Workplace Illumination - Part 8 - Perceptual Phenomena of Vision and Lighting

     All human visual capabilities are, ultimately, matters of perception.  Of course, the specific nature of each input to the visual system – the visual stimuli – is important; however, similar stimuli can result in significantly different visual perceptions.  It is the interpretation of visual stimuli, occurring in the brain, that defines our visual perceptions.
     In addition to wavelength, luminance, and related measures of illumination, the brain uses other information to interpret visual inputs.  Proximity of objects to one another, color variation, brightness, contextual clues, cultural norms, and other factors can influence the perception of, and reaction to, a visual scene.
     This installment explores some of the factors that influence visual perception.  Some factors defy measurement; some are relegated to the subconscious through training, experience, or other influence on an individual’s attentional budget.  Those of which observers are aware may still defy explanation of their impact, for many are unfamiliar with the mechanisms involved.  Nonetheless, all are important contributors to interpretations of our surroundings.

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
[email protected]