley & Sons, Inc., 2004.
[Link] The IESNA Lighting Handbook, 10ed.  David L. DiLaura, Kevin W. Houser, Richard G. Mistrick, Gary R. Steffy (eds).  Illuminating Engineering Society of North America; 2011.
[Link] Handbook of Human Factors and Ergonomics, 4ed.  Gavriel Salvendy (ed).  John Wiley and Sons; 2012.
[Link] Human Factors in Lighting, 3ed.  Peter R. Boyce.  CRC Press; 2014.
[Link]  Gulani Vision Institute.
[Link] The Eye Site.

Jody W. Phelps, MSc, PMP®, MBA
Principal Consultant
JayWink Solutions, LLC
jody@jaywink.com

Color
     In Part 7, the mechanics of color vision were discussed, building on the photoreceptor information presented in Part 3.  A color rendering index (CRI) provides an indication of the magnitude of a light source’s influence on our perception of colors through purely physical means.  Coordinated color temperature (CCT) links the physical appearance of light to our subconscious perceptions of the illumination provided.
     The nature of a space’s color and illumination can produce both psychological and physiological responses.  Spaces with vibrant colors and higher luminances tend to be more inviting and stimulating than those in which earthtones are prevalent or that are dimly lit.  Warm lighting (e.g. CCT < 3300 K) can cause a sensation of rising temperature due to excitation of the nervous system.  Cooler lighting (e.g. CCT > 4000 K) can have a calming effect, while causing a sensation of lower ambient temperature.

     At very low intensity, a stimulus appears to “lose” its color.  A typical demonstration involves an incandescent bulb whose supply voltage is reduced until it is extinguished.  As the voltage drops, the light’s color transitions to red, then fades.  Prior to extinguishment, the light enters its achromatic interval, the range of intensity in which color cannot be perceived in the light; it appears to be gray.  Rods are much more sensitive than cones (see Part 7, Exhibit 2), allowing perception of stimuli at intensities below which the color-rendering cones cease to function.

     Colors possess implicit significance in various contexts.  Red and yellow are often used to signify “danger” or “caution” in industrial settings; similar meaning is conveyed by these colors in traffic signals.  Colors can be used as shorthand for one’s emotional state, such as blue for sad or depressed, or red for angry.  Yellow has been used as a pejorative, describing someone who is timid or cowardly.
     Green is a versatile color; it is commonly used in reference to environmental or botanical matters (e.g. one has a “green thumb”).  It can also describe a person that is envious or ill.  Green can also be used as a slang term for money.  “I don’t have the green” offers more flair than “I can’t pay you.”
     Each of these uses, and many others, exploits a link between a color and an adopted meaning that has been correlated in our brains by various means.  Colors have no inherent significance, only the interpretations that we apply to them.

Relativity
     Many of our visual perceptions are effected by the relativity of visual stimuli.  A stimulus will be perceived differently depending on its luminance relative to an observer’s adaptation luminance.  For example, a flashlight may look exceptionally bright to a dark-adapted eye, though the same flashlight is unimpressive to the same eye when light-adapted.
     Relativity within the visual field is often defined by contrast – luminance, color, etc.  Contrast is the measure of a characteristic of a stimulus relative to that of its background.  The degree of contrast determines if a stimulus is differentiable from its surroundings, e.g. as a discrete object or as a detail (feature) of an object.  If a stimulus is too similar to its background – that is, the relative characteristic does not exceed a visual threshold – the visual scene can be misinterpreted.
     Relativity influences perception beyond determining the distinguishability of a stimulus from its background.  Consider the example in Exhibit 1.  Though it is consistent throughout, the gray annulus appears to be brighter when viewed against the black background than when viewed against the white background.  The effect is more pronounced when each pairing is viewed independently than when in the same scene.  Visual perception includes assessment of a stimulus relative to its surroundings.

Cues and Miscues
     Interpretation of a scene is aided by visual cues; it can also be hindered by miscues.  Cues are the elements or features of a visual scene that prompt a particular interpretation.  Cues often serve as shortcuts, or heuristics, increasing the speed at which the brain processes and interprets a scene.  When one of these heuristics leads to an incorrect interpretation, the input is called a miscue.  Miscues can occur naturally or can be intentionally created in a scene to manipulate its interpretation.
     In the absence of consistent cues, interpretation of visual stimuli can vary across time or observers.  Stated another way, shifting visual cues result in shifting interpretations.  A classic example of this phenomenon is presented in Exhibit 2, where two different perceptions can be derived from a single image.

     The addition of cues guides our interpretation of a scene.  In the example shown in Exhibit 3, some of the circular features are perceived as indentations (“dents”) and others as protrusions (“dings”).  The visual cue that guides our interpretation is the distributions of light within each circle.  The heuristic employed is that light shines from above.  Therefore, a feature illuminated at the top is a ding and one illuminated at the bottom is a dent due to the shadowing created by each type of feature.  Turning the image 180° causes the dents to become dings and vice versa.  If the light source location is known to be below, the heuristic is eliminated.  The interpretation of each feature is then opposite that when the light source location was assumed to be above.
     The preceding examples demonstrate perceptual anomalies in 2D.  In three-dimensional space, additional information contributes to a scene’s interpretation.  While a third dimension provides additional cues, it also creates potential for additional miscues.  Consider the example of a photograph of two people, standing abreast, that appear to be the same height.  Seeing the two in person, it is clear that one is significantly taller than the other.  When the photograph was taken, the taller person stood further from the camera; the two were not actually abreast.  This positioning exploited perspective while degrading other visual cues.  Perspective is the phenomenon by which objects at greater distance appear to be smaller.
     In the photograph example, the translation of 3D space to a 2D representation causes a loss of visual fidelity.  However, similar effects can be created while remaining in three-dimensional space.  One way to demonstrate this is to construct an Ames room; Exhibit 4 presents a schematic of Ames room construction and the resulting visual perception.  The room is constructed to counteract the visual effect of perspective, intentionally distorting the interpretation of the scene by way of fabricated miscues.  The Ames room deliberately alters visual perceptions; however, potential for similar distortions of perspective to occur unintentionally also exists.

Persistence and Loss
     The persistence of vision refers to the retention of a visual image after the stimulus has disappeared.  It only lasts approximately 0.1 s, but is sufficient to make intermittent images, such as video, appear fluid and continuous.
     The creation of an after-image is a phenomenon related to the persistence of vision.  A positive after-image occurs when closing one’s eye’s after viewing a bright object for a short time.  The after-image appears the same color as the object.  A positive after-image can also be experienced with open eyes, if looking into a sufficiently-darkened space.
     A negative after-image occurs when an observer looks away from the bright object to a neutral surface, such as white paper.  A negative after-image appears as the complementary color to that of the object. A complementary color is one that combines with the first to create white light.

     A loss of perception can be caused by any of several factors, independently or in combination.  Insufficient illuminance or excessive glare can cause a failure to perceive a target.  Incorrect perceptions can be caused by optical illusions or other miscues, a flickering light source (see below), light entering the visual field from unusual directions, or a lack of modelling (i.e. lighting fails to reveal three-dimensional characteristics of an object).  Both failures and errors are considered losses of perception.

More Physical Phenomena
     Direct and reflected glare were introduced in Part 5, focusing on their potential for negative impacts on visual performance.  There is, however, a form of glare with a positive connotation; it is called sparkle.  Sparkle is often associated with merchandising displays of gemstones and other finery.  It occurs when a source of high luminance subtends a small solid angle; a larger subtense is simply seen as glare.

     Variable output of a light source induced by alternating-current (AC) power causes flicker.  If flicker is not controlled, it can cause physiological responses, such as headache or nausea; an individual can suffer these ailments even if s/he is not consciously aware of flicker.  Awareness of flicker occurs below the critical flicker frequency (CFF), the rate of light output variation above which the output appears constant.  CFF ranges, approximately, from 15 Hz at low intensity to 60 Hz at high intensity.  It is the persistence of vision that allows the CFF to manifest.
     A flickering light source can create a stroboscopic effect that changes perceptions of motion.  In addition to annoyance, the stroboscopic effect can create a dangerous situation, such as when rotating equipment appears to be stationary.  When applied intentionally, a strobe can be used to check and adjust the operating speed of a machine or to attract attention to a critical situation.  Stroboscopic effect can also be generated by mechanical means, such as a fan located between a light source and an observer.

     The perception of a room’s dimensions can be influenced to a significant degree by painting surfaces within the space differently.  Exhibit 5 provides several examples of perceptual effects that can be achieved by variable painting.  This practice is common in residential spaces, but rare in commercial spaces; thus, it will not be discussed in detail.  However, it is possible for this technique to be useful in an unorthodox application and this warrants its inclusion.

     Studies of lighting system design options are more applicable to workplaces.  One such study, conducted in a conference room setting, compares photometric measures and subjective assessments of six lighting configurations.  A pictorial representation and description of each configuration is shown in the left panel of Exhibit 6, with corresponding photometric data provided in the center panel.
     Across the six configurations, a wide range of luminances was achieved for various surfaces in the room, resulting in significant variability in subjective assessments.  Some of those assessments are shown in the right panel of Exhibit 6, where the number of each configuration is placed on each scale to represent its mean rating.  Favorable assessments are shown on the left side of the scale and unfavorable on the right.
     As the mean ratings show, the first two are middling configurations, scoring near the fulcrum on almost every scale.  The third tips to the unfavorable, while the fourth is slightly favorable.  The assessment of configuration #5 is a mixed bag.  On the “evaluative” dimension, it scores somewhat unfavorably, but most-favorably on the “perceptual clarity” dimension.  It rates worst of the six on the “formality” dimension, while it is mid-pack on the remaining two dimensions.  Selection of this configuration could be seen as a strong signal of an organization’s priorities.
     The sixth configuration is rated most-favorably on the “evaluative” dimension and at or near best on almost all of the other scales shown (complete study results are not displayed).  Cursory review of this data suggests that #6 is a clear winner.  However, cost, organizational priorities, and the transferability of study results to one’s own space must be considered before reproducing this lighting system configuration.
     A study of this type is easier to execute, with much lower risk, in a conference room than in an industrial setting.  The structure, layout, and activity of a production department, for example, present challenges and requirements that are very different from those encountered in office and meeting spaces.  While much can be learned from this and similar studies, substantial additional analysis and translation is required to apply the results to an industrial workspace.


     Scouring the literature on human visual capabilities, lighting system design, and related topics would likely reveal several additional perceptual phenomena worth discussing.  However, were the series to be an exhaustive presentation, it would be too unwieldy to provide practical value.  Instead, this installment provides “highlights,” if you will, that practitioners should consider to keep perceptions as close as possible to reality.

     For additional guidance or assistance with Safety, Health, and Environmental (SHE) issues, or other Operations challenges, feel free to leave a comment, contact JayWink Solutions, or schedule an appointment.

     For a directory of “Workplace Illumination” volumes on “The Third Degree,” see Part 1:  An Introduction to Lighting (21Aug2024).

References
[Link] Lighting for Health and Safety.  N.A. Smith.  Butterworth-Heinemann; 2000
[Link] The IESNA Lighting Handbook, 9ed.  Mark S. Rea (ed).  Illuminating Engineering Society of North America; 2000.
[Link] The IESNA Lighting Handbook, 10ed.  David L. DiLaura, Kevin W. Houser, Richard G. Mistrick, Gary R. Steffy (eds).  Illuminating Engineering Society of North America; 2011.
[Link] Handbook of Human Factors and Ergonomics, 4ed.  Gavriel Salvendy (ed).  John Wiley and Sons; 2012.
[Link] Human Factors in Lighting, 3ed.  Peter R. Boyce.  CRC Press; 2014.
[Link]  “How to Change the Size of Your Room With Paint.”  Paintzen.com, October 20, 2020.

Jody W. Phelps, MSc, PMP®, MBA
Principal Consultant
JayWink Solutions, LLC
jody@jaywink.com

Color Vision
     The ability to see color requires sufficient luminance to activate cones in the retina.  Effective discrimination between similar colors requires higher luminance than that required to merely recognize color.  That is, color vision is unavailable in scotopic conditions and is limited in mesopic conditions.  Maximum acuity and discrimination requires photopic conditions.  Exhibit 1 provides a summary comparison of these conditions.

     As a consequence of the illumination levels in which they operate, cones exhibit much lower sensitivity to light than do rods.  The relative sensitivity of each photoreceptor type is shown in Exhibit 2.  The extreme differential sensitivity explains the importance of adaptation luminance to visual performance and the discomfort caused by rapid transitions between vastly-different illumination levels.
     As a person’s eyes are exposed to a high luminance (i.e. photopic conditions), their sensitivity declines, marking a shift from rods to cones as the predominant active photoreceptors.  This is the transition from dark-adaptation to light-adaptation.  As this transition occurs, the appearance, or perception, of colors also changes.  This phenomenon, called color adaptation, is caused by the changing rates of photopigment bleaching corresponding to the eyes’ overall sensitivity.

Spectral Distributions
     When mention is made of color, it is often assumed to refer to light entering our eyes.  However, characteristics of the source of light and its path to the eye must also be considered to develop an understanding of color vision.  Light sources, objects from which light reflects, and media through which light is transmitted each exhibit a spectral distribution that influences the light reaching our eyes.  Color is not inherent to light; it is its spectral distribution, as interpreted by the brain, that shapes our perception of color.
     The spectral power distributions (SPDs) of some light source examples are plotted in Exhibit 3.  The SPD is the source’s relative radiant power as a function of wavelength.  The appearance of a light source, when observed directly, can be predicted by studying its SPD plot.  For example, the plot for a high-pressure sodium light source, with mid-range wavelengths dominating its power distribution, explains the yellowish appearance of this type of light, often used for highway lighting.

     An object’s spectral reflectance distribution (SRD) describes its reflectance as a function of wavelength.  The SRD plots in Exhibit 4 use several common fruits as examples.  Note that all of the examples exhibit very low reflectance of short wavelengths; for this reason, none of these fruits appear to be blue or purple.
     A transmission medium may differentially influence light as it passes through, as described by its spectral transmittance distribution (STD), the medium’s transmittance as a function of wavelength.  Exhibit 5 plots the STDs of two examples of architectural glass.  Though the difference may not seem large, the lower transmittance of long-wavelength light offered by the standard-transmittance glass could have a significant aesthetic impact in some applications.
     A light source/object interaction accounts for the SPD of a light source and the SRD of an object to determine a reflected spectral distribution.  A graphical representation of this mathematical calculation is provided, in Exhibit 6, for one of the example fruits shown in Exhibit 4 illuminated by the light sources shown in Exhibit 3.  The calculation can also be expanded to include any effects of transmission media, whether between the source and object or between the object and observer, on the perceived appearance of an object by multiplying the appropriate distribution by the STD of the medium.

