Workplace Illumination - Part 9: Light-Related Maladies

     Like all human capabilities, visual performance is influenced by a number of factors.  Inborn imperfections in the eyes, physical injury, infection, and other ailments can affect visual capabilities.  Changes in visual capabilities are also experienced due to advancing age.  The variety of potential afflictions results in a wide range of severity and compensability.
     Focusing an image on the retina, differentiating colors, and distinguishing between objects or details are functions of the visual system that can be effected by various maladies.  Changes can also occur in the non-visual system, affecting circadian timing and related functions.  These changes may be more difficult to correlate with perceived effects, as links to non-visual system functions are typically less obvious than that of, say, a blurred image to visual system functions.
     An introduction to several light-related maladies, including innate and acquired conditions, is provided in this installment.  Effects on workplace visual performance, potential treatments, and countervailing techniques are presented in brief.  The prevalence and severity of these conditions among the workforce are parameters that must be considered in an analysis of lighting system design requirements.

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
[email protected]