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Glossary for Lighting Terminology

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A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

As the title implies, this last chapter contains terminology directly related to light and lighting practice. All terms are presented in alphabetical order and are followed by their standard symbols or abbreviations and their defining equations where applicable, by their definitions, and by other related terms of interest. No attempt has been made to provide information on pronunciations or etymologies. Definitions of electrical terms common to lighting and to other fields are available in the American National Standard Dictionary of Electrical and Electronics Terms (ANSI/IEEE 100-1984).

Any of the radiometric and photometric quantities that follow may be restricted to a narrow wavelength interval ?? by the addition of the word “spectral” and the specification of the wavelength ?. The corresponding symbols are changed by adding a subscript ?, as in Q? for a spectral concentration, or a ? in parentheses, as in K(?) for a function of wavelength. The appendix is a tabulated summary of standard units, symbols, and radiometric quantities. Other symbols, abbreviations, and conversion factors are also given in the appendix.

Most of the definitions in this glossary have been adapted from ANSI/IES RP-16-1996, Nomenclature and Definitions for Illuminating Engineering. Those marked with a dagger (†) do not appear in the ANSI standard.

Referenced:

The IESNA Lighting Handbook: Reference and Application / Ninth Edition/ 120 Wall Street, 17th Floor, New York, NY, 10005-4001 Mark S. Rea Ph.D., FIES, Editor-In-Chief

A

absolute luminance threshold luminance threshold for a bright object like a disk on a totally dark background.

absorptance, a = Fa/FI the ratio of the absorbed flux to the incident flux. See absorption.

Note The sum of the hemispherical reflectance, the hemispherical transmittance, and the absorptance is one.

absorption a general term for the process by which incident flux is converted to another form of energy, usually and ultimately to heat.

Note All of the incident flux is accounted for by the processes of reflection, transmission, and absorption.

accent lighting directional lighting to emphasize a particular object or surface feature or to draw attention to a part of the field of view. See directional lighting.

acceptance half-angle† the acceptance half-angle is set by the refractive indices of the core and cladding of fiber optics, and it is expressed in terms of the numerical aperture (NA). The numerical aperture is given as NA = n0 sin p.†

accommodation the process by which the eye changes focus from one distance to another.

actinic a term meaning photochemically active.

action spectrum the quantitative actinic response of a chemical or biological substance or living organism as a function of an appropriate spectral parameter such as wavelength or photon energy.

adaptation the process by which the retina becomes accustomed to more or less light than it was exposed to during an immediately preceding period. It results in a change in the sensitivity to light. See scotopic vision, photopic vision, and chromatic adaptation.

Note Adaptation is also used to refer to the final state of the process, such as reaching a condition of adaptation to a specific luminance level.

adaptive color shift the change in the perceived object color caused solely by change of the state of chromatic adaptation.

adverse weather lamp† See fog lamp.

aerodrome beacon an aeronautical beacon used to indicate the location of an aerodrome.

Note An aerodrome is any defined area on land or water–including any buildings, installations, and equipment–intended to be used either wholly or in part for the arrival, departure, and movement of aircraft.

aeronautical beacon an aeronautical ground light visible at all azimuths, either continuously or intermittently, to designate a particular location on the surface of the earth. See aerodrome beacon, airway beacon, hazard or obstruction beacon, and landmark beacon.

aeronautical ground light any light specially provided as an aid to air navigation, other than a light displayed on an aircraft. See aeronautical beacon, angle-of-approach lights, approach lights, approach-light beacon, bar (of lights), boundary lights, circling guidance lights, course light, channel lights, obstruction lights, perimeter lights, runway alignment indicator, runway end identification light, runway lights, taxi-channel lights, and taxiway lights.

aeronautical light any luminous sign or signal that is specially provided as an aid to air navigation.

after image a visual response that occurs after the stimulus causing it has ceased.

aircraft aeronautical light any aeronautical light specially provided on an aircraft. See anticollision light, ice detection light, fuselage lights, landing light, navigation light system, position lights, and taxi light.

airway beacon an aeronautical beacon used to indicate a point on the airway.

alphanumeric display (digital display) an electrically operated display of letters and/or digits. Tungsten filaments, gas discharges, light-emitting diodes, liquid crystals, projected numerals, illuminated numbers, fluorescent screens, and other principles of operation can be used.

altitude (in daylighting) the angular distance of a heavenly body measured on the great circle that passes, perpendicular to the plane of the horizon, through the body and through the zenith. It is measured positively from the horizon to the zenith, from 0° to 90°.

ambient lighting lighting throughout an area that produces general illumination.

anchor light (aircraft) an aircraft light designed for use on a seaplane or amphibian to indicate its position when at anchor or moored.

ampere† the intensity of electrical current flow. The symbol often used in equations is I, although A is also acceptable.

angle-of-approach lights aeronautical ground lights arranged so as to indicate a desired angle of descent during an approach to an aerodrome runway. (Also called optical glide path lights.)

angle of collimation the angle subtended by a light source at a point on an irradiated surface.

angstrom,ņ a unit of wavelength equal to 10-10 m (one ten-billionth of a meter).

anticollision light a flashing aircraft aeronautical light or system of lights designed to provide a red signal throughout 360° of azimuth for the purpose of giving long-range indication of an aircraft’s location to pilots of other aircraft.

aperture color† the perceived color of the sky or of a patch seen through an aperture and not identifiable as belonging to a specific object.

apostilb (asb) a lambertian unit of luminance equal to 1/p = 0.3183 cd/m2. This term is obsolete, and its use is deprecated.

approach-light beacon an aeronautical ground light placed on the extended centerline of the runway at a fixed distance from the runway threshold to provide an early indication of position during an approach to a runway.

Note The runway threshold is the beginning of the runway usable for landing.

approach lights a configuration of aeronautical ground lights located in extension of a runway or channel before the threshold to provide visual approach and landing guidance to pilots. See angle-of-approach lights, approach-light beacon, and VASIS.

arc discharge an electric discharge characterized by high cathode current densities and a low voltage drop at the cathode.

Note The cathode voltage drop is small compared with that in a glow discharge, and secondary emission plays only a small part in electron emission from the cathode.

arc lamp a discharge lamp in which the light is emitted by an arc discharge or by its electrodes.

Note The electrodes can be either of carbon (operating in air) or of metal.

artificial pupil a device or arrangement for confining the light passing through the pupil of the eye to an area smaller than the natural pupil.

atmospheric transmissivity the ratio of the directly transmitted flux incident on a surface after passing through unit thickness of the atmosphere to the flux that would be incident on the same surface if the flux had passed through a vacuum.

average luminance luminance is a property of a geometric ray. Luminance as measured by conventional meters is averaged with respect to two independent variables, area and solid angle; both must be defined for a complete description of a luminance measurement.

azimuth the angular distance between the vertical plane containing a given line or celestial body and the plane of the meridian.

B

back light illumination from behind (and usually above) a subject to produce a highlight along its edge and consequent separation between the subject and its background. See side-back light.

backing lighting the illumination provided for scenery in off-stage areas visible to the audience.

back-up lamp a lighting device mounted on the rear of a vehicle for illuminating the region near the back of the vehicle while moving in reverse. It normally can be used only while backing up.

bactericidal (germicidal) effectiveness the capacity of various portions of the ultraviolet (UV) spectrum to destroy bacteria, fungi, and viruses.

bactericidal (germicidal) efficiency of radiant flux the ratio of the bactericidal effectiveness of that wavelength to that of wavelength 265.0 nm, which is rated as unity.