Color Mixing
     Two methods of color mixing – additive and subtractive – are used to create the desired appearance of objects; both are in common use.  Additive color mixing can be used when the light source is controlled; subtractive color mixing can be used as an alternative to source control or when control of the light source is impossible.
     Additive color mixing creates a desired appearance by adjusting the SPD of incident light.  A common method mixes three primary colors in proportions necessary to generate the desired output.  This is often referred to as the RGB system, which is shorthand for the three primary colors red, green, and blue; it is the basis of many electronic displays.  Exhibit 7 depicts the overlapping of primary colors to create the secondary colors yellow, magenta, and cyan.  Where all three primary colors exist in equal proportion (i.e. center of image), white light results.

     Subtractive color mixing employs filters to selectively remove wavelengths from incident light, a technique employed extensively in photography.  For this method, depicted in Exhibit 8, the primary and secondary colors are the reverse of RGB additive color mixing.  That is, cyan, magenta, and yellow filters create areas of blue, red, and green where they overlap.  Where all three filters overlap, all wavelengths of light are removed, resulting in a black appearance.
     Paints, dyes, and inks also employ subtractive color mixing to achieve desired appearances.  In this case, pigments are used to selectively absorb unwanted wavelengths and reflect the remainder.  The fruit examples above demonstrate natural occurrences of selective reflection; the development of pigments allows us to produce similar effects at will.
     An important concept related to color mixing is that of metamerism.  Metamers are lights of differing SPDs that present the same color.  A certain color appearance can be achieved with multiple combinations of constituent colors.  In practice, this means that two light sources that appear identical when viewed directly can result in significantly different visual perceptions after transmission through a medium or reflection from an object.

Characterization of Color
The Munsell System
     Though a light’s appearance can be quantified by its spectral distribution, this is not a convenient method for many applications and practitioners.  Instead, colors are often described by a color order(ing) system, a prevalent one being the Munsell system.  The Munsell system defines color on three dimensions, pictorially represented in Exhibit 9:  hue, value, and chroma.
     Generic references to “color” describe the hue of observed light.  The hue scale, represented by the azimuth of the Munsell system model, consists of 100 gradations.  Major gradations are defined by five principal hues (purple, blue, green, yellow, red), abbreviated to the first letter of each (P, B, G, Y, R).  A set of five intermediate hues is also defined (purple-blue, blue-green, green-yellow, yellow-red, red-purple), also abbreviated by initials (PB, BG, GY, YR, RP).  [A note on convention:  an azimuthal scale is defined as increasing in a clockwise direction, as depicted in Exhibit 9.  However, the sequence of hues cited above progresses in counterclockwise fashion.  This was done to mimic the common presentation of the visible spectrum that progresses from violet on the left to red on the right (see Part 2, Exhibit 1).]

     The central spine in the Munsell system model represents the value scale.  This dimension consists of gradations ranging from black (0) to white (10).  “Colors” on this axis are achromatic (i.e. no hue) and are referenced as “Neutral X,” where “X” is the value number.
     Some sources, and other color order systems, refer to the value dimension as lightness or brightness.  Brightness is the term typically applied to self-luminous objects, while lightness typically refers to reflective surfaces.
     The radial dimension of the Munsell system model is the chroma scale.  It ranges from 0, for neutral (i.e. on the value axis), to 20, for highly-saturated colors, on the periphery.  This characteristic is often called saturation or strength; each of these terms references a color’s deviation from neutral, or grayscale.
     The Munsell system identifies each color by an alphanumeric code of the format hue/value/chroma.  For example, 10R/5/20 refers to highly-saturated, “pure” red – as red as red gets!  A corresponding point on the value axis – that is, absent hue and chroma, a “middle gray” – is called Neutral 5 or N5.  Possession by each of a Munsell Color Atlas allows disparate individuals to discuss a color sample as if they were looking at the same object.  Although individuals’ perception of a specified color differ, use of a Munsell color chip approximates that which would be experienced were the conversants collocated.
     Books containing a Munsell Color Atlas are available for physical comparisons of samples.  Pictorial models, such as that in Exhibit 9, remain useful conceptual representations.  The system can also be represented by the Munsell solid; it is often depicted as shown in Exhibit 10.  Munsell, in his seminal work, describes a color sphere, but the scales, as described, define a cylinder.  A cylinder also facilitates visualization of the entire range of colors, as the segments of a sphere become exceedingly small near the poles.

Color Matching Functions
     A color matching function (CMF) defines a color by the proportions of its constituents.  For the RGB system, color matching functions have been derived empirically.  Exhibit 11 provides plots of RGB CMFs and an example of their use.  A dashed line is drawn vertically at 480 nm; the intersection of this line with each CMF is projected to the vertical axis.  Light with the proportional contributions of the primary colors represented by the projections to the vertical axis simulates monochromatic light of 480 nm wavelength.

     To eliminate negative values, RGB CMFs are transformed into XYZ color matching functions that reliably identify metamers, though the “primary colors” used are fictitious.  Two sets of CMFs have been developed by the CIE (Commission International de l’Éclaraige or International Commission on Illumination), the globally-recognized authority on lighting, color, and related topics.  One is the 2° CMFs (1931), recommended for fields of view subtending 1 - 4°.  The other is the 10° CMFs (1964), recommended for angular subtense greater than 4°.  These sets of CMF are known as standard observers; plots are shown in Exhibit 12.
     The transformed CMFs are identified as x_bar(λ), y_bar(λ), and z_bar(λ) to represent the average visual response of observers.  Tristimulus values X, Y, and Z are analogous to RGB values; both represent the contribution of each primary color required to reproduce a light’s color.  Tristimulus values are found by calculating the products of the stimulus spectrum and each CMF; the areas under the resulting distribution curves are the tristimulus values.

Chromaticity Diagrams
     Though other chromaticity diagrams have been developed, the one of primary interest is the CIE 1931 2° Chromaticity Diagram shown in Exhibit 13.  Chromaticity coordinates are found by calculating the ratio of each tristimulus value to the sum of the tristimulus values:  x = X/(X + Y + Z); y = Y/(X + Y + Z); z = Z/(X + Y + Z).

     The chromaticity diagram is plotted in two dimensions – x and y.  The third is unnecessary; because x + y + z = 1, z can easily be determined from the x-y coordinates.  The colors shown in the diagram are “directional” only; hue and saturation are represented, but the perception of color is also influenced by lightness (the z dimension), which is not accounted for in the diagram.
     The curved boundary of the chromaticity diagram is the spectrum locus, along which the most-saturated colors lie.  The straight line enclosing the diagram is the purple boundary, so named because hues comprised of combinations of red and blue lie along it.
     The chromaticity diagram is also a useful backdrop for other information.  In Exhibit 14, regions are outlined within which stimuli are typically identified as the noted color.  MacAdam ellipses are shown in Exhibit 15; each ellipse represents chromaticities surrounding the coordinates of a given stimulus that are undifferentiable from it.  The variation in size and orientation of the ellipses is noteworthy.

Correlated Color Temperature
     The simplicity of single-number metrics make them popular, despite the loss of precision they often engender.  Sources of white light can be classified by their color temperature, the absolute temperature of an ideal radiator emitting light with the same chromaticity as the source.  These sources lie on the blackbody, or Planckian, locus, the arc shown in the interior of the chromaticity diagram in Exhibit 13.
     All sources of “white light” do not lie directly on the Planckian locus, however.  For those that do not, a correlated color temperature (CCT) is assigned.  The CCT of a light source is the absolute temperature of an ideal radiator emitting light with chromaticity nearest that of the source.  The lines crossing the Planckian locus in Exhibit 13, each corresponding to an absolute temperature, are isothermal lines, or isotherms.  A light source with chromaticity coordinates that lie on or near an isotherm are said to have a CCT equal to the isotherm’s absolute temperature.
     Full definition of a light source requires a second value, though it is rarely cited; CCT alone is deemed sufficient for most applications.  The second value is the distance from the source’s chromaticity coordinates to the nearest point on the Planckian locus.  This distance is called Duv; positive values are assigned to chromaticities lying above the Planckian locus and negative values are assigned to those lying below it.  To be considered “white light,” a source’s Duv value must be in the range of ± 0.006.
     In lieu of citing a CCT, a light source may be assigned a more-generic descriptor of its temperature-correlated appearance.  Unfortunately, these descriptors are counterintuitive, as they invert the CCT scale.  That is, higher CCT light sources provide “cooler” light.  Lights with CCT up to 3300 K are considered “warm;” these typically have a yellowish tinge.  From 3300 – 4000 K CCT, light sources are “intermediate” and from 4000 – 5300 K CCT they are “intermediate-cool.”  “Cold” light sources have CCTs above 5300 K; a blue tinge becomes evident in light with higher CCT.

The Color Gamut
     While other color-characterization methods compare a light source’s properties to those of a reference light source, the color gamut approach uses a set of test colors.  The chromaticity coordinates of eight standard test colors are plotted on a chromaticity diagram; the area enclosed by connecting the eight points is called the color gamut of the light source.  The size and shape of a color gamut represent the extent to which color discrimination is possible under the light source.  For example, the color gamut of the high-pressure sodium light source, shown among other examples in Exhibit 16, is localized in the yellow region.  This provides similar insight into the nature of this type of light source to that of its SPD, discussed previously.

Color Rendering
     Color rendering is the term used to describe the effect a light source has on the chromaticity of observed colors when compared to the same object viewed under a reference light source.  A color rendering index (CRI) is used to quantify a light source’s effect.  Higher CRIs indicate that colors appear “truer to the originals;” a CRI of 100, the value assigned to the reference light source, indicates that no color distortion occurs.  A CRI > 80 is considered “good” for most applications.
     Computation of the CIE color rendering index involves comparing the color gamut of the subject light source to that of the reference light source.  CIE defines fourteen test colors, summarized in Exhibit 17.  The first eight colors, used to compute CRI, are pastels covering the full range of hues.  The remaining six are special test colors, including samples representing skin tones and foliage.

     A graphical representation of the method of computation of CRI is shown in Exhibit 18, where a high-pressure sodium light source is, once again, the example.  The magnitude of the chromaticity shift of each test color is called a CIE special CRI.  The CIE general CRI, the value cited on light source packaging, for example, is the average of the CIE special CRIs of the eight pastel test colors.  Large chromaticity shifts result in a CRI of 16 for the example light source in Exhibit 18; its CCT is 1975 K.  Though several deficiencies have been identified in the CIE test color method, the CIE CRI continues to be the index of choice and is commonly found in lighting catalogs.


     Interested readers can find much more information on additional chromaticity diagrams, color order systems, and related topics in the references cited below and other sources.  The scope of this installment, like others in the series, must be limited to avoid overwhelming readers with excessive detail that may not be immediately applicable.  However, the information presented here provides sufficient background to facilitate understanding of forthcoming discussions of other visual phenomena and lighting system design.

     For additional guidance or assistance with Safety, Health, and Environmental (SHE) issues, or other Operations challenges, feel free to leave a comment, contact JayWink Solutions, or schedule an appointment.

     For a directory of “Workplace Illumination” volumes on “The Third Degree,” see Part 1:  An Introduction to Lighting (21Aug2024).

References
[Link] A Color Notation.  A. H. Munsell.  Geo. H. Ellis Co.; 1905.
[Link] Lighting for Health and Safety.  N.A. Smith.  Butterworth-Heinemann; 2000
[Link] The IESNA Lighting Handbook, 9ed.  Mark S. Rea (ed).  Illuminating Engineering Society of North America; 2000.
[Link] The IESNA Lighting Handbook, 10ed.  David L. DiLaura, Kevin W. Houser, Richard G. Mistrick, Gary R. Steffy (eds).  Illuminating Engineering Society of North America; 2011.
[Link] Handbook of Human Factors and Ergonomics, 4ed.  Gavriel Salvendy (ed).  John Wiley and Sons; 2012.
[Link] Human Factors in Lighting, 3ed.  Peter R. Boyce.  CRC Press; 2014.

Jody W. Phelps, MSc, PMP®, MBA
Principal Consultant
JayWink Solutions, LLC
jody@jaywink.com

     Before exploring new concepts, it is useful to revisit some terms introduced in previous installments of the series.  The objective is for the information to be comprehensible and applicable; the assessment of vision is deemed the appropriate context in which to provide additional detail in service of this goal.

Accommodation
     In Part 3, the physical structure of the eye and the components involved in accommodation were presented, where the relative terms “near” and “distant” described the need for accommodative capability.  The range of this capability is defined by its extremes, the near point and far point, as shown in Exhibit 1.

     The near point is the location of maximum accommodation, where the eye’s maximum refractive ability can just focus an image on the retina.  At birth, this point is as close as 1 cm (0.4 in) from the eye.  As a person ages, the near point retreats; in middle-age, this retreat becomes a prominent symptom of presbyopia.  With advanced age, the crystalline lens loses elasticity, limiting its accommodative capability.
     The far point is the location at which accommodation is essentially zero; the eye is “at rest.”  At this point, the slightest adjustment of lens curvature provides a focused image.  For the normal-sighted, infinity is typically the far point.
     The distance between the near point and the far point is called the range of accommodation.  To assign a practical value to this range, for purposes of an eye exam, for example, 6 m or 20 ft is commonly equated with infinity.
     The range of accommodation is an external measure.  A related measure, internal to the eye, quantifies the refractive capability of a lens or system.  The unit of measure, diopter (D), is defined as the strength of lens required to focus parallel light rays at a distance of 1 m (3 ft).
     Variable dioptric power is achieved by accommodation of the eye and is expressed as the inverse of the focal length of the lens.  As dioptric power increases, focal length decreases.  For example, the focal length of a 5 D lens is 0.2 m (8 in), while a 10 D lens focuses at a distance of 0.1 m (4 in).
     The human eye, at rest, exhibits a total dioptric power of approximately 60 D.  This is comprised of approximate contributions of 48 D by the cornea and aqueous humor and 12 D by the crystalline lens (flattened when at rest); vitreous humor contributes less than 1 D to the eye’s total dioptric power.  A young person can add approximately 14 D by accommodation; as noted previously, this ability declines with age.

