Workplace Illumination – Part 6: Quality of Vision

     Several aspects of vision contribute to a subjective assessment of clarity or visibility of an object.  Though many characteristics of vision are quantifiable, ultimately, subjective assessments often dominate discussions of lighting needs.  The challenge for lighting designers and industrial hygienists is to combine subjective feedback and objective data to extrapolate lighting system requirements.
     In Part 5, objective measures of light were introduced; the discussion involved the source of light (e.g. luminance) and its path (e.g. refraction, reflection).  In this installment, the receiver of light, the human visual system, is once again considered.  Factors contributing to visual capabilities and quantifiable measures are explored.

     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
[email protected]