Workplace Illumination – Part 4: Visual and Non-Visual Systems

     The functions of the eye presented in Part 3 often get the most attention.  This is simply because they are closely associated with the most-salient features of our sensory perceptions.  That is to say that the eye’s role as a component of the visual system is most familiar; the previously discussed functions may even be mistakenly conceived as the entirety of the visual system.  The exploration in this installment demonstrates that this is far from the reality.
     In addition, the eye plays an important role in the non-image-forming system; the term non-visual system is used here for brevity and to maintain an obvious contrast between functional attributes.  The non-visual system affects sleep/wake cycles, alertness, and related circadian rhythms.

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
[email protected]