Discrimination
     Visual discrimination is the ability to detect and distinguish between stimuli within the visual field.  Discrimination is often discussed in terms of visual thresholds, such as those introduced in Part 4.  Visual stimuli can be characterized by five parameters:  luminance contrast, chromatic contrast, visual size, retinal illumination, and retinal image quality.
     Conceptually, luminance contrast is the “brightness” of a target relative to its surroundings.  In context, this translates to the luminance of a target relative to that of its immediate surroundings.  For a target viewed amidst a uniform background, this is quantified as C = |Lt – Lb|/Lb, where C is the luminance contrast, Lt is the luminance of the target, and Lb is the luminance of the background.  For targets that are darker (i.e. lower luminance) than the background, 0 ≤ C ≤ 1, while 0 ≤ C < ∞ for targets that are brighter than the background.
     When the target consists of a periodic pattern, luminance contrast is calculated as C = (Lmax – Lmin)/(Lmax + Lmin), where Lmax and Lmin are, respectively, the maximum and minimum luminances of the target.  In this case, 0 ≤ C ≤ 1.
     Chromatic contrast refers to a difference in color between a target and its background.  Typically, it only becomes important when luminance contrast is critically low (C < ~0.2).  Measures of chromatic contrast have been proposed, but none are universally accepted.  However, it can be influenced by lighting choices; specifically, the spectral output of a light source can affect the appearance of a target.  Color vision and related lighting guidelines are discussed in upcoming installments of the series.
     A target’s visual size is defined by the solid angle it subtends at the viewer’s eye for purposes of detection.  To resolve detail, visual size is defined as the solid angle subtended at the viewer’s eye by a target’s critical dimension.
     The critical dimension varies by the type of target, or stimulus, considered.  For two points, the critical dimension is the distance between them; for two parallel lines, it is the separation distance.  The width of the gap in a Landolt ring, used in acuity testing (see below), is its critical dimension.
     Retinal illumination is the product of the average luminance (cd/m^2) of the visual field and the area of the pupil (mm^2); et = L Ap.  Its unit of measure is the troland (Td), defined as the illuminance produced by a stimulus of 1 cd/m^2, or nit, when the pupil’s area is 1 mm^2.  This formulation typically refers to photopic conditions, though it can also be used for scotopic conditions.  Constant retinal illumination corresponds to an eye’s adaptation luminance.  A stimulus that creates higher retinal illumination will be more distinguishable from its background.
     The sharpness or blurriness of an image is known as retinal image quality.  High spatial frequencies yield sharper images; spatial frequency is measured in cycles/m or m^-1.  To understand spatial frequency, consider the specification of the number of lines of resolution in a video display.  This number divided by the size of the display is the spatial frequency of the display.  For example, a 4K TV that is one meter tall has a spatial frequency of 4000 cycles/m; a 4K display of 0.5 m height yields a spatial frequency of 8000 cycles/m, and so on.
     Retinal image quality improves slightly with decreased pupil size, due to greater depth of field.  At equal luminances, short-wavelength light results in smaller pupil sizes.  Image quality is reduced when light travels through media that scatter or disperse it.

Acuity
     Visual acuity can be defined or measured in various ways.  Readers are likely familiar with a Snellen chart, commonly used by optometrists; an example is shown in Exhibit 2.  Viewing the chart from an effective distance of 20 ft or 6 m, an individual identifies the smallest line of letters that is legible.  If that line can be read at a distance of 30 ft by a person with normal vision, the subject’s visual acuity is rated as 20/30 (approximately 6/9 vision using the metric system).

     Visual acuity can also be quantified by a target’s visual angle.  As shown in Exhibit 3, visual acuity = 1/θ, where θ is the visual angle of a target at the maximum distance at which details are discernible.  A detail’s visual angle, measured in minutes of arc, is the angle subtended at the point of observation.
     Another definition of visual acuity is the angle subtended at an observer’s eye by a detail of such size that it is correctly identified in 50% of its presentations.  This type of test typically uses acuity targets such as a Landolt ring or Landolt C.  A Landolt ring is shown in Exhibit 4 alongside other types of acuity targets; an E, parallel bars, and a disc are also shown. The critical dimension of each is identified as ‘d.’  Landolt rings are often presented in an array with each ring randomly oriented.

     Visual acuity is degraded in low levels of illumination where vision is reliant on rods.  Acuity improves with increasing illuminance to a limit where it plateaus.  The location of this plateau varies according to task characteristics, but illuminances of 250 – 300 lx are often cited.

Relative Visual Performance
     The relative visual performance (RVP) model uses task performance metrics to assess visual capabilities.  One formulation employs a numerical verification task in which two columns of numbers are compared.  Performance is evaluated on the time taken to complete the task, the number of discrepancies missed, and the number of false positives.  Data from one such test is shown in Exhibit 5, where these metrics are plotted against the luminance contrast of the task (i.e. ink on paper).  Evident in the plots is that, at low luminance contrast, increasing contrast improves visual performance significantly and that at C > ~0.4, performance is essentially constant.

     Another RVP test measures an individual’s response time to the onset of a stimulus.  This test was developed to isolate, to the extent possible, the visual component of a task from the cognitive and motor components.  A comparison of surface plots depicting performance for each of four stimuli of varying visual size is shown in Exhibit 6.
     For each stimulus size, relative visual performance is plotted against retinal illumination and luminance contrast.  The surface plots show that visual performance is nearly constant over a wide range of conditions; this region is called the plateau of visual performance.  The rapid decline in performance, at the edge of the plateau, is called the escarpment of visual performance.  These terms allude to the plots’ resemblance to geological formations to convey the severity of performance degradation in adverse conditions.
     RVP or similar tests are rarely conducted in industrial settings; precise data are not available for most applications.  However, awareness of the parameters that exert the greatest influence on visual performance can lead lighting system designers to conduct appropriate sensitivity analyses and verification studies.  A small investment in analysis could prevent the installation of an ineffective lighting system and the corresponding expense of upgrading or replacing it, lost productivity, and potential health effects.

     Other models and measures of visual performance are available, though they are not presented here.  The most common and practical are those presented.  Lighting systems are often designed and installed without benefit of comprehensive data.  Practitioners must rely on their understanding of interactions among lighting parameters, visual performance, and affected individuals to specify lighting that meets organizational objectives.

     For additional guidance or assistance with Safety, Health, and Environmental (SHE) issues, or other Operations challenges, feel free to leave a comment, contact JayWink Solutions, or schedule an appointment.

     For a directory of “Workplace Illumination” volumes on “The Third Degree,” see Part 1:  An Introduction to Lighting (21Aug2024).

References
[Link] Lighting for Health and Safety.  N.A. Smith.  Butterworth-Heinemann; 2000
[Link] The IESNA Lighting Handbook, 9ed.  Mark S. Rea (ed).  Illuminating Engineering Society of North America; 2000.
[Link] The IESNA Lighting Handbook, 10ed.  David L. DiLaura, Kevin W. Houser, Richard G. Mistrick, Gary R. Steffy (eds).  Illuminating Engineering Society of North America; 2011.
[Link] Handbook of Human Factors and Ergonomics, 4ed.  Gavriel Salvendy (ed).  John Wiley and Sons; 2012.
[Link] Human Factors in Lighting, 3ed.  Peter R. Boyce.  CRC Press; 2014.

Jody W. Phelps, MSc, PMP®, MBA
Principal Consultant
JayWink Solutions, LLC
jody@jaywink.com

Photometric Quantities
     Several fundamental terms in photometry are interrelated and, thus, best understood when considered in conjunction.  Some sources define the extent of photometry as the measurement of the fundamental quantity luminous flux.
     Luminous flux (Φ) is the rate of flow of optical radiation, or the rate of energy emission by a light source.  It is calculated spectrally by integrating the source’s radiant power per wavelength, quantifying total output in all directions in lumens (lm).  A lumen is defined the luminous flux emitted by a point source with 1 candela luminous intensity within a unit solid angle.
     Luminous intensity (I) is a directional measure of light output defined as the luminous flux per unit solid angle in a specified direction.  Its unit of measure, the candela (cd), is equal to one lumen per steradian (1 lm/sr).  The term candela has supplanted candle, the historical source and measure of light.
     A steradian (sr) is the angle subtended by unit area on the surface of a sphere of unit radius at the sphere’s center.  To aid comprehension of this term, note that a triangle’s side subtends the opposite angle.  It is the unit of measure of solid angle (ω), the ratio of the surface area of a sphere intercepted by a cone, with its vertex at the sphere’s center, to the square of the sphere’s radius.
     Verbal definitions of complex terms can be cumbersome; pictorial representations are provided in Exhibit 1 and Exhibit 2 to enhance clarity of some of those presented.  Some difficulty arises from the circular, or self-referential, nature of related terms.  This characteristic can make the concepts of photometry and related subjects difficult to comprehend; substantial experience with practical applications may be needed to become sufficiently comfortable with the use and meaning of several relevant terms.

     The luminous flux per unit area of a surface is its illuminance (E).  Its unit of measure is the lux (lx); one lux is equivalent to one lumen per square meter (1 lx = 1 lm/m^2).  The luminous intensity per unit area of emitted light is called luminance (L).  Its unit of measure is candela per square meter (cd/m^2).
     With properties of an illuminated surface known, the luminance of a light source is determined according to the following relation:  L = (E x ρ)/π, where L is the luminance of the light source (cd/m^2), E is the surface illuminance (lx), ρ is the surface reflectance, and the surface has a matte, or diffusely-reflecting, surface.  Reflectance of a surface is the ratio of reflected luminous flux to incident luminous flux.
     For a non-matte, or specularly-reflecting, surface, the corresponding relation is L = (E x β)/π, where L and E are the same as before and β is the luminance factor.  Luminance factor is a ratio of the luminances of two reflecting surfaces viewed from the same direction under identical lighting conditions.  The first is the surface of interest (numerator); the second is a “perfect white uniformly diffusing” surface, a standard reference (denominator).

     Refraction is the “bending” of light as it passes from one homogeneous medium to another.  An example of the effect of refraction on visual perception is shown in Exhibit 3.  The magnitude of the displacement of a perceived image from its actual location can be determined if the refractive index of each medium is known.

     A medium’s refractive index (μ) is the ratio of the speed of light in vacuum to the speed of light in the medium.  The magnitude of the direction change as light crosses a boundary between two media is proportional to the ratio of their refractive indices:  μi/μr = sin θr/sin θi, where subscripts i and r refer to the incident and refracted rays, respectively, μ is the refractive index of the medium, and θ is the angle of the ray from the normal.
     When light enters a nonhomogeneous medium, diffuse transmission may occur.  That is, instead of a ray passing through at a modified angle, light is dispersed in all directions.  This effect can be used to increase the available light in an area while minimizing the impact of glare when a high-intensity light source is in use.

     Qualitative assessments of lighting are often dominated by the existence of glare, potentially in multiple forms.  Glare is the phenomenon that occurs when luminances within the visual field significantly exceed that to which the eye is adapted.  The effect of glare is dependent upon its severity, ranging from a minor inconvenience, to a cause of discomfort, to a loss of visual perception of objects in the visual field.
     Direct glare occurs when the source of high luminance is in the visual field, such as a lamp or window transmitting daylight.  If outdoors, sunlight can also cause direct glare; it often refers to light sources outside the area of attention (e.g. work surface).
     Reflected glare is caused by highly-reflective surfaces in the visual field that direct high illuminances to the eye.  Use of this term usually refers to reflections within, or in close proximity to, the area of attention.  When reflected glare reduces visibility of an object’s details, it may also be called veiling reflection.  A veiling reflection can also obscure an object completely, such as when looking through glass with high-illuminance reflected glare in the line of sight.
     Shadow is the absence of light; it is often used to reference the level of illumination in comparison to the average or required level in an area.  It may be thought of as the opposite of glare, though it is not strictly true.  Unlike glare, shadow is not typically a direct cause of discomfort, though a resultant loss of visual acuity may be an indirect cause of discomfort or annoyance.
     The existence of glare or shadow indicates a lack of lighting uniformity.  Nonuniform lighting may be intentionally generated for aesthetic purposes; however, safety, productivity, and flexibility demands of workplaces are best accommodated by uniform lighting.  Lighting uniformity is discussed further in subsequent installments of this series presenting types of light fixtures and lighting system design guidelines.

Measurement Equipment
     The basic unit of photometric instrumentation is the photometer, used to measure luminous flux.  Specialized versions are also available to measure other quantities or to correct for nonstandard conditions.
     An integrating photometer is used to measure geometrically total luminous flux.  A common type employs a sphere with its interior covered with a diffuse white reflective coating.  A small opening allows entry of light from the source of interest and another serves as a measurement port.  Diffusion of light inside the sphere allows direct measurement of total luminous flux by eliminating the directional component.
     An illuminance meter measures illuminance on a plane.  Color-corrected versions normalize the spectral distribution of light and cosine-corrected versions compensate for deviations of incidence from the normal.  A luminance meter measures the average luminance over a defined area.
     A reflectometer measures the reflectance of a surface, while a transmissometer measures the transmittance of a medium.  A medium’s transmittance (τ) is the ratio of luminous flux transmitted through it to that incident upon it (τ = Φt/Φi).  A spectrophotometer measures reflectance and transmittance as a function of wavelength.
     A goniophotometer is used to measure the directional distribution of a light source or reflective surface.  Evaluations of the uniformity of lighting system output may make extensive use of this type of instrument to supplement other data, such as flux and intensity.
     Characteristics of any instrument must be considered before data collected with it can be trusted for decision-making.  Its sensitivity, correction ability, defined application limitations, and other properties influence the value a device can add to an investigation.


     This installment has focused on the properties of light and media, with little consideration of how these properties support visual performance.  This will be addressed in upcoming installments presenting colorimetry, measures of visual acuity, and other related topics.

     For additional guidance or assistance with Safety, Health, and Environmental (SHE) issues, or other Operations challenges, feel free to leave a comment, contact JayWink Solutions, or schedule an appointment.

     For a directory of “Workplace Illumination” volumes on “The Third Degree,” see Part 1:  An Introduction to Lighting (21Aug2024).