Note Tentative bactericidal efficiency of various wavelengths of radiant flux are given in Chapter 5, Nonvisual Effects of Radiant Energy.

bactericidal (germicidal) exposure the product of bactericidal flux density on a surface and time. It usually is measured in bactericidal µW × min/cm2 or bactericidal W × min./ft2.

bactericidal (germicidal) flux radiant flux evaluated according to its capacity to produce bactericidal effects. It usually is measured in microwatts of UV radiation weighted in accordance with its bactericidal efficiency. Such quantities of bactericidal flux would be in bactericidal microwatts.

Note Ultraviolet radiation of wavelength 253.7 nm usually is referred to as “ultraviolet microwatts” or “UV watts.”

bactericidal (germicidal) flux density the bactericidal flux per unit area of the surface being irradiated. It is equal to the quotient of the incident bactericidal flux divided by the area of the surface when the flux is uniformly distributed. It usually is measured in µW/cm2 or W/ft2 of bactericidally weighted UV radiation (bactericidal µW/cm2 or bactericidal W/ft2).

bactericidal lamp a UV lamp that emits a significant portion of its radiative power in the UV-C band (100 to 280 nm).

baffle a single opaque or translucent element to shield a source from direct view at certain angles, to absorb or block unwanted light, or to reflect and redirect light.

balcony lights luminaires mounted on the front edge of an auditorium balcony.

ballast a device used with an electric-discharge lamp to obtain the necessary circuit conditions (voltage, current, and waveform) for starting and operating. See reference ballast.

ballast factor the fractional flux of a fluorescent lamp operated on a ballast compared to the flux when operated on the standard (reference) ballast specified for rating lamp lumens.

Note The lamp is at specified ambient temperature conditions for photometric testing

ballast-lamp photometric factor ratio of fluorescent luminaire lumen output using given ballast and lamp types (under photometric test conditions) to the lumen output using the lamp and ballast types used to generate a photometric test.

Note This factor is applicable when “energy-conserving” lamps and ballasts are used in a luminaire photometered with standard lamps and conventional ballasts; it is also applied in the converse situation.

bar
(of lights) a group of three or more aeronautical ground lights placed in a line transverse to the axis, or extended axis, of the runway. See barrette.

bare
(exposed) lamp a light source with no shielding.

barn doors
a set of adjustable flaps–usually two, four, or eight–which can be attached to the front of a luminaire (usually a Fresnel spotlight) in order to partially control the shape and spread of the light beam.

barrette
(in aviation) a short bar in which the lights are closely spaced so that from a distance they appear to be a linear light.
Note Barrettes are usually no longer than 4.6 m (15 ft) in length.

base light
uniform, diffuse, near-shadowless illumination sufficiently intense for a television or film picture of acceptable quality at a desired lens opening. Acceptable base level of unaccented stage illumination.

beacon
a light (or mark) used to indicate a geographic location. See aerodrome beacon, aeronautical beacon, airway beacon, approach-light beacon, hazard or obstruction beacon, identification beacon, and landmark beacon.

beam angle
the angle between the two directions for which the intensity is 50% of the maximum intensity as measured in a plane through the nominal beam centerline. For beams that do not possess rotational symmetry, the beam angle is generally given for two planes at 90°, typically the maximum and minimum angles.
Note In certain fields of application, the beam angle was formerly measured to 10% of maximum intensity.

beam axis of a projector
a line midway between two lines that intersect the intensity distribution curve at points equal to a stated percentage of its maximum (usually 50%).

beam lumens
the total flux in that region of space where the intensity exceeds 50% of the maximum intensity.

beam projector
a luminaire with the light source at or near the focus of a paraboloidal reflector, producing near-parallel rays of light in a beam of small divergence. Some are equipped with spill rings to reduce spill and glare. In most types, the lamp can be moved toward or away from the reflector to vary the beam spread.

beam spread
(in any plane) the angle between the two directions in the plane in which the intensity is equal to a stated percentage of the maximum beam intensity.

biconical reflectance
, ρ(ωi;ωr) ratio of the reflected flux collected through a conical solid angle to the incident flux limited to a conical solid angle.
Note The directions and extent of each cone must be specified; the solid angle need not be a right circular cone.

biconical transmittance, τ(ωi;ωt) ratio of transmitted flux collected through a conical solid angle to the incident flux limited to a conical solid angle.
Note The directions and extent of each cone must be specified.

bidirectional reflectance, ρ(θi,φi;θr,φr) ratio of reflected flux collected over an element of solid angle surrounding the given direction to essentially collimated incident flux.
Note The directions of incidence and collection and the size of the solid angle “element” of collection must be specified. In each case of conical incidence or collection, the solid angle is not restricted to a right circular cone, but can be of any cross section, including a rectangle, a ring, or a combination of two or more solid angles.

bidirectional reflectance distribution function (BRDF), fr the ratio of the differential luminance of a ray dLr(θr,φr) reflected in a given direction (θr,φr) to the differential luminous flux density dEi(θi,φi) incident from a given direction of incidence, (θi,φi), that produces it.
where dΩ ≡ dω cos θ
Notes (i) This distribution function is the basic parameter for describing (geometrically) the reflecting properties of an opaque surface element (negligible internal scattering). (ii) It can have any positive value and approaches infinity in the specular direction for ideally specular reflectors. (iii) The spectral and polarization aspects must be defined for complete specification, since the BRDF as given above defines only the geometric aspects.

bidirectional transmittance, τ(θi,φi;θt,φt) ratio of incident flux collected over an element of solid angle surrounding the given direction to essentially collimated incident flux.
Note The direction of incidence and collection and the size of the solid angle element must be specified.

bidirectional transmittance distribution function (BTDF), ft the ratio of the differential luminance dLt(θt,φt) for a ray transmitted in a given direction (θt,φt) to the differential luminous flux density dEi(θi,φi) incident from a given direction of incidence (θi,φi) that produces it:
where dΩ ≡ dω cos θ
Notes (i) This distribution is the basic parameter for describing (geometrically) the transmitting properties of a thin scattering film (with negligible internal scattering) so that the transmitted radiation emerges from a point that is not significantly separated from the point of incidence of the incident ray(s). The governing considerations are similar to those for application of the bidirectional reflectance distribution function (BRDF), rather than the bidirectional scattering-surface reflectance distribution function (BSSRDF). (ii) It can have any positive value and approaches infinity in the direction for regular transmission (possibly with refraction but without scattering). (iii) The spectral and polarization aspects must be defined for complete specification, since the BTDF as given above defines only the geometrical aspects.

bihemispherical reflectance
, ρ(2π; 2π) ratio of reflected flux collected over the entire hemisphere to the flux incident from the entire hemisphere.

bihemispherical transmittance, τ(2π; 2π) ratio of transmitted flux collected over the entire hemisphere to the incident flux from the entire hemisphere.

binocular portion of the visual field that portion of space where the fields of the two eyes overlap.

biological rhythm
a characteristic periodic change in a living organism or life-related process. Some biological rhythms are induced and/or synchronized by light.

