Workplace Illumination – Part 1: An Introduction to Lighting

     Light, like heat, noise, and vibration, can seem innocuous at first.  However, the characteristics of light in a workplace, its consistency, and its delivery can profoundly affect the health, safety, and productivity of workers.  Thus, workplace illumination warrants further investigation and, yes, its own series in “The Third Degree.”
     Many similarities exist between the human experiences of light and sound.  It likely comes as no surprise, then, that presentation of fundamental information in this series parallels that in the “Occupational Soundscapes” series.  Links between the two also exist, for example, when used in conjunction to increase the effectiveness of communication.
     These similarities and combined uses are explored, along with physiological and psychological impacts of lighting design choices and related topics.  This introductory installment provides a glossary of terms and, like those before it, a directory of published installments for convenient access to all series content.

     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
[email protected]
 
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)