References
[Link] Fundamentals of Industrial Ergonomics, 2ed.  B. Mustafa Pulat.  Waveland Press; 1997.
[Link] Lighting for Health and Safety.  N.A. Smith.  Butterworth-Heinemann; 2000
[Link] The IESNA Lighting Handbook, 9ed.  Mark S. Rea (ed).  Illuminating Engineering Society of North America; 2000.
[Link] Kodak's Ergonomic Design for People at Work.  The Eastman Kodak Company (ed).  John Wiley & Sons, Inc., 2004.
[Link] The IESNA Lighting Handbook, 10ed.  David L. DiLaura, Kevin W. Houser, Richard G. Mistrick, Gary R. Steffy (eds).  Illuminating Engineering Society of North America; 2011.
[Link] Handbook of Human Factors and Ergonomics, 4ed.  Gavriel Salvendy (ed).  John Wiley and Sons; 2012.
[Link] Human Factors in Lighting, 3ed.  Peter R. Boyce.  CRC Press; 2014.

Jody W. Phelps, MSc, PMP®, MBA
Principal Consultant
JayWink Solutions, LLC
jody@jaywink.com

The Visual System
     The visual system is responsible for our visual perceptions.  That is, it creates images in our brains by adjusting to and processing information provided by light incident upon the eyes.  The adjustments, processing, and translating of incident light to create perceptual images is known, collectively and generically, as vision.

Visual Pathways
     Electrical impulses generated by photoreceptors in the retina are transmitted along the visual pathways.  The optic nerves from both eyes meet at the optic chiasma, a crossover point where signals are reorganized.  In the chiasma, information, from both eyes, about each side of the visual field is collected and sent to the pathway on the opposite side of the brain.  Collected information is sent from the chiasma to the lateral geniculate nucleus (LGN) on each side.  The LGN distributes the optical signals, via optic radiations, to various points in the visual cortex, where our perception of sight is generated.
An overview of the visual pathways is provided in Exhibit 1.  Discussion in greater depth is beyond the scope of this series; for starters, it would require detail that was foregone in the presentation of the structure of the eye.  For purposes of this presentation, it shall suffice to share information without detailed explanation.

     The layout of ganglion cells in the retina (see Part 3, Exhibit 2), through which signals are sent to the optic nerve, is preserved in the LGN and the visual cortex.  In the visual cortex, located at the rear of the brain, this arrangement reinforces the supremacy of the fovea in the discrimination of fine visual detail by dedicating approximately 80% of cortical cells to just 10° of the visual field (foveal vision).  This distribution of visual-processing resources in the cortex is known as cortical magnification.
     Image-processing responsibility in the visual cortex is also divided by objectives.  Areas within the visual cortex are, independently, or nearly so, responsible for perception of motion, analysis of color, and facial recognition, for example.
     The tremendous complexity of the visual system requires that our objective be limited to a high-level understanding of its major functions.  Mere awareness of the structural and functional sophistication into which detailed exploration is lacking here (“known unknowns”) can lead to successful investigations at appropriate times, when details become relevant and applicable.  To minimize information overload in the interim, discussion of the brain’s structure, cell types and functions, and other biological details is curtailed.  Instead, the discussion returns to practical matters of the visual system in operation.

Accommodation and Adaptation
     The eye’s process of accommodation, described in Part 3, is the first step in translating the visual field into a coherent image.  Any condition, internal or external, that inhibits foveal vision, such as dim lighting, also limits the accommodation range capability of the eye.  An extreme case is one in which the visual field is completely uniform, such as when one is in total darkness or dense fog.  In such conditions, the eye’s accommodation typically “defaults” to a focal distance of approximately 70 cm (27.6 in).
     While accommodation allows the visual system to adjust to the range of focal distances needed for normal vision, the process of adaptation serves a related function for varying lighting conditions.  The range of illuminances to which humans are regularly exposed is extremely broad (i.e. sunlight to darkness), requiring adjustments in the visual system to process information effectively.  The primary mechanism of adaptation is to assume one of three states corresponding to the visual system’s characteristic sensitivity.
     The lowest sensitivity is required when illuminance is high.  In this state, photopic vision fully utilizes cone photoreceptors, providing color vision and discrimination of fine detail.  These are characteristics of foveal vision in the light-adapted eye.
     In low-light conditions, high sensitivity is required.  In the dark-adapted eye, only rod photoreceptors are active.  The fovea is inactive, eliminating its color vision and detail discrimination capabilities; only grayscale images can be formed.  This adaptive state is called scotopic vision.
     Between these extremes is the state of mesopic vision, in which intermediate levels of illumination activate both rod and cone photoreceptors to varying degrees.  The ability to discriminate colors and fine details depends on where in the mesopic range the eye is adapted (i.e. level of illumination).  As illumination decreases, approaching scotopic conditions, fine details blur and the discriminable color palette is reduced.  As illumination increases, more details become discriminable and more color variations are differentiable.
     Due to the variable activation of photoreceptors, the wavelength at which sensitivity peaks is different in each state.  Plots of the spectral sensitivity of rods and cones are shown in Part 3, Exhibit 3 and Exhibit 4, respectively.  Recall that all rods contain a single photopigment, resulting in a uniform sensitivity peak at approximately 505 nm.  With only rods active in scotopic conditions, peak sensitivity of the visual system corresponds to rod sensitivity.
     In photopic conditions, visual system sensitivity is a composite value, comprised of the sensitivities of the three cone types.  This can be thought of as a weighted average; the high number of M- and L-cones “pulls” the peak sensitivity up to approximately 555 nm.
     The transition between photopic and scotopic vision, or between light-adaptation and dark-adaptation, is called the Purkinje shift.  This shift in sensitivity, depicted in Exhibit 2, is not instantaneous; rapid changes in lighting conditions can cause severe difficulties.  Implications of the Purkinje shift, as it relates to job design, are discussed further in subsequent installments of this series.

     Peak sensitivity of the visual system in mesopic conditions varies between the photopic and scotopic peaks.  Pinpointing the peak sensitivity under any particular set of conditions, however, can be very difficult.  The effort may not be worthwhile, as other parameters are available for practical application.
     The mechanism by which the eye’s photoreceptors contribute to visual system adaptation is called photochemical adaptation.  Photochemical adaptation is the process of achieving an equilibrium condition in the breakdown, or bleaching, and regeneration cycle of photopigment.  Photoreceptor sensitivity is proportional to the amount of photopigment available for bleaching.  If insufficient photopigment is available for bleaching by the light to which it is exposed, because equilibrium has not been established in the photoreceptor, sensitivity is reduced.
     Another mechanism of adjustment is neural adaptation.  Neural interactions in the retina provide rapid sensitivity adjustments in the moderate light levels typical of electrically-lighted environments.  It is this rapid adjustment capability that allows us to scan an unevenly-lit space without perceiving a need for further adaptation.
     The size of the pupil, adjusted by the iris (see Part 3), also plays a role in adaptation.  Though its range of effectiveness is narrow in comparison to the full illumination range in which the visual system regularly operates, the pupil’s influence on the amount of light incident on the retina makes a small contribution to adaptation.

Visual Thresholds
     In generalized terms, a visual threshold is the magnitude of stimulus that must be exceeded to generate visual perception.  It is often defined as the stimulus with 50% probability of detection by 50% of any population.  Parameters typically used to characterize a visual threshold include:

• Wavelength of incident light.
• Size of stimulus.
• Duration of stimulus.
• State of adaptation.
• Region of the retina irradiated.

These parameters can be used to describe three types of visual threshold:  spatial, temporal, and color.
     Spatial thresholds describe the ability to differentiate a target from its background or to discriminate detail within a target. Commonly-used measures of these abilities are, respectively, luminance contrast and visual acuity.
     Luminance contrast can be quantified in various ways, depending on whether the target is brighter or darker than its background or is self-luminous.  For simplicity, a qualitative description suffices:  the visibility of a target relative to its immediate surroundings.  The higher the luminous contrast, the more easily a target is detected.
     Visual acuity can also be quantified in various ways, including the “20/20” format of an optometrist’s exam results.  Again, a qualitative description suffices:  the ability to discriminate details of a fixed-luminance-contrast target.  The greater one’s visual acuity, the finer the detail that can be seen clearly.  The concept of visual acuity is made salient to many by reading printed text; poor visual acuity requires a larger font to be read comfortably.
     Temporal thresholds measure the responsiveness of the visual system to fluctuations in illumination.  The relevant parameters are the magnitude, or amplitude, and frequency of illumination changes.  Temporal thresholds are relevant to the design of visual signals, such as warning lights, and in preventing noticeable, often disturbing, fluctuations in a lighting system’s output.
     Conceptually, color thresholds define the number of differentiable colors in one’s “visual color palette.”  A more-detailed exploration of color vision is forthcoming; for now, it shall suffice to say that a color threshold describes the ability to differentiate between targets of similar color appearing simultaneously or sequentially, the latter being significantly more difficult.

The Non-Visual System
     Understanding the operation of the non-visual, or “non-image-forming,” system, of which the eye is also a critical component, is less intuitive than that of the visual system.  Effects of light on the non-visual system can be positive or negative.  The same effect can be advantageous in one situation and detrimental in another.  Image forming, on the other hand, is generally neutral; the image created (“seen”) may be objectionable, but the process of creating it is not.
     Circadian timing can be effected by varying exposures to light.  Human biorhythms (i.e. circadian rhythms) operate on a 24-hour cycle; an important component of which is the pattern of daylight and darkness.  While there are other influences on circadian rhythms, this discussion is limited to that of light.
     For the vast majority of human existence, the only sources of light were the sun and fire, the flame becoming less “open” with the invention of the oil lamp.  The advent of electric lighting, and its subsequent development, has changed the nature and availability of light.  With it, the ability to influence the circadian timing system, both inadvertently and, later, intentionally, has also evolved.
     The circadian timing system receives information about lighting conditions from photoreceptors differing from those in the visual system (i.e. rods and cones).  The photopigment within these photoreceptors, melanopsin, has a peak sensitivity to light at approximately 480 nm, slightly higher than cyanolabe (S-cones).
     Further details of the physiological structure of the non-visual system are foregone here.  Its influence on workplace lighting design is indirect or secondary to other design parameters; great detail may, once again, be an unnecessary distraction from the series’ objectives.  It shall suffice to say that the non-visual and visual systems are intertwined; they are not fully independent, nor is it a single system performing two functions.  Relevant characteristics and functions of the non-visual system are presented below.
     Humans’ natural sleep/wake cycles are quite consistent until disrupted by changes in circadian signals.  A disrupted sleep/wake pattern is one possible manifestation of phase shifting.  Phase shifts occur when cues differ from the norm or are absent.  The term refers to the fundamental operating condition to which the human body is adapting; i.e. light or darkness, day or night.  Physiological responses attempt to align the functions of human biosystems to the perceived circadian phase.  This is relevant to work scheduling, an important component of job design.
     A phase shift is a slow response to light exposure.  The effect of melatonin suppression occurs much more rapidly.  Secretion of melatonin effects other endocrine activity and raises core body temperature.  Exposure to bright light reduces melatonin secretion, resulting in a higher level of alertness.  Thus, exposure to light, particularly with strong shortwave components (i.e. blue light), makes it more difficult to sleep.
     Like muscles, hormones often operate in opposing pairs in the human body.  The hormone that balances melatonin is cortisol.  As the ratio of melatonin to cortisol increases, the calmer, sleepier one becomes.  Melatonin suppression lowers this ratio (i.e. increases the relative amount of cortisol); as it decreases, one becomes more “awake” or alert.
An effect of changing patterns of light and darkness that occurs on a much longer time scale is seasonal affective disorder (SAD).  Seasonal reductions in the amount of sunlight available can result in decreased energy and increased need for sleep, feelings of despair, and depression.  Artificial light can be used to counteract SAD, but it must be done carefully to avoid other circadian disruptions.
     Workplace lighting is an important, but easily overlooked, contributor to employee morale.  All else equal, the lighting of a space can have a profound impact on one’s subconscious assessment of it.  This psychological response, when the space is viewed favorably, is called positive affect.  When the space is disliked, it is called negative affect.  In either case, individuals may find it difficult to articulate reasons for their feelings about the space.


     To reiterate, great detail has been omitted from this presentation.  An overview best suits the needs of the series by developing awareness of the broad scope and complexity of related subject matter without burdening the reader with extraneous information that is unlikely to be a direct aid to practical application.

     For additional guidance or assistance with Safety, Health, and Environmental (SHE) issues, or other Operations challenges, feel free to leave a comment, contact JayWink Solutions, or schedule an appointment.

     For a directory of “Workplace Illumination” volumes on “The Third Degree,” see Part 1:  An Introduction to Lighting (21Aug2024).

References
[Link] Fundamentals of Industrial Ergonomics, 2ed.  B. Mustafa Pulat.  Waveland Press; 1997.
[Link] Lighting for Health and Safety.  N.A. Smith.  Butterworth-Heinemann; 2000
[Link] The IESNA Lighting Handbook, 9ed.  Mark S. Rea (ed).  Illuminating Engineering Society of North America; 2000.
[Link] The IESNA Lighting Handbook, 10ed.  David L. DiLaura, Kevin W. Houser, Richard G. Mistrick, Gary R. Steffy (eds).  Illuminating Engineering Society of North America; 2011.
[Link] Handbook of Human Factors and Ergonomics, 4ed.  Gavriel Salvendy (ed).  John Wiley and Sons; 2012.
[Link] Human Factors in Lighting, 3ed.  Peter R. Boyce.  CRC Press; 2014.

Jody W. Phelps, MSc, PMP®, MBA
Principal Consultant
JayWink Solutions, LLC
jody@jaywink.com

Physical Structure
     Parts of the eye are often grouped or classified for a specific context of discussion.  Some overlap may exist in these groupings when an overview is the objective, as is the case here.  A cross-section of a human eye is shown in Exhibit 1, identifying many of its constituent parts.

     The dioptric system, or the light-transmitting element, consists of components of the eye through which light travels on its way to the light-receiving element, the retina.  Along this path, components of the dioptric system cause a series of refractions at their interfaces:

• from air to the anterior (forward or outer) surface of the cornea;
• from the posterior (rearward or inner) surface of the cornea to the aqueous humor in the anterior chamber;
• from the aqueous humor to the anterior surface of the crystalline lens;
• from the posterior surface of the lens to the vitreous humor in the vitreous cavity.
• Light travels through the vitreous humor to impinge on the retina at the rear of the eye.