blackbody
a temperature radiator of uniform temperature whose radiant exitance in all parts of the spectrum is the maximum obtainable from any temperature radiator at the same temperature. Such a radiator is called a blackbody because it absorbs all the radiant energy that falls upon it. All other temperature radiators can be classed as nonblackbodies. Nonblackbodies radiate less in some or all wavelength intervals than a blackbody of the same size and the same temperature.
Note The blackbody is practically realized over limited solid angles in the form of a cavity with opaque walls at a uniform temperature and with a small opening for observation. It is variously called a standard radiator, an ideal radiator, or a complete radiator.

blackbody
(planckian) locus the locus of points on a chromaticity diagram representing the chromaticities of blackbodies having various (color) temperatures.

black light
the popular term for UV energy near the visible spectrum.
Note For engineering purposes the wavelength range 320 to 400 nm has been found useful for rating lamps and their effectiveness upon fluorescent materials (excluding phosphors used in fluorescent lamps). By confining black light applications to this region, germicidal and erythemal effects are, for practical purposes, eliminated.

black-light flux
radiant flux within the wavelength range 320 to 400 nm. It is usually measured in milliwatts. See fluoren.
Note The floren is used as a unit of black-light flux and is equal to one milliwatt of radiant flux in the wavelength range 320 to 400 nm. Because of the variability of the spectral sensitivity of materials irradiated by black light in practice, no attempt is made to evaluate black-light flux according to its capacity to produce effects.

black-light flux density
black-light flux per unit area of the surface being irradiated. It is equal to the incident black-light flux divided by the area of the surface when the flux is uniformly distributed. It usually is measured in milliwatts per unit area of black-light flux.

black-light lamp
an ultraviolet lamp that emits a significant portion of its radiative power in the UV-A band (315 to 400 nm).

blending lighting
general illumination used to provide smooth transitions between the specific light areas on a stage.

blinding glare
glare that is so intense that for an appreciable length of time after it has been removed, no object can be seen.

Blondel-Rey
law the ratio of the thresholds of a square-form flashing light (Ea) and of a steady light (Eo), in point vision conditions at night. The ratio depends on the duration in seconds of the flash (t):

bollard† luminaires having the appearance of a short, thick post, used for walkway and grounds lighting. The optical components are usually top-mounted.

borderlight
a long continuous striplight hung horizontally above a stage and aimed down to provide general diffuse illumination or to light the cyclorama or a drop; usually wired in three or four color circuits. Also available in portable versions.

borderline between comfort and discomfort
(BCD) the average luminance of a source in a field of view that produces a sensation between comfort and discomfort.

boundary lights
aeronautical ground lights delimiting the boundary of a land aerodrome without runways. See range lights.

bowl
an open-top diffusing glass or plastic enclosure used to shield a light source from direct view and to redirect or scatter the light.

bracket
(mast arm) an attachment to a lamp post or pole from which a luminaire is suspended.

brightness (of a perceived aperture color) the attribute by which an area of color of finite size is perceived to emit, transmit, or reflect a greater or lesser amount of light. No judgment is made as to whether the light comes from a reflecting, transmitting or self-luminous object. See also brightness of a perceived light source color, luminance, subjective brightness, and veiling brightness.

brightness contrast threshold
when two patches of color are separated by a brightness contrast border as in the case of a bipartite photometric field or of a disk-shaped object surrounded by its background, the border between the two patches is a brightness contrast border. The contrast that is just detectable is known as the brightness contrast threshold.

brightness of a perceived light source color
† the attribute in accordance with which the source seems to emit more or less luminous flux per unit area.
bulb† See lamp.

C

candela, cd the SI unit of luminous intensity, equal to one lumen per steradian (lm/sr). Formerly candle. See Chapter 2, Measurement of Light and Other Radiant Energy.
Note The fundamental luminous intensity definition in the SI is the candela. The candela is the luminous intensity in a given direction of a source that emits monochromatic radiation of frequency 540 × 1012 Hz that has a radiant intensity in that direction of 1/683 watt per steradian. The candela so defined is the base unit applicable to photopic quantities, scotopic quantities, and quantities to be defined in the mesopic domain. From 1909 until January 1, 1948, the unit of luminous intensity in the United States, as well as in France and Great Britain, was the international candle, which was maintained by a group of carbon-filament vacuum lamps. For the present unit as defined above, the internationally accepted term is candela. The difference between the candela and the old international candle is so small that only measurements of high precision are affected. From 1948 to 1979, the unit of luminous intensity was defined in terms of a complete (blackbody) radiator. From this relation, Km and K’m, and consequently the lumen, were determined. One candela was defined as the luminous intensity of 1/600,000 m2 of projected area of a blackbody radiator operating at the temperature of solidification of platinum, at a pressure of 101,325 newtons per square meter (N/m2 = Pa).

candlepower
(cp), I = dφ/dω luminous intensity expressed in candelas.

carbon-arc lamp
an electric-discharge lamp employing an arc discharge between carbon electrodes. One or more of these electrodes can have cores of special chemicals that contribute importantly to the radiation.

cavity ratio
(CR) a number indicating cavity proportions. See ceiling cavity ratio, floor cavity ratio, and room cavity ratio.

For cavities of irregular shape:
Note The relationship between cavity ratio and room coefficient should be noted. If the entire room is considered as a cavity, the room height becomes Heightcavity and CR = 10 × Kr

ceiling area lighting
a general lighting system in which the entire ceiling is, in effect, one large luminaire.
Note Ceiling area lighting includes luminous ceilings and louvered ceilings.

ceiling cavity
the cavity formed by the ceiling, the plane of the luminaires, and the wall surfaces between these two planes.

ceiling cavity ratio
(CCR) a number computed by using the distance from the plane of the luminaire to the ceiling (hc) as Heightcavity in the equations given for cavity ratio.

ceiling projector
a device designed to produce a well-defined illuminated spot on the lower portion of a cloud for the purpose of providing a reference mark for the determination of the height of that part of the cloud.

ceiling ratio
the ratio of the luminous flux reaching the ceiling directly to the upward component from the luminaire.

central
(foveal) vision the seeing of objects in the central or foveal part of the visual field, approximately 2° in diameter. It permits seeing much finer detail than does peripheral vision.

central visual field
that region of the visual field that corresponds to the foveal portion of the retina.

channel
an enclosure containing the ballast, starter, lamp holders, and wiring for a fluorescent lamp. Can also be a similar enclosure on which filament lamps (usually tubular) are mounted.

channel lights
aeronautical ground lights arranged along the sides of a channel of a water aerodrome. See taxi-channel lights.

characteristic curve
a curve that expresses the relationship between two variable properties of a light source, such as candlepower and voltage or flux and voltage.

chromatic adaptation
the process by which the chromatic properties of the visual system are modified by the observation of stimuli of various chromaticities and luminances. See state of chromatic adaptation.

chromatic color
perceived color possessing a hue. In everyday speech, the word color is often used in this sense in contradistinction to white, gray, or black.

chromatic contrast threshold
(color contrast threshold) a threshold of chromaticity difference between two patches of color juxtaposed and separated only by a color contrast border, below which they cannot be perceived as different in chromaticness or separated by a contrast border. A contrast border can involve differences both in luminance and in chromaticity between the sides.

chromaticity coordinates of a color
, x, y, z the ratios of each of the tristimulus values of the color to the sum of the three tristimulus values.

chromaticity diagram
a plane diagram formed by plotting one of the three chromaticity coordinates against another.

chromaticity difference threshold
the smallest difference in chromaticity between two colors of the same luminance that makes them perceptibly different. The difference can be a difference in hue or saturation, or a combination of the two.

chromaticity of a color
the dominant or complementary wavelength and purity aspects of the color taken together, or of the aspects specified by the chromaticity coordinates of the color taken together.

chromaticness
† the attribute of a visual sensation according to which the (perceived) color of an area appears to be more or less chromatic.