     The retina is the innermost layer, or “coat,” of the eye, exhibiting a complex structure of its own.  As shown in Exhibit 2, light must travel through other types of cells before reaching the photoreceptors.  The photoreceptors are of greatest interest because of the critical roles they play in image formation and visual perception.

      There are two types of photoreceptors distributed in the retina; these are referred to as rods and cones.  Rod photoreceptors contain a single photopigment, rhodopsin, that results in the spectral sensitivity curve shown in Exhibit 3.  Rods provide low-resolution visual capability, with no color perception, in low-light conditions.  In such conditions, perception of changes in the visual field, such as those caused by movement, is the primary function, as visual acuity is very low.

     There are three types of cone photoreceptors, each with a unique spectral sensitivity, as shown in Exhibit 4.  These photoreceptors are called S-, M-, and L-cones, abbreviations for short-, medium-, and long-wavelength, respectively.  The characteristic spectral sensitivity of each type of cone is derived from the photopigment it contains:  cyanolabe in S-cones, chlorolabe in M-cones, and erythrolabe in L-cones.  Peak sensitivities occur at approximately 450 nm, 525 nm, and 575 nm wavelengths, respectively.  Peak rod sensitivity (see Exhibit 3) occurs at an approximate wavelength of 505 nm.
     In addition to their unique sensitivity curves, distribution of photoreceptors throughout the retina is highly uneven.  Concentrations of rods and cones, as a function of position relative to the fovea, is depicted in Exhibit 5.  The fovea is the small, central region of the retina where the visual axis intersects.  Stated another way, the fovea is the terminus on the retina of transmitted light in the normal line of sight (NLS) that provides maximum visual acuity.  Maximum visual acuity results from the high concentration of cones and absence of low-resolution rods in the fovea.  However, the relatively-low sensitivity of cones requires a relatively-high level of illumination to operate effectively.

     Concentration of the highly-sensitive rods peak at approximately 20° in each direction from the fovea.  This corresponds to a maximum visual sensitivity in low-light conditions at approximately 20° from the NLS.  The high density of rods throughout the non-foveal area of the retina, relative to cone concentrations, yields a much higher total.  Estimates range from 100 – 110 million rods compared to 5 – 40 million cones in a typical human eye.
     Though the types of cone photoreceptors are not differentiated in the plot of Exhibit 5, their distribution is also quite uneven.  The peak concentration of S-cones occurs adjacent to the fovea, while being nearly absent within it, where M- and L-cones are abundant.  The relative concentrations of cones in the foveal region are approximately one S-cone to 16 M-cones to 32 L-cones.

     The outermost layer of the eye consists, largely, of a tough, fibrous membrane, the sclera, covering the majority of the eyeball and maintaining its shape.  The remainder is comprised of transparent tissue, the cornea, that allows unadulterated light transmission into the eye while covering the iris and pupil.
     The intermediate layer, or “middle coat,” of the eye consists of the choroid, the ciliary body, and the iris.  The choroid contains a dense vascular network that services the retina.  A chief component of the ciliary body is the ciliary muscle that is connected to the lens.  As the ciliary body extends from the sclera, it becomes the iris, the pigmented ring visible in the eye.  The opening at the center of the iris that determines the amount of light entering the eye is the pupil.
     Exhibit 6 depicts the interactive functions of the pupil, iris, lens, ciliary muscle, and retina activated to achieve accommodation.  Accommodation is the process used to change the focal distance of the eye or the condition of the eye associated with vision focused at a desired distance.

     To focus on a distant object, the ciliary muscle relaxes, enlarging the pupil and flattening the lens.  Light received from a distance requires less refraction to focus on the fovea; a flatter lens reduces refraction of incoming light.  To transition focus to a nearer object, the ciliary muscle contracts, reducing pupil size and causing the lens to become rounder.  The combined effect is to collect the divergent light and refract it sufficiently to focus on the fovea.

     The aqueous humor nourishes the cornea and lens in similar fashion to blood in other areas of the body.  Blood cannot serve this function in the anterior chamber of the eye because it would occlude light transmission.  Transparency of the aqueous humor, like that of the cornea, is critical to our visual perceptions.
     Vitreous humor is gelatinous material that fills the majority of an eye’s volume.  It pressurizes the vitreous cavity to maintain the eye’s near-spherical shape.  Vitreous humor must also be transparent, lest our visual perceptions be distorted by its influence on incident light.

     An important component of the eye not yet mentioned is the macula.  It is a yellow-tinted area, approximately 5 mm in diameter, covering the central portion of the retina, including the fovea.  Its tinting absorbs short-wavelength (i.e. blue, ultraviolet) light, providing a measure of protection against potentially damaging exposures.
     Another important aspect of the eye’s physical structure is the existence of a blind spot.  The point at which the optic nerve enters the eyeball is devoid of all photoreceptors (see Exhibit 5).  Therefore, light incident on this point cannot be translated to an electrical signal; i.e. an image cannot be formed in this region.
     The blind spot is approximately 15° from the center of the visual field; binocular vision compensates for the gap in each eye’s visual information by overlaying the information gathered in both eyes to create a single image.  This method of image processing provides the additional benefit of depth perception (stereopsis).
     The visual field created by binocular vision is depicted in Exhibit 7.  The blind spot of each eye is well within the area visible by both eyes (unshaded central region).  A typical visual field for a single eye contains the range from 60° nasally (i.e. inboard) to 85° laterally (i.e. outboard) and from 65° down to 45° up.

     Although the eyelid is not part of the eye, it is critical to its health and proper function.  The eyelid protects the eye from physical intrusion by dust, other debris, or objects, while maintaining hydration of the corneal surface.  It also shields the eye from unwanted irradiance, such as incident light of great intensity or other bothersome characteristic.

Additional Functions of the Eye
     Eye movements are induced by six muscles, organized in opposing pairs.  These will not be discussed in detail here, as the “how” of eye movement is peripheral (unfortunate pun not intended) to the relevant scope of “why” eye movements may be necessary and “what” happens when we move our eyes.  The type of eye movement that occurs depends on the visual task being performed.
     Fixation is the process of maintaining focus on a specific object or at a specific distance, or the condition of such constant focus.  The term fixation implies that eye movement ceases once the target is in focus; however, this is not the case.  Small, uncontrolled oscillations of eye position, called tremor, always occur.  These small adjustments are necessary to maintain a coherent image, behaving like a self-test algorithm is a closed-loop control system.
     If the target of fixation moves, rapid eye movement may be initiated to restore focus.  These movements, called saccades, can be as fast as 1000°/s, but exhibit latency of approximately 0.2 s.  During saccadic movements, visual function is quite limited.  The changes in fixation points, or points of focus, required to read this line of text are also achieved via saccades.
     In contrast to saccades, pursuit eye movements are relatively slow (up to 40°/s) and smooth.  Such movements are used to track a fixation target, often at a distance.  Erratic movements and high-speed targets cannot be effectively tracked with pursuit eye movements; the change in position must be smooth and occur at low speed.
     Movements of the eyes are not independent of one another; they are coordinated to obtain a single image containing maximum information.  Version movements cause both eyes to travel in the same direction to maintain convergence of both eyes’ lines of sight on a desired target.  Pursuit eye movements are a type of version movement.
     Moving the eyes in opposite directions to change the fixation focal distance requires vergence movements.  Compared to other eye movements, vergence movements are rather slow (up to 10°/s); they can occur smoothly, such as when tracking a single target, or as “jump” movements, such as when switching fixation targets.

     As is often the case, decisions must be made to limit the scope of a single installment or an entire series.  As the image in Exhibit 2 makes clear, discussion of the structure of the retina could be greatly expanded.  The same is true of other topics under the umbrella of the eye’s structure and function.  However, pursuing ever-greater detail in all topics serves to distract and detract from the series’ practical objectives.  The preceding presentation is deemed sufficient to act as a useful building block, documenting progress toward the ultimate goal of designing safe and effective lighting systems.

     For additional guidance or assistance with Safety, Health, and Environmental (SHE) issues, or other Operations challenges, feel free to leave a comment, contact JayWink Solutions, or schedule an appointment.

     For a directory of “Workplace Illumination” volumes on “The Third Degree,” see Part 1:  An Introduction to Lighting (21Aug2024).

References
[Link] Lighting for Health and Safety.  N.A. Smith.  Butterworth-Heinemann; 2000
[Link] The IESNA Lighting Handbook, 9ed.  Mark S. Rea (ed).  Illuminating Engineering Society of North America; 2000.
[Link] The IESNA Lighting Handbook, 10ed.  David L. DiLaura, Kevin W. Houser, Richard G. Mistrick, Gary R. Steffy (eds).  Illuminating Engineering Society of North America; 2011.
[Link] Human Factors in Lighting, 3ed.  Peter R. Boyce.  CRC Press; 2014.
 
Jody W. Phelps, MSc, PMP®, MBA
Principal Consultant
JayWink Solutions, LLC
jody@jaywink.com

The Electromagnetic Spectrum
     The term “light” can be used in many ways.  In the context of workplace illumination, it is typically assumed to refer to “visible light,” shortened for convenience.  Visible light comprises a small portion of the electromagnetic spectrum, as depicted in Exhibit 1.  The visible spectrum is a narrow band of radiation with wavelengths in the approximate range of 380 ~ 760 nm.  This, and other cited ranges, is approximate because, as implied by the term spectrum, gradual transitions take place between types of radiation.  For this reason, there is slight disagreement in cited ranges among various references and, occasionally, within a reference.  Variability of individuals’ visual capabilities contributes to the imprecision of the visual spectrum’s boundaries.

     The spectrum of visible light and the audible range of sound (see “Occupational Soundscapes” series) contrast in significant ways.  The first is a matter of convention; while audible sound is typically referenced by its frequency (~20 ~ 20,000 Hz), visible light is typically referenced by its wavelength (~380 ~ 760 nm).  Conversion between the two parameters is simple, according to the relation c = λf, where c is the speed, λ is the wavelength, and f is the frequency, each of light or sound accordingly.
     The approximate speed of sound in air, used in the earlier series, is 343 m/s (1125 ft/s).  The approximate speed of light in vacuum is 3.00 x 108 m/s (186,000 mi/s).  From this, alternative specifications of the visible spectrum (~395 ~ 790 THz) and the audible range (~0.02 ~ 17 m) can be calculated, though their use is exceedingly rare.  However, a comparison of the frequency/wavelength ranges of human-perceptible light and sound, in addition to the nominal transmission speeds cited, quickly reveals how disparate these sensory inputs are.
     In the series on sound, an octave was defined as a range of frequencies in which the highest frequency was twice that of the lowest.  By this definition, the audible range of sound spans approximately ten octaves, while the spectrum of visible light spans only one octave.  However, the visual sense is generally deemed far superior to the auditory sense in humans due to the ability to differentiate between inputs throughout the perceptible range when presented simultaneously.  This capability is explored further in the discussion of the structure of the eye.
     In addition to the vastly different travel speeds (several orders of magnitude) and frequency/wavelength ranges, the conventional definitions of nominal travel speeds highlight another key difference between light and sound.  Sound is propagated by pressure waves; thus a medium is required for transmission.  As a form of radiation, light is subject to no such limitation.  Despite the disparate natures of light and sound and the sensory organs that process these inputs, parallels nonetheless remain between our experience of each, as is explored throughout this series.
     The region of the electromagnetic spectrum consisting of ultraviolet (UV), visible light, and infrared (IR) (~10 ~ 10^6 nm) is known as optical radiation.  Interestingly, ultraviolet is often referred to as light, while infrared is typically referred to as radiation.  Radiation is an accurate term for both, as both lie outside the visible spectrum; that is, neither is detectable by the unaided eye.  Though not directly responsible for normal vision, both are important for other reasons, to be discussed later.
     For practical purposes, there are two potential sources of light:  natural light, provided by the sun, and artificial light, provided by electric lamps.  Though sunlight is considered, much of the discussion of workplace illumination is biased toward electric lighting for two key reasons:
1) Sunlight is not always available, due to temporal (i.e. nighttime) and structural (i.e. building architecture) limitations.
2) With the advent of ubiquitous, reliable electrical grids, electric lamps became the default light source, supplanting open flame and oil lanterns.

Black Body Radiation
     An object emits radiation in relation to its temperature.  A black body is a theoretical object that, when maintained at constant temperature, absorbs all incident radiation and emits radiation, according to Planck’s law, at the maximum intensity possible for any body at that temperature.  Due to its supreme emission capability, a black body is also called an ideal radiator and similar monikers.
            Planck’s law describes the distribution of electromagnetic radiation emitted by a black body, at a given temperature, when in equilibrium in terms of matter and energy.  Planckian distribution curves for several temperatures are shown in Exhibit 2.  As seen in Exhibit 2, as temperature increases, peak radiation occurs at shorter wavelengths.  The wavelength at which this peak occurs is estimated by Wien’s law (or Wien’s approximation):  λpeak = (2.898 x 10^6)/T, where λpeak is the wavelength of highest-intensity radiation (nm) and T is the black body’s stable temperature (K).

     The temperature of the sun varies greatly among its layers.  At its core, the temperature is approximately 15 million K; at the outermost layer of the sun’s atmosphere, the corona, temperatures are as high as 2 million K.  Between these two extremes lies the visible “surface” of the sun, the photosphere, so named for its emission of optical radiation.  The effective temperature of the photosphere is between 5,500 K and 6,000 K, placing its peak emission in the visible spectrum.
     The immense amount of energy available in the sun results in radiation of extreme intensity.  The visible light emitted is so intense that it cannot be viewed directly without damaging the photoreceptors in the eye.  Radiation with wavelengths outside the visible spectrum can be useful, but also pose risks.
     Ultraviolet radiation can cause tissue damage; this characteristic is explored further in discussions of the eye and exposure limits.  At shorter wavelengths, exposure becomes more hazardous.  X-rays and gamma rays are forms of ionizing radiation capable of penetrating the human body.  Whereas damage caused by exposure to UV radiation is limited in depth (i.e. skin) and often visually apparent, penetrating radiation can cause internal, cellular-level tissue damage.  Such an exposure can be difficult to diagnose, given the lack of visible surface damage.  Sadly, it is unlikely to give one superhuman strength or even green-tinted skin that could signal an exposed individual to seek appropriate medical care.
     Infrared radiation plays an important role in the maintenance of body temperature, as discussed in the “Thermal Work Environments” series (Part 2 and Part 6 on thermoregulation in hot and cold environments, respectively).  Microwaves can cause burns with sufficient exposure.  Fortunately, this threshold is relatively high.  We are inundated with radiation with wavelengths longer than microwaves with little perceptible danger.  Any time we turn on a radio or television, the signal is waiting for us.  Radar is used extensively in air traffic control, speed measurement, and similar applications.
     The distribution curves in Exhibit 2 show that the intensity of radiation drops precipitously from its peak, particularly at higher temperatures.  The relatively narrow band of wavelengths in which we receive high-intensity exposure from sunlight reduces the danger of ionizing radiation to tolerable levels.
     The relevance of black body radiation distribution becomes clear when characterizing light sources for comparison.  Meaningful evaluations are facilitated by quantitative metrics; the radiation distribution provides a mechanism for creating such a metric.  However, there is much to explore before we get to that!