CIE (L*,
a*, b*) uniform color space, CIELAB a transformation of CIE tristimulus values X, Y, Z into three coordinates (L*, a*, and b*) that define a space in which equal distances are more nearly representative of equal magnitudes of perceived color difference. This space is specially useful in cases of colorant mixtures (for example, dye-stuffs, paints).

CIE (L*,
u*, v*) uniform color space, CIELUV a transformation of CIE tristimulus values X, Y, Z into three rectangular coordinates (L*, u*, and v*) that define a space in which equal distances are more nearly representative of equal magnitudes of perceived color difference. This space is specially useful in cases where colored lights are mixed additively, for example, color television.

CIE standard chromaticity diagram
one in which the x and y chromaticity coordinates are plotted in rectangular coordinates.

circling guidance lights
aeronautical ground lights provided to supply additional guidance during a circling approach when the circling guidance furnished by the approach and runway lights is not adequate.

clear sky
a sky that has less than 30% cloud cover.

clearance lamp
lighting devices for the purpose of indicating the width and height of a vehicle.

clerestory
that part of a building that rises clear of the roofs or other parts and whose walls contain windows for lighting the interior.

cloudy sky
a sky that has more than 70% cloud cover.

coefficient of attenuation
(at a point in a given direction), μ the decrement in flux per unit distance in a given direction within a medium. It is defined by the relation φx = φ0e−μx where φx is the flux at any distance x from a reference point having flux φ0. More generally, where the coefficient varies from point to point; μ = μ(x) along the path.

coefficient of beam utilization (CBU) the ratio of the luminous flux (lumens) reaching a specified area directly from a floodlight or projector to the total beam luminous flux (lumens).

coefficient of utilization
(CU) the ratio of luminous flux (lumens) calculated as received on the work plane to the total luminous flux (lumens) emitted by the lamps alone. It is equal to the product of room utilization factor and luminaire efficiency. See Chapter 9, Lighting Calculations.

coffer
a recessed panel or dome in the ceiling.

cold-cathode lamp
an electric-discharge lamp whose mode of operation is that of a glow discharge and that has electrodes so spaced that most of the light comes from the positive column between them.

color
† the characteristic of light by which a human observer can distinguish between two structure-free patches of light of the same size and shape. See light source color and object color.

color difference thresholds
the difference in chromaticity and/or luminance between two colors that makes them just perceptibly different. The difference can be a difference in hue, saturation, brightness (lightness for surface colors), or a combination of the three.

color comparison or color grading
the judgment of equality, or of the amount and character of difference, of the color of two objects viewed under identical illumination.

color contrast threshold
† See chromaticity difference threshold.

color correction
(of a photograph or printed picture) the adjustment of a color reproduction process to improve the perceived-color conformity of the reproduction to the original.

color discrimination
the perception of differences between two or more colors.

color matching
the action of making a color appear the same as a given color.

color-matching functions
(spectral tristimulus values), ¯x(λ) = Xλ/φeλ,¯y(λ) = Yλ/φeλ, ¯z(λ) = Zλ/φeλ the tristimulus values per unit wavelength interval and unit spectral radiant flux.
Note Color-matching functions have been adopted by the Commission Internationale de l’Eclairage (CIE). They are tabulated as functions of wavelength throughout the spectrum and are the basis for the evaluation of radiant energy as light and color. The standard values adopted by the CIE in 1931 are given in Chapter 4, Color. The ¯y values are identical with the values of the spectral luminous efficiency for photopic vision. The ¯x, ¯y, and ¯z values for the 1931 Standard Observer are based on a 2° bipartite field and are recommended for predicting matches for stimuli subtending between 1 and 4°. Supplementary data based on a 10° field were adopted in 1964 for use for angular subtends greater than 4°.

color preference index
(of a light source), Rp Measure appraising a light source for enhancing the appearance of an object or objects by making their colors tend toward people’s preferences. Judd’s “Flattery Index” is an example. See flattery index.

color rendering
† a general expression for the effect of a light source on the color appearance of objects in conscious or subconscious comparison with their color appearance under a reference light source.

color rendering improvement
(of a light source)† the adjustment of spectral composition to improve color rendering.

color rendering index
(of a light source) (CRI) a measure of the degree of color shift objects undergo when illuminated by the light source as compared with those same objects when illuminated by a reference source of comparable color temperature.

color temperature of a light source
the absolute temperature of a blackbody radiator having a chromaticity equal to that of the light source. See also correlated color temperature and distribution temperature.

colorfulness
† See chromaticness.

colorfulness of a perceived color
the attribute according to which it appears to exhibit more or less chromatic color. For a stimulus of a given chromaticity, colorfulness normally increases as the absolute luminance is increased.

colorimetric purity
(of a light), pc the ratio L1/L2, where L1 is the luminance of the single-frequency component that must be mixed with a reference standard to match the color of the light, and L2 is the luminance of the light. See excitation purity.

colorimetric shift
the change of chromaticity and luminance factor of an object color due to change of the light source. See adaptive color shift and resultant color shift.

colorimetry
the measurement of color.

compact-arc lamp
† See short-arc lamp.

compact source iodide lamp
(CSI) an arc source utilizing a mercury vapor arc with metal halide additives to produce illumination typically in the 5000 to 6000 K range. Requires a ballast and ignitor-system for opera-
tion.

comparison lamp
a light source having a constant, but not necessarily known, luminous intensity with which standard and test lamps are successively compared.

complementary wavelength
(of a light), λc the wavelength of radiant energy of a single frequency that, when combined in suitable proportion with the light, matches the color of a reference standard. See dominant wavelength.

complete diffusion
that in which the diffusing medium completely redirects the incident flux by scattering so that no incident flux can remain in an image-forming state.

cones
retinal receptors that dominate the retinal response when the luminance level is high and provide the basis for the perception of color.

configuration factor
, C12 the ratio of the illuminance on a surface at point 1 (due to the flux directly received from lambertian surface 2) to the exitance of surface 2. It is used in flux transfer theory. Also, the ratio of the differential flux directly received by surface 2 (and due to element 1) to the total differential flux emitted by differential lambertian surface element 1:
Note In the literature this ratio is also called the angle factor, illumination factor, point configuration factor, and sky factor.

conical-directional reflectance
, ρ(ωi; θr, φr) ratio of reflected flux collected over an element of solid angle surrounding the given direction to the incident flux limited to a conical solid angle.
Note The direction and the extent of the cone must be specified, and the direction of collection and size of the solid angle “element” must be specified.

conical-directional transmittance, τ(ωi; θt, φt) ratio of transmitted flux, collected over an element of solid angle surrounding the direction to the incident flux limited to a conical solid angle.
Note The direction and extent of the cone must be specified, and the direction of collection and size of the solid angle “element” must be specified.

conical-hemispherical reflectance, ρ(ωi; 2π) ratio of reflected flux collected over the entire hemisphere to the incident flux limited to a conical solid angle.
Note The direction and extent of the cone must be specified.