     There is much more that can be said about visible light, but our objectives are best-served by limiting the scope of discussion.  For example, a discussion of the wave and particle natures of light is not entered herein.  Practical application benefits little from such depth of knowledge that requires substantial effort to attain and may distract readers from the mission of this series.  A plethora of resources, including an assortment of physics textbooks, are available, should this subject be of interest.
     Additional information is shared throughout the series, of course, but in context to improve clarity.  The preceding overview of the electromagnetic spectrum and black body radiation provides the foundation necessary to pursue advanced topics.  This pursuit begins, in the next installment, with a presentation of the structure and function of the human eye.


     For additional guidance or assistance with Safety, Health, and Environmental (SHE) issues, or other Operations challenges, feel free to leave a comment, contact JayWink Solutions, or schedule an appointment.

     For a directory of “Workplace Illumination” volumes on “The Third Degree,” see Part 1:  An Introduction to Lighting (21Aug2024).

References
[Link] “Electromagnetic Spectrum.”  InfoHow; November 27, 2013.
[Link] “Planck’s law.”  Wikipedia.
[Link] “Planck’s law and Wien’s displacement law.”  Tec-Science; February 17, 2020.
[Link] Lighting for Health and Safety.  N.A. Smith.  Butterworth-Heinemann; 2000
[Link] The IESNA Lighting Handbook, 9ed.  Mark S. Rea (ed).  Illuminating Engineering Society of North America; 2000.
[Link] The IESNA Lighting Handbook, 10ed.  David L. DiLaura, Kevin W. Houser, Richard G. Mistrick, Gary R. Steffy (eds).  Illuminating Engineering Society of North America; 2011.
[Link] Human Factors in Lighting, 3ed.  Peter R. Boyce.  CRC Press; 2014.
 
Jody W. Phelps, MSc, PMP®, MBA
Principal Consultant
JayWink Solutions, LLC
jody@jaywink.com

     Discussions of illumination and lighting design utilize a unique lexicon, within which are several occurrences of similar words that often occur in similar contexts.  This creates a situation in which words and definitions can easily be confused, limiting comprehension of the information presented.  To mitigate this situation, the bulk of this installment is dedicated to providing a convenient reference for important terms.  The glossary will be updated as the series progresses, adding terms encountered in subsequent installments.
*Note:  Terms with qualifiers, or differentiators, are grouped with the undifferentiated term to facilitate review of related terms and concepts.  For example, the term disability glare is entered as “glare, disability” to retain proximity to the undifferentiated, or “generic,” term glare and other related terms.