conical-hemispherical transmittance, τ(ωt; 2π) ratio of transmitted flux collected over the entire hemisphere to the incident flux limited to a conical solid angle.
Note The direction and extent of the cone must be specified.

conspicuity the capacity of a signal to stand out in relation to its background so as to be readily discovered by the eye.

contrast† See luminance contrast.

contrast rendition factor (CRF) the ratio of the visual task contrast with a given lighting environment to the contrast with sphere illumination. Also known as the contrast rendering factor.

contrast sensitivity the ability to detect the presence of luminance differences. Quantitatively, it is equal to the reciprocal of the brightness contrast threshold.

cornice lighting lighting comprising sources shielded by a panel parallel to the wall and attached to the ceiling and distributing light over the wall.

correlated color temperature (of a light source) (CCT) the absolute temperature of a blackbody whose chromaticity most nearly resembles that of the light source.

cosine law a law stating that the illuminance on any surface varies as the cosine of the angle of incidence. The angle of incidence is the angle between the normal to the surface and the direction of the incident light. The inverse square law and the cosine law can be combined as E = (I cos θ)/d2. See cosine-cubed law and inverse square law.

cosine corrected light meter† a light meter that properly accepts and measures light from the hemisphere above the detector proportional to the cosine of the incident angle.

cosine-cubed law an extension of the cosine law in which the distance d between the source and surface is replaced by h/cos θ, where h is the perpendicular distance of the source from the plane in which the point is located. It is expressed by E = (I cos3 θ)/h2.

counter-key light illumination on a subject from a direction that is opposite to that of the key light.

country beam† See upper (driving) beams.

course light† an aeronautical ground light, supplementing an airway beacon, for indicating the direction of the airway and to identify by a coded signal the location of the airway beacon with which it is associated.

cove lighting lighting comprising sources shielded by a ledge or horizontal recess, and distributing light over the ceiling and upper wall.

criteria rating a technique that determined the probability that a specific criterion will be met anywhere in a defined area. The name of the criteria rating includes the name of the criterion being rated. It is expressed in shorthand notation by listing the rating in percent followed by the criterion itself and separated by “@”. For example, a lighting system producing a luminance of 100 cd/m2 over 60% of the specified area could have its luminance rating expressed as 60%@100 cd/m2.

critical flicker frequency (CFF)† See flicker fusion frequency.

critical fusion frequency (CFF)† See flicker fusion frequency.

cross lighting illumination from two sources on opposite sides of the subject. Often different color media are used in the luminaires for the same area to give the illusion of shadow while providing sufficient illumination for good visibility.

cucoloris an opaque cutout panel mounted between a light source (sun or arc) and a target surface in order to project a shadow pattern (clouds or leaves are typical) upon scenery, cyclorama, or acting area.

cutoff angle (of a luminaire) the angle, measured up from nadir, between the vertical axis and the first line of sight at which the bare source is not visible.

D

dark adaptation the process by which the retina becomes adapted to a luminance less than about 0.034 cd/m2 = 2.2 × 10−5 cd/in.2 = 0.01 fL.

daylight availability the luminous flux from sun plus sky at a specific location, time, date, and sky condition.

daylight factor a measure of daylight illuminance at a point on a given plane, expressed as the ratio of the illuminance on the given plane at that point to the simultaneous exterior illuminance on a horizontal plane from the whole of an unobstructed sky of assumed or known luminance distribution. Direct sunlight is excluded from both interior and exterior values of illuminance.

daylight lamp a lamp producing a spectral distribution approximating that of a specified daylight.

densitometer a photometer for measuring the optical density (common logarithm of the reciprocal of the transmittance or reflectance) of materials.

dichroic filter† a filter that transmits certain wavelengths and reflects those not transmitted; the absorption is small.

diffuse reflectance the ratio of the flux leaving a surface or medium by diffuse reflection to the incident flux.
Note Provision for the exclusion of regularly reflected flux, which is nearly always present, must be clearly described.

diffuse reflection that process by which incident flux is redirected over a range of angles.

diffuse transmission that process by which the incident flux passing through a surface or medium is scattered.

diffuse transmittance the ratio of the diffusely transmitted flux leaving a surface or medium to the incident flux.
Note Provision for the exclusion of regularly transmitted flux must be clearly described.

diffused lighting lighting provided on the work plane or on an object that is not incident predominantly from any particular direction.

diffuser a device to redirect or scatter light from a source, primarily by the process of diffuse transmission.

diffusing panel a translucent material covering the lamps in a luminaire in order to reduce the brightness by distributing the flux over an extended area.

diffusing surfaces and media those surfaces and media that redistribute at least some of the incident flux by scattering. See complete diffusion, incomplete diffusion, narrow-angle diffusion, perfect diffusion, and wide-angle diffusion.

digital display† See alphanumeric display.

dimmer a device used to control the intensity of light emitted by a luminaire by controlling the voltage or current available to it.

direct component that portion of the light from a luminaire that arrives at the work plane without being reflected by room surfaces. See indirect component.

direct glare glare resulting from high luminances or insufficiently shielded light sources in the field of view. It is usually associated with bright areas, such as luminaires, ceilings, and windows, that are outside the visual task or region being viewed. A direct glare source can also affect performance by distracting attention.

direct-indirect lighting a variant of general diffuse lighting in which the luminaires emit little or no light at angles near the horizontal.

direct lighting lighting involving luminaires that distribute 90 to 100% of the emitted light in the general direction of the surface to be illuminated. The term usually refers to light emitted in a downward direction.

direct ratio the ratio of the luminous flux that reaches the floor of a room cavity directly to the downward component from the luminaire.

directional-conical reflectance, ρ(θi, φi; ωr) ratio of reflected flux collected through a conical solid angle to essentially collimated incident flux.
Note The direction of incidence must be specified, and the direction and extent of the cone must be specified.

directional-conical transmittance, τ(θi, φi; ωt) ratio of transmitted flux collected through a conical solid angle to essentially collimated incident flux.
Note The direction of incidence must be specified, and the direction and extent of the cone must be specified.

directional-hemispherical reflectance, ρ(θi, φi; 2π) ratio of reflected flux collected over the entire hemisphere to essentially collimated incident flux.
Note The direction of incidence must be specified.

directional-hemispherical transmittance, τ(θi, φi; 2π) ratio of transmitted flux collected over the entire hemisphere to essentially collimated incident flux.
Note The direction of incidence must be specified.

directional lighting lighting provided on the workplane
or on an object. Light that is predominantly from a preferred direction. See accent lighting, key light, and cross light.

disability glare the effect of stray light in the eye whereby visibility and visual performance are reduced. A direct glare source that produces discomfort can also produce disability glare by introducing a measurable amount of stray light in the eye.

disability glare factor (DGF) a measure of the visibility of a task in a given lighting installation in comparison with its visibility under reference lighting conditions, expressed in terms of the ratio of luminance contrasts having an equivalent effect upon task visibility. The value of the DGF takes account of the equivalent veiling luminance produced in the eye by the pattern of luminances in the task surround.

discomfort glare† glare that produces discomfort. It does not necessarily interfere with visual performance or visibility.

discomfort glare factor the numerical assessment of the capacity of a single source of brightness, such as a luminaire, in a given visual environment for producing discomfort (this term is obsolete and is retained only for reference and literature searches). See glare and discomfort glare.

discomfort glare rating (DGR) a numerical assessment of the capacity of a number of sources of luminance, such as a luminaire, in a given visual environment for producing discomfort. See discomfort glare factor. See also Chapter 9, Lighting Calculations.

distal stimuli in the physical space in front of the eye one can identify points, lines and surfaces, and three-dimensional arrays of scattering particles that constitute the distal physical stimuli that form optical images on the retina. Each element of a surface or volume to which an eye is exposed subtends a solid angle at the entrance pupil. Such elements of solid angle make up the field of view, and each has a specifiable luminance and chromaticity. Points and lines are specific cases that have to be dealt with in terms of total intensity and intensity per unit length. Distal stimuli are sometimes referred to simply as lights or colors.

distribution temperature (of a light source) the absolute temperature of a blackbody whose relative spectral distribution is most nearly the same in the visible region of the spectrum as that of the light source.

dominant wavelength (of a light), λd the wavelength of radiant energy of a single frequency that, when combined in suitable proportion with the radiant energy of a reference standard, matches the color of the light. See complementary wavelength.

downlight a small direct lighting unit that directs the light downward and can be recessed, surface-mounted, or suspended.

downward component that portion of the luminous flux from a luminaire that is emitted at angles below the horizontal. See upward component.

driving beam† See upper (driving) beams.

dual headlighting system two double headlighting units, one mounted on each side of the front end of a vehicle. Each unit consists of two sealed-beam lamps mounted in a single housing. The upper or outer lamps have two filaments that supply the low beam and part of the high beam, respectively. The lower or inner lamps have one filament that provides the primary source of light for the high beam.

dust-proof luminaire a luminaire so constructed or protected that dust does not interfere with its successful operation.

dust-tight luminaire a luminaire so constructed that dust does not enter the enclosing case.

E

effective ceiling cavity reflectance, ρCC a number giving the combined reflectance effect of the wall and ceiling reflectance of the ceiling cavity. See ceiling cavity ratio.

effective floor cavity reflectance, ρFC a number giving the combined reflectance effect of the wall and floor reflectance of the floor cavity. See floor cavity ratio.

effective intensity, Ie The effective intensity (Ie) of a flashing light is, conventionally,

where the source has constant magnitude I0 over a time duration t. For a time-varying source I(t):
expressed in candelas. The times t1 and t2 are so chosen as to maximize the calculated effective intensity. It is then found that the actual intensity It at time t1 or t2 is equal to the calculated equivalent intensity Ie.

efficacy† See luminous efficacy of a source of light and spectral luminous efficacy of radiant flux.

efficiency† See luminaire efficiency, luminous efficacy of a source of light, and spectral luminous efficiency of radiant flux.

egress† See means of egress.

egress lighting† emergency lighting for egress.

electric discharge† See arc discharge, gaseous discharge, and glow discharge.

electric-discharge lamp a lamp in which light (or radiant energy near the visible spectrum) is produced by the passage of an electric current through a vapor or gas. See carbon arc lamp, cold-cathode lamp, fluorescent lamp, glow lamp, high intensity discharge (HID) lamp, and hot cathode lamp.
Note Electric-discharge lamps can be named after the filling gas or vapor that is responsible for the major portion of the radiation, for example, mercury lamps, sodium lamps, neon lamps, and argon lamps. A second method of designating the electric-discharge lamps is by physical dimensions of operating parameters, for example, short-arc lamps, high-pressure lamps, and low-pressure lamps. A third method of designating electric discharge lamps is by their application. In addition to lamps for illumination there are photochemical lamps, bactericidal lamps, blacklight lamps, sun lamps, and others.

electroluminescence the emission of light from a phosphor excited by an electromagnetic field.

electromagnetic spectrum† a continuum of electric and magnetic radiation encompassing all wavelengths. See regions of the electromagnetic spectrum.

elevation the angle between the axis of a searchlight drum and the horizontal. Elevation is positive for angles above the horizontal, and negative below the horizontal.

ellipsoidal reflector spotlight a spotlight in which a lamp and an ellipsoidal reflector are mounted in a fixed relationship directing a beam of light into an aperture, where it can be shaped by a pattern, iris, shutter system, or other insertion. The beam then passes through a single or compound lens system that focuses it as required, producing a sharply defined beam with variable edge definition.

emergency† any condition, external or internal to the premises, that compromises the effectiveness of the lighting in an occupied area for safe movement within and out of that area and safe operation of equipment within the space. An emergency can include any or all of the following:

  • Utility power failure
  • Utility power voltage reduction (brownout) below the minimum required to support the arc in fluorescent or HID lamps
  • Power interruption within the premises including total power loss or individual phase or branch circuit failure
  • Fire or smoke.

emergency exit† a way out of the premises that is intended to be used only during an emergency.

emergency lighting lighting designed to supply illumination essential to the safety of life and property in the event of failure of the normal supply.

emissivity, ε the ratio of the radiance (for directional emissivity) or radiant exitance (for hemispherical emissivity) of an element of surface of a temperature radiator to that of a blackbody at the same temperature.

emittance, ε The ratio of radiance in a given direction (for directional emittance) or radiant exitance (for hemispherical emittance) of a sample of a thermal radiator to that of a blackbody radiator at the same temperature.

enclosed and gasketed† See vapor-tight luminaire.

equal interval (isophase) light a rhythmic light in which the light and dark periods are equal.

equipment operating factor the ratio of the lumens of a high-intensity discharge (HID) lamp-ballast-luminaire combination in a given operating position to the lumens of the lamp-ballast-luminaire combination (a) operated in the position for rating lamp lumens, and (b) using the standard (reference) ballasting specified for rating lamp lumens.
Note If the given lamp operating position is not the same as the lamp rating position, the lumens ratio for the operating ballast to standard rating ballast is determined in the given operating position. This ratio is multiplied by the lamp position factor to obtain the equipment operating factor.

equivalent contrast, ˜Ca numerical description of the relative visibility of a task. It is the contrast of the standard visibility reference task giving the same visibility as that of a task whose contrast has been reduced to threshold when the background luminances are the same. See visual task evaluator.

equivalent contrast, ˜Ce the actual equivalent contrast in a real luminous environment with nondiffuse illumination. This actual equivalent contrast ˜Ce is less than the equivalent contrast due to veiling reflection. ˜Ce = ˜C × CRF. See contrast rendition factor.

equivalent luminous intensity of an extended source at a specified distance the intensity of a point source that would produce the same illuminance at that distance. Formerly, apparent luminous intensity of an extended source.

equivalent sphere illumination (ESI) the level of sphere illumination that would produce task visibility equivalent to that produced by a specific lighting environment.

equivalent veiling luminance the luminance of the reflected image of a bright surface that is superimposed on a test object to measure the veiling effect equivalent to that produced by stray light in the eye produced by a disability glare source. The disability glare source is turned off when the reflected image is turned on.

erythema a temporary reddening of the skin such as produced by exposure to actinic UV radiation. UV-induced erythema is due to actinic action and is a delayed effect occurring several hours after exposure. This differs from IR-induced erythema, a thermal effect occurring only for the duration of time that the skin temperature is elevated.
Note The degree of erythema is used as a guide to dosages applied in UV therapy.

erythemal effectiveness the capacity of various portions of the ultraviolet spectrum to produce erythema.

erythemal efficiency of radiant flux (for a particular wavelength) the ratio of the erythemal effectiveness of a particular wavelength to that of wavelength 296.7 nm, which is rated as unity.
Note This quantity formerly was called relative erythemal factor.