Glossary
accommodation:  the process used to change the eye’s focal distance; the condition of being focused at a desired distance.
accommodation, range of:  distance between near point and far point.
achromatic:  without color or hue.
achromatic interval:  the range of intensity in which color cannot be perceived in a light.
acuity, visual:  the ability to discriminate fine details in the visual field.
adaptation:  adjustment of the visual system to the prevailing illuminance level.
adaptation, color:  the change in the appearance of colors as one’s eye adapts to higher luminance.
adaptation, neural:  rapid adjustments in the retina that enable uninterrupted vision in unevenly-lit spaces.
adaptation, photochemical:  the process of achieving equilibrium in the bleaching/regeneration cycle of photopigment.
affect, negative:  generally unfavorable disposition toward an object, space, etc., often without articulable causes.
affect, positive:  generally favorable disposition toward an object, space, etc., often without articulable causes.
after-image, negative:  retained image of an object appearing in its complementary color.
after-image, positive:  retained image of an object appearing in its original color.
Ames room:  a purpose-built structure designed to counteract the effects of perspective and other visual cues.
angle, beam:  the angle from vertical (nadir) of the central axis of the cone of light emitted from a luminaire.
angle, visual:  the angle subtended by an object or detail at the point of observation, measured in minutes of arc.
asthenopia:  see eyestrain.
astigmatism:  a type of refractive error in which the focal point of light varies with the angle at which it enters the eye caused by an irregularity in the curvature of the lens or cornea; correctable with proper orientation of a cylindrical lens. 
atrophy:  loss of cells.
axis, visual:  a theoretical line through the center of the lens to the center of the fovea; light entering the eye is refracted to be parallel with this axis.
backlighting:  placement of a primary light source below or behind the workplane.
ballast:  a magnetic or electronic current-limiting device used to maintain the arc in a discharge lamp.
binocular:  creation of a single image by combining visual information from both eyes.
black body:  a theoretical object that, when maintained at constant temperature, absorbs all incident electromagnetic radiation and emits radiation, according to Planck’s law, at the maximum intensity possible for any body at that temperature. Also blackbody.
bleaching:  chemical reaction of photopigment that generates electrical impulses that are translated into visual perceptions.
blind spot:  the area of the retina devoid of photoreceptors where the optic nerve exits the eyeball.
brightness:  equivalent to value in the Munsell system when referencing a self-luminous object.
bulb:  outer glass shell of a lamp that contains the emitter (discharge tube, filament, etc.), fill gas, mechanical supports, etc.
burn time:  duration of use, usually in hours, of a lamp.
candela (cd):  unit of luminous intensity; one lumen per steradian (lm/sr).
cataract:  opacity of an area or the entirety of the crystalline lens.
cavity, anterior:  the space between the cornea and the lens containing aqueous humor.
cavity, vitreous:  the space between the lens and the retina containing vitreous humor.
chiasma, optic:  crossover point of the visual pathway where signals from both eyes are combined for each side of the visual field.
chlorolabe:  photopigment contained in M-cones, with peak sensitivity at approximately 525 nm.
choroid:  the portion of the eye, within the “middle coat,” containing the vascular network that services the retina.
chroma (C):  deviation from neutral; the 20-division radial scale of the Munsell system ranging from neutral (0) to highly-saturated (20).  Also saturation or strength.
chromaticity:  the combination of hue and saturation of a color.
chromaticity coordinate:  the ratio of a tristimulus value (X, Y, or Z) to the sum of tristimulus values (X + Y + Z) for a given color.
chromaticity diagram:  a two-dimensional plot of chromaticity coordinates, usually x and y.
chromaticity shift:  the change in chromaticity coordinates of a color when viewed under test lighting compared to reference lighting.
ciliary:  of the structures in the eye responsible for accommodation.
circadian:  relating to the 24-hour cycle of the earth’s rotation, particularly its effect on human behavior and biosystem functions.
clerestory:  a window located above eye-level, often immediately below the roof.
color:  a perception of light, dependent on its wavelength, created in the brain, by which sources and objects, by reflection, can be distinguished.
colorblindness:  any of several forms of defective, or deficient, color vision; see deuteranomalous, deuteranope, dichromat, cone monochromat, rod monochromat, protanomalous, protanope, trichromat, anomalous trichromat, tritanomalous, tritanope.
color, complementary:  the color with which existing light is combined to result in white light.
color confusion, line of:  a locus of points on a chromaticity diagram along which color discrimination is difficult or impossible for someone with a specific color vision deficiency.
color gamut:  the area of a chromaticity diagram circumscribed by the chromaticity coordinates of the first eight CIE test colors.
colorimetry:  measurement of the characteristics of color; e.g. hue.
color matching function (CMF):  spectral tristimulus values; the relative contribution of a primary color required to produce light with the same chromaticity as a reference light.
color mixing:  the selective control of the spectral distribution of light to achieve a desired appearance; can be additive (i.e. control of light source) or subtractive (i.e. filters or pigments).
color order(ing) system:  defined system of color identification and comparison.
color, primary:  one of three colors of light (red, green, blue) used in additive color mixing; one of three colors of filter (cyan, magenta, yellow) used in subtractive color mixing.
color rendering:  the effect of illumination on a color’s chromaticity compared to that under reference illumination.
color rendering index (CRI):  a value representing the magnitude of the change in chromaticity when an object is viewed under test lighting compared to that when the same object is viewed under reference lighting of comparable color temperature.
color rendering index, general (Ra):  the CRI of a single test color.
color rendering index, special (Ri):  the average of the CRIs of the first eight CIE test colors.
color, secondary:  one of three colors of light (cyan, magenta, yellow) created by a combination of two primary colors; one of three colors of light (blue, green, red) created by the overlap of two filters.
color temperature:  the absolute temperature of an ideal radiator with chromaticity equal to that of the light source.
cone:  low-sensitivity photoreceptor responsible for perception of color and discrimination of fine detail.
contrast, chromatic:  the color difference between a target and its background.
contrast, luminance:  the brightness of a target relative to its immediate surroundings.
cornea:  transparent portion of the sclera covering the iris and pupil.
correction, color:  function of a photometric instrument that normalizes the spectral distribution of light being measured.
correction, cosine:  function of a photometric instrument that compensates for deviations of incident light from the normal (i.e. angle of incidence > 0).
correlated color temperature (CCT):  the absolute temperature of an ideal radiator with chromaticity nearest that of the light source.
cortex, visual:  region of the brain responsible for generating visual perception.
cortisol:  hormone that fosters alertness; balanced by melatonin.
critical flicker frequency (CFF):  the rate of light source output variation above which the output appears to be constant.
critical fusion frequency (CFF):  see critical flicker frequency.
cue:  element or feature of a visual scene that prompt a particular interpretation.
cutoff:  the border of illumination provided by a luminaire, beyond which there is no direct glare.
cyanolabe:  photopigment contained in S-cones, with peak sensitivity at approximately 450 nm.
dark-adapted:  adjusted for optimum visual system performance in low illumination; scotopic.
daylight:  optical radiation from the sun comprised of sunlight, skylight, and groundlight; a component of natural light.
daylight factor (DF):  the ratio of the illuminance due to daylight of an interior point on the workplane to the illuminance on a horizontal plane with an unobstructed view of the entire sky under the given sky condition.
daylighting:  the practice of designing and constructing an interior space to be lit, in whole or in part, by natural light to achieve aesthetic, visual performance, energy consumption, and/or other objectives.
dazzle:  a form of glare in which the luminance of a large portion of the visual field exceeds the eye’s adaptation ability; also saturation glare.
depth, daylight penetration (Ddaylight-penetration):  maximum distance from a window at which daylight provides sufficient illuminance; dependent on window-head height and use of shading devices.
depth, limiting room (Dlrd or DLynes):  distance from a window beyond which daylighting creates an excessive illuminance contrast ratio or illuminance gradient.
depth, no skyline (Dno-skyline):  distance from a window beyond which there is no view of the sky from the workplane.
depth of daylit area (Ddaylit-area):  distance from window that can be successfully daylit; the minimum of limiting room depth, no skyline depth, or daylight penetration depth.
deuteranomalous:  referring to an anomalous trichromat characterized by reduced sensitivity to medium-wavelength (green) light.
deuteranope:  a type of dichromat that lacks M-cone photoreceptors.
diabetic retinopathy:  a condition characterized by dark or blank spots in the visual field due to retinal damage caused by long-term uncontrolled diabetes.
dichromat:  an individual that possess two types of cones, but lacks the third; see deuteranope, protanope, tritanope.
dimension, critical:  magnitude of the relevant feature of a target that determines its visual size.
dimming:  reducing the luminous flux of a lamp by controlled modification of its electrical input.
diode:  a device that allows electrical current to flow in only one direction.
diopter (D):  unit of dioptric power; the strength of lens required to focus parallel light rays at a distance of 1 m (3 ft).
diplopia:  a condition in which coordination of eye movements has failed, causing each eye to focus on a different point in space; also double vision.
discharge lamp:  a type of light source in which an electrical arc ionizes a gas, resulting in the emission of optical radiation.
discrimination:  the extent to which a stimulus can be detected or fine details resolved; the act of identifying distinct stimuli.
distribution, reflected spectral:  the product of the spectral power distribution of incident light and the spectral reflectance distribution of an object or surface; the result of a light source/object interaction.
distribution, spectral:  any characteristic of light as a function of wavelength.
distribution, spectral power (SPD):  the relative radiant power of a light source as a function of wavelength.
distribution, spectral reflectance (SRD):  the reflectance of an object or surface as a function of wavelength.
distribution, spectral transmittance (STD):  the transmittance of a medium as a function of wavelength.
down-conversion:  the process by which a phosphor absorbs UV radiation and emits visible light.
driver:  a constant-voltage or constant-current device that converts AC line power to DC power for use by an LED.
Duv:  the distance of a light source’s chromaticity coordinates from the Planckian locus.
efficacy, luminous:  the ratio of the luminance of a lamp to its input power (lm/W); called system efficacy when the energy use of all components is included in the value.
efficacy, system:  see luminous efficacy.
electron hole:  an empty position on an atom’s valence band where an electron could reside.
energy, radiant:  see radiation.
erythrolabe:  photopigment contained in L-cones, with peak sensitivity at approximately 575 nm.
extraocular muscles:  the set of three opposing pairs of muscles responsible for eye movement.
eyestrain:  fatigue of the muscles of the eye caused by prolonged visual discomfort and characterized by irritation and inflammation of the eyes, blurred vision, and headache; also asthenopia.
factor, luminance (β):  ratio of illuminance of a surface or medium to the illuminance of a perfect surface or medium viewed under identical conditions.
factor, maintenance (M):  luminaire performance factor that accounts for the effects of burn time and contamination on output; may also include effects of material degradation in luminaire components.  Also called light loss factor.
factor, utilization (U):  luminaire performance factor equal to the proportion of specified lamp output that is available on the task plane; accounts for luminaire type, light distribution components, and possibly material degradation in luminaire components.
failure to perceive:  inability to identify a target within the visual field or to distinguish a target from its background.
far-sightedness:  see hypermetropia.
field, visual:  the area from which the eye receives light, translating it to visual images.
filament:  component of incandescent lamp heated by electrical current to emit visible light.
fill gas:  the combination of gases contained in a lamp’s bulb; it may include inert gas to inhibit corrosion, ionizing gas to sustain a discharge arc, etc.
filter:  method or device used in subtractive color mixing.
fixation:  the process of maintaining focus at a specific distance or on a specific object, or the condition of such maintained focus.
flicker:  variable output of a light source caused by its power supply or physical interruption of a light beam.
flicker fusion frequency (FFF):  see critical flicker frequency.
fluorescent:  emitting optical radiation in the visible spectrum due to the absorption of short-wavelength (UV) radiation.
flux, luminous (Φ):  flow rate of optical radiation emitted by a light source, measured in lumens.
fovea:  the cone-rich central portion of the retina that provides maximum visual acuity.
fritting:  pattern of lines, dots, or other shapes that maintains the view through a glazing while diffusing some of the incident light.
glare:  the phenomenon experienced when luminances in the visual field significantly exceed that to which the eye is adapted; a component of nuisance light.
glare, adaptation:  a temporary form of glare caused by a rapid, large increase in luminance of a large portion of the visual field, eliminated by light-adaptation of the eye.
glare, direct:  high luminance in the visual field generated by a source outside the area of attention, such as a work surface.
glare, disability:  direct or reflected glare that hinders visual performance.
glare, discomfort:  direct or reflected glare that causes discomfort, but does not directly affect visual performance.
glare, reflected:  high luminance in the visual field caused by a highly-reflective surface in or near the area of attention.
glare, saturation:  see dazzle.
glaucoma:  a condition characterized by a narrowing of the visual field caused by increased intraocular pressure.
glazing:  the light-transmitting material in a window, usually glass; may be transparent or translucent.
goniophotometer:  a photometric instrument used to measure the directional distribution of a light source or reflective surface.
groundlight:  optical radiation from the sun received after reflection from terrestrial surfaces, such as terrain, vegetation, or structures; a component of daylight.
high-bay:  describing the space or luminaires typically used in such a space, in which luminaire mounting height is at least 25 ft (7.6 m) and the spacing to mounting height ratio is 1.0 or less.
hue (H):  the attribute related to the perception of light as purple, blue, green, yellow, red, or an intermediary color; the 100-division azimuthal scale of the Munsell system.
humor, aqueous:  transparent fluid in the anterior chamber of the eye that nourishes the cornea and lens.
humor, vitreous:  transparent gelatinous material in the vitreous chamber that maintains the eye’s shape.
hypermetropia:  a form of refractive error in which the focal point is behind the retina, resulting in lower visual acuity for near objects than for distant objects; also far-sightedness or hyperopia.
hyperopia:  see hypermetropia.
illumination:  the emission of optical radiation that makes perception possible or the provision of light to aid in perception of objects (i.e. “vision”).  Generic use of the term predominates; when precision is required, the term illuminance is preferred.
illumination, retinal (et):  the amount of light entering the eye; the product of visual field luminance and pupil area.
illuminance (E):  luminous flux per unit area incident at a point on a surface or object, measured in lux.
illuminance, horizontal:  luminous flux per unit area incident at a point on a horizontal surface, measured in lux.
illuminance ratio:  the ratio of illuminances of any two areas in a larger space.
illuminance, vertical:  luminous flux per unit area incident at a point on a vertical surface, measured in lux.
image preservation:  characteristic of glazing that maintains the view through a window.
incandescent:  emitting optical radiation due to thermal excitation of a material’s atoms.
index, refractive (μ):  ratio of the speed of light in vacuum to the speed of light in a medium.
infrared (IR):  the portion of the electromagnetic spectrum in the approximate range of 770 nm ~ 10^6 nm (1 mm).
injection luminescence:  the emission of optical radiation generated by the recombination of electrons and electron holes in a diode.
intensity, luminous (I):  luminous flux per unit solid angle in a specified direction, measured in candela.
intraocular pressure:  fluid pressure within the eye that maintains its shape and structural integrity.
iris:  pigmented ring of muscle tissue that adjusts the size of the pupil to moderate the amount of light entering the eye.
isotherm:  a line connecting points of constant temperature.
isothermal line:  see isotherm.
L-cone:  cone photoreceptor, containing erythrolabe, with greatest sensitivity to long-wavelength light.
lamp:  an electric light source, consisting of several components, including an emitter (e.g. filament or discharge tube), base, bulb, fill gas, and, in some cases, a ballast or driver.
Landolt ring or Landolt C:  a C-shaped acuity target in which the gap is the critical dimension.
lateral geniculate nucleus (LGN):  organ within the visual pathway where optical signals are distributed to various points in the visual cortex. Also body.
law, combination:  see cosine-cubed law.
law, cosine:  fundamental law of illumination that quantifies the reduction in illuminance of a plane when rotated out of perpendicular to incident light rays.
law, cosine-cubed:  fundamental law of illumination that combines the inverse square law and the cosine law to quantify the reduction in illuminance of a plane attributed to both its orientation relative to a light source and its distance from it.
law, inverse square:  fundamental law of illumination that quantifies the reduction in illuminance of a plane as its distance from a light source increases.
LED:  see light-emitting diode.
length, focal:  the distance at which a lens focuses images.
lens, crystalline:  the optical disk behind the pupil that is deformed to refract light received from various distances to focus it on the fovea.
light:  various vernacular uses, such as a shortened form of visible light or light source, or an informal substitute for illumination or illuminant.
light-emitting diode (LED):  a semiconductor diode that emits optical radiation by way of injection luminescence.
light intrusion:  a form of light trespass in which errant light enters a window of a nearby structure.
light, natural:  optical radiation from the sun received directly or after scattering and/or reflection; comprised of daylight and moonlight.
light, nuisance:  errant light that is detrimental to visual performance, aesthetics, ecological processes, etc.; consists of sky glow, trespass, and glare.
light pipe:  flexible “tubular skylight” used to transmit light from a rooftop to an interior space along a curvilinear path.
light pollution:  unwanted effects of artificial light; see nuisance light.
light shelf:  passive device mounted on the exterior or interior of a space to shade the lower portion of a window while reflecting daylight onto the ceiling through the upper portion.
light spill:  the emission and/or reflection of light into unintended spaces.
light, spill:  light emitted or reflected into intended spaces.
light-to-solar-gain ratio (LSG):  ratio of visible transmittance to solar heat gain coefficient; used to assess the desirability of a glazing in a particular climate.
light trespass:  the detrimental illuminance of a space in the vicinity of the intended target area; a component of nuisance light..
light, wasted:  any light emitted that does not contribute to its intended purpose.
lighting, area:  uniform illumination of a large area for purposes of safe navigation and simple tasks.
lighting, egress:  components of escape lighting, such as exit signs, used to aid navigation of a facility in normal conditions.
lighting, emergency:  a system of lamps, luminaires, auxiliary power sources, and self-luminous signs used to aid evacuation of a facility and/or execution of critical response procedures when normal operations cannot be sustained and/or dangerous conditions exist.
lighting, escape:  a type of emergency lighting used to aid evacuation of a facility.
lighting, general:  see area lighting.
lighting, local:  see task lighting.
lighting, localized:  illumination of a task area and its immediate surroundings.
lighting, office:  illumination of a space in which occupants rely on information presented on display devices for satisfactory task performance.
lighting, safety:  a system of lamps, luminaires, controls, and material properties used to mitigate risks associated with an activity or inherent in a space.
lighting, security:  a system of lamps, luminaires, controls, and material properties used to deter crime and unauthorized access to a site or space and increase the probability of detection, identification, and apprehension of perpetrators, thereby enhancing the perception of safety of persons and property.
lighting, standby:  a type of emergency lighting used to aid execution of critical response procedures in emergency situations.
lighting, task:  illumination of a relatively small area, such as a single workstation, without significant illumination of its surroundings.
light loss factor (LLF):  see maintenance factor.
lightness:  equivalent to Munsell value when referencing a reflective surface.
light source/object interaction:  the influence of a light source’s spectral power distribution and an object’s spectral reflectance distribution on the light reaching an observer, quantified as the product of the two.
light, visible:  the portion of the spectrum of electromagnetic radiation, in the approximate wavelength range of 380 – 780 nm, capable of creating a visual sensation via excitation of photoreceptors in the human eye; often used without differentiator (i.e. “light”) as a generic term.
light-adapted:  adjusted for optimum visual system performance in high illumination; photopic.
line of sight, normal:  visual axis created when eye is at rest.
locus, blackbody:  the line connecting the chromaticity coordinates of ideal radiators at various temperatures.
locus, Planckian:  see blackbody locus.
locus, spectrum:  the line connecting the chromaticity coordinates of monochromatic colors; the upper boundary of the chromaticity diagram.
louver:  a baffle mounted on a luminaire to reduce glare; it may also be used as a reflector for light distribution control.
low-bay:  describing the space or luminaires typically used in such a space, in which luminaire mounting height is less than 25 ft (7.6 m) and the spacing to mounting height ratio exceeds 1.0.
lumen (lm):  unit of luminous flux; the luminous flux emitted by a point source with 1 candela luminous intensity within a unit solid angle; 1/683 W for light of λ = 555 nm.
lumen depreciation:  the reduction in a lamp’s output attributed to burn time.
lumen maintenance:  the level of output retained by a lamp or a general term for a lamp’s use-effected level of output; see lumen depreciation.
lumen method:  calculation procedure used to determine the number of luminaires needed in a specified area to meet its illuminance requirement.
luminaire:  a complete lighting unit consisting of a lamp, the physical structure needed to mount, aim, and protect the lamp, electrical control devices (e.g. ballast or driver), and distribution control devices (e.g. diffuser, reflector).
luminance (L):  luminous intensity per unit area of emitted light, measured in candela per square meter (cd/m^2).
luminance, adaptation:  the prevailing illuminance level to which the visual system has adjusted; the level that determines the visual condition (scotopic, mesopic, photopic), sensitivity of the eye, and the appearance of colors.
luminance ratio:  the ratio of luminances of any two areas in the visual field.
lux (lx):  unit of illuminance; one lumen per square meter (lm/m^2).
M-cone:  cone photoreceptor, containing chlorolabe, with greatest sensitivity to medium-wavelength light.
MacAdam ellipse:  the shape enclosing chromaticities surrounding the coordinates of a given stimulus that are undifferentiable from it.
macula lutea:  yellow-tinted central area of the retina that absorbs short-wavelength optical radiation; often shortened to macula.
macular degeneration, age-related (AMD):  deterioration of the macula, developing in later life, characterized by a loss of central vision; see dry macular degeneration, wet macular degeneration.
macular degeneration, dry:  a condition involving thinning of the macula and the growth of protein clumps, pigment abnormalities, and cell loss.
macular degeneration, wet:  a condition involving abnormal blood vessel growth under the retina, in which leakage and scarring cause vision loss.
magnification, cortical:  increased visual acuity of the fovea resulting from preferential distribution of visual-processing resources in the cortex.
matte:  producing predominantly diffuse reflection.
melanopsin:  photopigment contained in non-visual system photoreceptors, with peak sensitivity at approximately 480 nm.
melatonin:  hormone that fosters calmness or sleepiness; balanced by cortisol.
mesopic:  related to intermediate conditions, between photopic and scotopic; vision with moderate acuity and limited color-discrimination ability.
metamer:  lights of the same color, produced by different spectral power distributions.
miscue:  a misleading cue.
modelling:  the effect of lighting that reveals three-dimensional characteristics of an object.
monochromat:  an individual with one type of cone photoreceptor, or no cones, characterized by very limited color vision or a total lack thereof; see cone monochromat, rod monochromat.
monochromat, cone:  an individual with only one type of cone photoreceptor, usually S-cones, providing very limited color perception.
monochromatic:  containing a single wavelength of light.
monochromat, rod:  an individual with no cone photoreceptors, characterized by the ability to perceive only differences in brightness, or “grayscale vision.”
moonlight:  optical radiation from the sun received after reflection from the moon; a component of natural light.
Munsell Color Atlas:  a book of physical specimens used for color identification and comparison.
Munsell solid:  a three-dimensional model of the Munsell system of color identification.
Munsell (color) system:  a scheme that identifies colors by their coordinates on three scales, or dimensions – hue, value, and chroma.
myopia:  a form of refractive error in which the focal point is in front of the retina, resulting in lower visual acuity for distant objects than for near objects; also near-sightedness.
near-sightedness:  see myopia.
nerve, optic:  key component of the visual pathway that carries electrical impulses from the retina.
neutral:  exhibiting a complete absence of hue and chroma; lying on the value axis of the Munsell system model.
night blindness:  a condition resulting from Vitamin A deficiency and occurring in low levels of illumination in which exposure to a high luminance depletes the supply of rhodopsin, leaving the eye blind until it is regenerated.
nit:  non-SI unit of luminance equivalent to 1 candela per square meter (cd/m^2).
nonverbal communication:  facial expressions and body language exhibited by a person, often unintentionally, while engaged in an interaction with another.
observer, standard:  one of the CIE color matching functions (2° or 10°).
office:  a space in which satisfactory task performance depends significantly on the ability to view information displayed on screens, gauges, or similar devices.
optometry:  the practice of measuring the power and range of vision and prescribing lenses to correct errors in refraction in the eye.
parallax:  apparent change in position of an object caused by a change in the direction from which it is viewed.
pathway, visual:  the organs and nerves involved in absorbing visible light, converting it to electrical signals, transmitting and organizing these signals, and interpreting them as visual perceptions.
perception, incorrect:  faulty interpretation of a visual stimulus.
perception, loss of:  a failure to perceive or an incorrect perception.
perception theory:  visual communication theory asserting that all visual communication has a neurological basis; vision occurs in the brain where emotion supersedes reason.
perception, visual:  the brain’s construction of a recognizable image from the light incident on the retina.
perceptual confusion:  difficulty interpreting a visual scene caused by an inconsistency between the patterns of illuminances and surface reflectances.
performance, visual:  the measure of speed and accuracy when conducting a visual task.
perspective:  the phenomenon by which objects at greater distance appear to be smaller.
phase shift:  disruption of natural circadian rhythm caused by the absence of or changes in normal cues, particularly patterns of light and darkness.
photometer:  instrument used to measure photometric quantities, such as luminous flux.
photometer, integrating:  instrument used to directly measure total luminous flux of a light source.
photometry:  measurement of quantities related to lighting; use is expanded here to include qualitative assessments of lighting in addition to quantitative measures.
photopic:  related to conditions of high illumination; vision capable of perceiving a wide range of colors with maximum acuity.
photopigment:  chemical compound contained in photoreceptors with unique light-absorption characteristics.  See cyanolabe, chlorolabe, erythrolabe, and rhodopsin.
photoreceptor:  cells in the retina that preferentially absorb incident light depending on the photopigment contained within.
photosphere:  the visible “surface” of the sun, primarily responsible for emission of the optical radiation received on Earth.
Planck’s law:  an equation, expressed in wavelength form or frequency form, that describes the distribution of electromagnetic radiation emitted by a black body, at a given temperature, when in equilibrium in terms of matter and energy.  Also Planck radiation law.
point, far:  location of target at which zero accommodation is required to focus an image on the retina or the eye is “at rest.”
point, near:  location of target where maximum accommodation can just focus an image on the retina.
polarity, negative:  display of bright text, graphics, or other information or objects on a dark background.
polarity, positive:  display of dark text, graphics, or other information or objects on a bright background.
power, dioptric:  strength of a lens or system; extent of refraction induced as light passes through a lens or system.
power, radiant:  the amount of radiant energy emitted by a light source.
presbyopia:  a form of far-sightedness that develops in middle age, characterized by a reduction in the range of accommodation caused by a loss of elasticity of the crystalline lens.
protanomalous:  referring to an anomalous trichromat characterized by reduced sensitivity to long-wavelength (red) light.
protanope:  a type of dichromat that lacks L-cone photoreceptors.
pupil:  opening at the center of the iris, the size of which determines how much light enters the eye.
Purkinje shift:  transition between peak wavelength sensitivities of light- and dark-adaptation.
purple boundary:  the straight line connecting the ends of the spectrum locus to create the lower boundary of the chromaticity diagram, along which hues that contain red and blue components lie.
quality, retinal image:  sharpness of image projected onto the retina.
radiation:  energy transmitted in the form of waves.
radiation, ionizing:  high-frequency electromagnetic waves with sufficient energy to separate electrons from substances such as air, water, and living tissue.
radiation, optic:  see radiation, visual.
radiation, optical:  electromagnetic radiation spanning the ultraviolet, visible light, and infrared portions of the spectrum (~10 ~ 10^6 nm).
radiation, visual:  component of the visual pathway, connecting the lateral geniculate nucleus to the visual cortex.
radiator, ideal:  see black body.
ratio, diversity:  a measure of illuminance consistency within a defined area, equal to maximum illuminance/minimum illuminance.
ratio, uniformity:  a measure of illuminance consistency within a defined area, equal to minimum illuminance/mean illuminance.
reception theory:  visual communication theory concerned with how images convey messages rather than the meanings derived from them.
reflectance (ρ):  ratio of luminous flux reflected from a surface to that incident upon it.
reflection, veiling:  reflected glare that obscures details or limits visibility of an object.
reflectometer:  instrument used to measure surface reflectance.
refraction:  the change in a light ray’s direction of travel as it passes from one medium to another.
refractive error:  a condition in which light is not properly focused (form a coherent image) on the retina; see astigmatism, hypermetropia, myopia.
relative visual performance (RVP):  a model that uses task performance metrics to assess visual capabilities.
representation theory:  visual communication theory that treats images as viable substitutes for the subjects of those images.
restrike time:  time required after a lamp is extinguished before it can be restarted.
retina:  the “inner coat” of the eye containing photoreceptors and nerve cells that transmit the translated visual information as electrical impulses to the optic nerve.
rhodopsin:  photopigment contained in rods, with peak sensitivity at approximately 505 nm.
rod:  highly sensitive, low-resolution photoreceptor, containing rhodopsin, used in low-light conditions.
roof monitor:  section of a structure raised to accommodate daylighting apertures above the surrounding roof.
S-cone:  cone photoreceptor, containing cyanolabe, with greatest sensitivity to short-wavelength light.
saccade:  rapid eye movement initiated to focus on a new fixation target.
safety:  the mitigation of risk associated with an activity.
safety culture:  a set of norms that establish safe work practices and personal protection as organizational priorities.
safety, psychological:  see security.
saturation:  purity or intensity of a color; chroma.
sclera:  outer layer of the eye, consisting of tough, fibrous material.
scotopic:  related to conditions of low illumination; vision of limited acuity, incapable of perceiving color.
seasonal affective disorder (SAD):  psychological condition, that may include depression, caused by reduced availability of sunlight in winter months.
security:  the mitigation of concern for risk of criminal victimization, physical injury, etc.
self-luminous display (SLD):  computer monitor, backlit screen, lighted gauge, or other device used to display information in which the light required for visibility of the information is provided by the device.
semiconductor:  a material with intermediate resistance (between a conductor and an insulator) that can be modified by application of an electric field.
semiotics:  the study of signs and symbols.
sensitivity, spectral:  the differential absorption characteristics of a photopigment, often displayed as a curve relative to wavelength or summarized by a wavelength of peak sensitivity.
sidelighting:  method of daylighting that uses glazing adjacent to the workplane.
size, visual:  the solid angle subtended by a target at the observer’s eye (detection) or the solid angle subtended by a target’s critical dimension at the observer’s eye (resolution).
sky angle (θ):  the angle between any horizontal obstruction of daylight and the vertical obstruction created by any overhang of the window, measured at the center of the window in degrees.
sky glow:  luminance of the night sky in excess of that produced by natural sources, such as moonlight or starlight; a component of nuisance light.
Sky Quality Meter (SQM):  a measure of sky brightness in magnitudes per square arc-second.
skylight:  (1) optical radiation from the sun received after scattering in the atmosphere; a component of daylight. (2) a glazed aperture, sometimes domed, in the roof of structure; a type of toplight.
Snellen chart:  standardized display of 11 lines of progressively smaller letters used by optometrists to assess visual acuity.
solar heat gain:  heat gain in an interior space attributed to solar radiation incident on its windows.
solar heat gain coefficient (SHGC):  proportion of optical radiation incident on a glazing that is transferred to the interior by any means (i.e. transmission, reradiation, conduction, convection); values range from 0.0 to 1.0.
solid angle (ω):  ratio of the surface area of a sphere intercepted by a cone with its vertex at the center of the sphere to the square of the sphere’s radius, measured in steradians.
sparkle:  a type of glare, often viewed favorably, created when a source of high luminance subtends a small solid angle.
spatial frequency:  the number of high-low luminance cycles, or “lines of resolution,” per unit length of an image, measured in cycles per meter (cycles/m) or inverse meters (m^-1); the number of high-low luminance cycles per degree of visual angle, measured in cycles per degree (cycles/degree).
spectral:  as a function of wavelength.
spectrophotometer:  instrument used to measure reflectance and transmittance as a function of wavelength.
spectrum, electromagnetic:  the continuum comprised of all wavelengths of electric and magnetic radiation.
spectrum, visible:  the portion of the electromagnetic spectrum consisting of visible light (~380 ~ 760 nm).
specular:  producing predominantly regular reflection, in which the angle of reflection equals the angle of incidence.
steradian (sr):  unit of solid angle;  the angle subtended at a sphere’s center by an area on its surface equal to the square of its radius.
stereopsis:  depth perception created by comparing the images generated by two eyes looking at an object from slightly different angles.
strategy, control:  the mechanism(s) in use to adjust the output of a lighting system based on an area’s occupancy, a programmed time schedule, or other parameters.
strategy, lighting:  the type of lighting system in use; general, localized, or local lighting.
strategy, split:  the use of multiple lighting and/or control strategies in a single space.
strength:  see saturation.
stroboscopic effect:  the distortion of visual cues, particularly those of motion, resulting from rapid fluctuations in luminance.
sunlight:  optical radiation received directly from the sun; a component of daylight.
sunpath:  the perceived path of the sun across the sky, relative to a specific point on Earth, on a specific day of the year.
system, dioptric:  the transparent, light-transmitting components of the eye, including the cornea, lens, and humors.
system, non-image-forming:  see system, non-visual.
system, non-visual:  the organs and nerves that use information from incident light to influence other biosystems, such as endocrine activity.
system, visual:  the organs and nerves involved in the process of absorbing incident light to create visual perceptions, including the eye, the visual pathway, and the visual cortex.
task area:  the space occupied by additional materials relevant to task performance, such as additional tools, documents, etc.; “within reach, but secondary.”
task margin:  illumination transition area between a task area and non-task areas or other workstations; a lighting buffer zone.
task proper:  the space occupied by items directly involved in task performance; “where the hands and eyes operate.”
task, visual:  components of work that require vision to perform.
threshold, color:  a measure of the ability to differentiate between targets of similar color appearing simultaneously or sequentially.
threshold, spatial:  a measure of the ability to differentiate a target from its background or to discriminate detail within a target; examples include luminance contrast and visual acuity.
threshold, temporal:  a measure of the responsiveness of the visual system to fluctuations in illumination. 
threshold, visual:  the magnitude of stimulus that must be exceeded to generate visual perception, often defined as the stimulus with 50% probability of detection by 50% of any population.
toplighting:  method of daylighting that uses glazing located above the workplane.
transmission, diffuse:  scattering of light transmitted through a nonhomogeneous medium.
transmissometer:  instrument used to measure transmittance of a medium.
transmittance (τ):  ratio of luminous flux transmitted through a medium to that incident upon it.
tremor:  small, uncontrolled eye movements that the “visual control system” uses to maintain a coherent image.
trichromat:  an individual with normal color vision.
trichromat, anomalous:  an individual in which the spectral sensitivity of one photopigment is lower than normal or expected.  See deuteranomalous, protanomalous, tritanomalous.
tristimulus values:  the relative contributions of three primary colors required to reproduce the chromaticity of a light.
tritanomalous:  referring to an anomalous trichromat characterized by reduced sensitivity to short-wavelength (blue) light.
tritanope:  a type of dichromat that lacks S-cone photoreceptors.
troland (Td):  unit of retinal illumination; the retinal illumination produced by a stimulus with 1 cd/m^2 (nit) luminance and pupil area of 1 mm^2.
U-factor (U):  a measure of a glazing’s heat conductivity, the inverse of its R-value; measured in W/h/m2/K.
ultraviolet (UV):  the portion of the electromagnetic spectrum in the approximate range of 10 ~ 400 nm.
uniformity:  an expression of the variation of lighting characteristics within an area or between areas, often expressed in ratios, e.g. illuminance ratio.
value (V):  the vertical scale of the Munsell system model ranging from 0 (black) to 10 (white).
vergence:  movement of the eyes in opposite directions to effect a change in focal distance.
version:  movement of the eyes in the same direction to maintain convergence of the eyes’ lines of sight.
video display terminal (VDT):  see self-luminous display.
vision:  creation of perceptual images from light incident on the eyes.
vision loss:  any deviation from normal vision.
vision, persistence of:  the retention of a visual image after the stimulus has disappeared.
visual cognition:  visual communication theory supporting visual primacy.
visual comfort:  an abstract term used to describe the level of satisfaction with lighting conditions; visual discomfort is used to refer to dissatisfaction with lighting conditions, such as the existence of glare or flicker in the visual field.
visual communication:  prescribed messages in designed visual formats, transmitted with clarity and intent.
visual display unit (VDU):  see self-luminous display.
visual primacy:  prioritization of visual processing; dominance of vision over other sensory capabilities created by the structure and function of the brain.
visual stimulus:  any perceptible light entering the eye.
warm-up time:  time required for a lamp to reach its maximum, or rated, output.
Wien’s law:  an equation used to approximate the wavelength at which peak radiation intensity occurs, given in the form  λpeak = (2.898 x 10^6)/T, where λpeak is the wavelength of highest-intensity radiation (nm) and T is the black body’s stable temperature (K).  Also Wien’s displacement law or Wien’s approximation.
window-head height (hw-h or hwindow-head-height):  distance from the floor to the top of a window, measured in meters.
window-to-wall ratio (WWR):  proportion of a space’s wall area that must be glazed to achieve the desired daylight factor.
workplane:  plane on which visual tasks are normally performed and illuminances are specified; typically 0.76 m (30 in) above the floor.