erythemal exposure the product of erythemal flux density on a surface and time. It usually is measured in μW × min/cm2.
Note For average untanned skin a minimum perceptible erythema requires about 300 μW × min/cm2 of radiation at 296.7 nm.

erythemal flux radiant flux evaluated according to its capacity to produce erythema of the untanned human skin. It usually is measured in microwatts of UV radiation weighted in accordance with its erythemal efficiency. Such quantities of erythemal flux are said to be in erythemal microwatts. See erythemal efficiency of radiant flux and erythemal unit.
Note A commonly used practical unit of erythemal flux is the erythemal unit (EU) or E-viton (erytheme), which is equal to the amount of radiant flux that produces the same erythemal effect as 10 μW of radiant flux at wavelength 296.7 nm.

erythemal flux density the erythemal flux per unit area of the surface being irradiated. It is equal to the quotient of the incident erythemal flux divided by the area of the surface when the flux is uniformly distributed. It usually is measured in μW/cm2 of erythemally weighted UV radiation (erythemal μW/cm2). See finsen.
Note A suggested practical unit of erythemal flux density is the finsen, which is equal to one E-viton per square centimeter.

erythemal threshold† See minimal perceptible erythema.

erythemal unit (EU)† a unit of erythemal flux that is equal to the amount of radiant flux that produces the same erythemal effect as 10 μW of radiant flux at wavelength 296.7 nm. Also called E-viton.

E-viton (erytheme)† See erythemal unit.

exit† the portion of a means of egress that segregates all other spaces in the building or structure by fire-resistant construction in order to provide a protected way of travel to the exit discharge. Exits include exterior exit doors, exit passageways, horizontal exits, and separated exit stairs or ramps.

exit access† the portion of a means of egress that leads to an exit.

exit discharge† the portion of a means of egress between the conclusion of an exit and a public way.

exit sign† a graphic device including words or symbols that indicates or identifies an escape route or the location of, or direction to, an exit or emergency exit.

exitance† See luminous exitance and radiant exitance.

exitance coefficient, EC the ratio of the average (time zero) wall or ceiling cavity exitance to the quotient of the total lamp flux divided by the floor area.
Note (i) Exitance is measured in lumens per unit area, where the units of area agree with those of the floor area. (ii) Average wall or ceiling cavity luminances can be determined by noting the underlying assumption of lambertian room surfaces where L = M/π; L is in candela per unit area, where the units of area agree with those of M. (iii) Exitance coefficients and former luminance coefficients are numerically identical.

excitation purity of a light, pe the ratio of the distance on the CIE chromaticity diagram between the reference-point and the light-point to the distance in the same direction between the reference-point and the spectrum locus or the purple boundary. See colorimetric purity.

explosion-proof luminaire a luminaire that is completely enclosed and capable of withstanding an explosion of a specific gas or vapor that can occur within it, and preventing the ignition of a specific gas or vapor surrounding the enclosure by sparks, flashes, or explosion of the gas or vapor within. It must operate at such an external temperature that a surrounding flammable atmosphere is not ignited thereby.

externally illuminated exit sign† an exit sign with an externally mounted light source. The exit legend and background are typically opaque and rely on reflected light for visibility.

eye light illumination on a person to produce a specular reflection from items such as eyes, teeth, and jewelry without significantly increasing the total illumination of the subject.

F

far (long-wavelength) infrared† the region of the electromagnetic spectrum extending from 5000 to 1,000,000 nm.

far ultraviolet† the region of the electromagnetic spectrum extending from 100 to 200 nm.

fay light a luminaire that uses incandescent parabolic reflector lamps with a dichroic coating to provide “daylight” illumination.

fenestra method a procedure for predicting the interior illuminance received from daylight through windows.

fenestration any opening or arrangement of openings (normally filled with media for control) for the admission of daylight.

field angle the angle between the two directions for which the intensity is 10% of the maximum intensity as measured in a plane through the nominal beam centerline. For beams that do not possess rotational symmetry, the beam angle is generally given for two planes at 90°, typically the maximum and minimum angles. Note that in certain fields of applications the angle of the 10% of maximum directions was formerly called beam angle.

fill light illumination added to reduce shadows or contrast range.

film (or aperture) color† the perceived color of the sky or a patch of color seen through an aperture.

filter a device for changing, by transmission or reflection, the magnitude or spectral composition of the flux incident upon it. Filters are called selective (or colored) or neutral, according to whether or not they alter the spectral distribution of the incident flux. Alternatively, a component of an electronic dimmer used to control electromagnetic or radio-frequency interference.

filter factor the transmittance of black light by a filter.
Note The relationship between glow factor and filter factor is illustrated by the following formula for determining the luminance (L) of fluorescent materials exposed to black light:

where
E = [fluorens × m−2]
fg = glow factor
ff = filter factor.
When integral-filter black-light lamps are used, the filter factor is dropped from the formula because it already has been applied in assigning fluoren ratings to these lamps.

finsen† a suggested practical unit of erythemal flux density equal to one E-viton per square centimeter.

fixed light a light having a constant luminous intensity when observed from a fixed point.

fixture† See luminaire.

flashing light a rhythmic light in which the periods of light are of equal duration and are clearly shorter than the periods of darkness. See group flashing light, interrupted quick-flashing light, and quick-flashing light.

flashtube a tube of glass or fused quartz with electrodes at the ends and filled with a gas, usually xenon. It is designed to produce high-intensity light flashes of extremely short duration.

flattery index (of a light source), Rf a measure appraising a light source for appreciative viewing of colored objects, for promoting an optimistic viewpoint by flattery (making the view more pleasant), or for enhancing the perception of objects in terms of color. Also sometimes called color preference index (CPI).

flicker fusion frequency (FFF) the frequency of intermittent stimulation of the eye at which flicker disappears. It also is called the critical fusion frequency (CFF) or critical flicker frequency (CFF).

flicker index a measure of the cyclic variation in output of a light source, taking into account the waveform of the light output. It is the ratio of the area under the light output curve that is above the average light output level to the total area under the light output curve for a single cycle. See Chapter 6, Light Sources.

flicker photometer† See visual photometer.

floodlight a projector designed for lighting a scene or object to a luminance considerably greater than its surroundings. It usually is capable of being pointed in any direction and is of weatherproof construction.
Note The beam spread of floodlights can range from narrow field angles (10°) to wide ones (more than 100°). See beam angle, field angle, heavy-duty floodlight, general-purpose (GP) floodlight, ground-area open floodlight, and ground-area open floodlight with reflector insert.

floodlighting a system designed for lighting a scene or object to a luminance greater than its surroundings. It can be for utility, advertising, or decorative purposes.

floor cavity the cavity formed by the workplane, the floor, and the wall surfaces between those two planes.

floor cavity ratio (FCR) a number computed by using
the distance from the floor to the workplane (hf) as Heightcavity in the equations given in cavity ratio. See Chapter 9, Lighting Calculations.

floor lamp a portable luminaire on a high stand suitable for standing on the floor. See torchère.

fluoren† a unit of black-light flux equal to one milliwatt of radiant flux in the wavelength range 320 to 400 nm.

fluorescence the emission of light as the result of, and only during, the absorption of radiation of shorter wavelengths (time scale less than approximately 10−8 s).