            For additional guidance or assistance with Safety, Health, and Environmental (SHE) issues, or other Operations challenges, feel free to leave a comment, contact JayWink Solutions, or schedule an appointment.
 
References
[Link] “Lighting Glossary.” Flos USA.
[Link] “Lighting Terminology.”  Wisconsin Energy Efficiency and Renewable Energy, Division of Extension, University of Wisconsin-Madison.
[Link] “Lighting Terminology.”  HyLite LED.
[Link] The IESNA Lighting Handbook, 9ed.  Mark S. Rea (ed).  Illuminating Engineering Society of North America; 2000.
 
Jody W. Phelps, MSc, PMP®, MBA
Principal Consultant
JayWink Solutions, LLC
jody@jaywink.com
 
Directory of “Workplace Illumination” entries on “The Third Degree.”
Part 1:  An Introduction to Lighting (21Aug2024)
Part 2:  Mechanics of Light (4Sep2024)
Part 3:   The Human Eye (18Sep2024)
Part 4:  Visual and Non-Visual Systems (2Oct2024)
Part 5:  Quantity and Quality of Light (16Oct2024)
Part 6:  Quality of Vision (30Oct2024)
Part 7:  Colorimetry and Color Vision (13Nov2024)
Part 8:  Perceptual Phenomena of Vision and Lighting (27Nov2024)
Part 9:  Light-Related Maladies (11Dec2024)
Part 10:  Visual Communication (8Jan2025)
Part 11:  Luminaires (22Jan2025)
Part 12:  General Lighting Characteristics (5Feb2025)
Part 13:  Daylighting (19Feb2025)
Part 14:  Task Lighting (5Mar2025)
Part 15:  Office Lighting (19Mar2025)
Part 16:  Safety, Security, and Emergency Lighting (2Apr2025)
Part 17:  Nuisance Light (16Apr2025)
Part 18:  Lighting Cost Analysis (30Apr2025)