fluorescent lamp a low-pressure mercury electric-discharge lamp in which a fluorescing coating (phosphor) transforms some of the UV energy generated by the discharge into light. See instant-start fluorescent lamp, preheat (switch-start) fluorescent lamp, and rapid-start fluorescent lamp.

flush-mounted or recessed luminaire a luminaire that is mounted above the ceiling (or behind a wall or other surface) with the opening of the luminaire level with the surface.

flux transfer theory a method of calculating the illuminance in a room by taking into account the interreflection of the light flux from the room surfaces based on the average flux transfer between surfaces.

fog (adverse-weather) lamps units that can be used in lieu of headlamps or in connection with the lower-beam headlights to provide road illumination under conditions of rain, snow, dust, or fog.

follow spot (light) any instrument operated so as to follow the movement of an actor. Follow spots are usually high-intensity, controlled-beam luminaires.

footcandle, fc a unit of illuminance equal to 1 lm/ft2 or 10.76 lx.

footcandle meter† See illuminance (lux or footcandle) meter.

footlambert, fL a lambertian unit of luminance equal to 1/π candela per square foot. This term is obsolete, and its use is deprecated.

footlights a set of striplights at the front edge of the stage platform used to soften face shadows cast by overhead
luminaires and to add general toning lighting from
below.

form factor, f12 the ratio of the flux directly received by surface 2 (and due to lambertian surface 1) to the total flux emitted by surface 1. It is used in flux transfer theory.
Also, the ratio of the average illuminance on surface 1 to the causative exitance of lambertian surface 2:
Note In the literature, this quantity is also called the angle factor, configuration factor, geometrical factor, I-factor, illumination factor, and shape modulus.

formation light a navigation light especially provided to facilitate formation flying.

fovea a small region at the center of the retina, subtending about 2°, that contains cones but no rods and that forms the site of most distinct vision.

foveal vision† See central (foveal) vision.

Fresnel spotlight a luminaire containing a lamp and a Fresnel lens (stepped flat lens with a textured back) that has variable field and beam angles obtained by changing the spacing between lamp and lens (flooding and spotting). Produces a smooth, soft-edged, defined beam of light.

fuselage lights aircraft aeronautical lights, mounted on the top and bottom of the fuselage, used to supplement the navigation lights.

G

gas-filled lamp an incandescent lamp in which the filament operates in a bulb filled with one or more inert gases.

gaseous discharge the emission of light from gas atoms excited by an electric current.

general color rendering index, Ra Measure of the average shift of eight standardized colors chosen to be of intermediate saturation and spread throughout the range of hues. If the color rendering index is not qualified as to the color samples used, Ra is assumed.

general diffuse lighting lighting involving luminaires that distribute 40 to 60% of the emitted light downward and the balance upward, sometimes with a strong component at 90° (horizontal). See direct-indirect lighting.

general lighting lighting designed to provide a substantially uniform level of illuminance throughout an area, exclusive of any provision for special local requirements. See ceiling area lighting, direct lighting, direct-indirect lighting, general diffuse lighting, indirect lighting, localized general lighting, semidirect lighting, and semi-indirect lighting.

general-purpose floodlight (GP) a weatherproof unit so constructed that the housing forms the reflecting surface. The assembly is enclosed by a cover glass.

germicidal effectiveness† See bactericidal (germicidal) effectiveness.

germicidal efficiency of radiant flux† See bactericidal (germicidal) efficiency of radiant flux.

germicidal exposure† See bactericidal (germicidal) exposure.

germicidal flux and flux density† See bactericidal (germicidal) flux and bactericidal (germicidal) flux density.

germicidal lamp a low-pressure mercury lamp in which the envelope has high transmittance for 254-nm radiation. See bactericidal lamp.

glare the sensation produced by luminances within the visual field that are sufficiently greater than the luminance to which the eyes are adapted, which causes annoyance, discomfort, or loss in visual performance and visibility. See blinding glare, direct glare, disability glare, and discomfort glare.
Note The magnitude of the sensation of glare depends on such factors as the size, position, and luminance of a source; the number of sources; and the luminance to which the eyes are adapted.

globe a transparent or diffusing enclosure intended to protect a lamp, to diffuse and redirect its light, or to change the color of the light.

glossometer an instrument for measuring gloss as a function of the directionally selective reflecting properties of a material in angles near to and including the direction giving specular reflection.

glow discharge an electric discharge characterized by a low, approximately constant current density at the cathode (on the order of 10 μA/mm2) at low cathode temperature and a high voltage drop (typically 50 V or more). Secondary emission from the cathode is much greater than the thermionic emission.
Note A distinction is made between the normal cathode drop (potential difference due to space charge near the cathode) that occurs when the glow does not cover the cathode completely (with constant current density) and that is independent of the discharge current, and the abnormal cathode drop that occurs when the glow covers the cathode completely (with increased current density) and that depends on the discharge current.

glow factor a measure of the visible light response of a fluorescent material to black light. It is equal to π times the luminance in cd/m2 produced on the material divided by the incident black-light flux density in mW/m2. It can be measured in lm/mW.

glow lamp an electric-discharge lamp whose mode of operation is that of a glow discharge and in which light is generated in the space close to the electrodes.

goniophotometer a photometer for measuring the directional light distribution characteristics of sources, luminaires, media, and surfaces.

graybody a temperature radiator whose spectral emissivity is less than unity and the same at all wavelengths.

ground-area open floodlight (O) a unit providing a weatherproof enclosure for the lamp socket and housing. No cover glass is required.

ground-area open floodlight with reflector insert (OI) a weatherproof unit so constructed that the housing forms only part of the reflecting surface. An auxiliary reflector is used to modify the distribution of light. No cover glass is required.

ground light visible radiation from the sun and sky reflected by surfaces below the plane of the horizon.

group flashing light a flashing light in which the flashes are combined in groups, each including the same number of flashes, and in which the groups are repeated at regular intervals. The duration of each flash is clearly less than the duration of the dark periods between flashes, and the duration of the dark periods between flashes is clearly less than the duration of the dark periods between groups.

H

hard light light that causes an object to cast a sharply defined shadow.

hazard or obstruction beacon an aeronautical beacon used to designate a danger to air navigation.

hazardous location an area where ignitable vapors or dust can cause a fire or explosion created by energy emitted from lighting or other electrical equipment or by electrostatic generation.

headlamp a major lighting device mounted on a vehicle and used to provide illumination ahead of it. Also called a headlight. See multiple-beam headlamp and sealed-beam headlamp.

headlight† an alternative term for headlamp.

heat extraction thermal factor the fractional lumen loss or gain due to passage of room air being returned to the plenum through the lamp compartment of the luminaire.

heavy-duty floodlight (HD) a weatherproof unit having a substantially constructed metal housing into which is placed a separate and removable reflector. A weatherproof hinged door with cover glass encloses the assembly but provides an unobstructed light opening at least equal to the effective diameter of the reflector.

hemispherical-conical reflectance, ρ(2π; ωr) the ratio of reflected flux collected over a conical solid angle to the incident flux from the entire hemisphere.

Note The direction and extent of the cone must be specified.

hemispherical-conical transmittance
, τ(2π; ωt) the ratio of transmitted flux collected over a conical solid angle to the incident flux from the entire hemisphere.
Note The direction and extent of the cone must be specified.

hemispherical-directional reflectance, ρ(2π; θr, φr) the ratio of reflected flux collected over an element of solid angle surrounding the given direction to the incident flux from the entire hemisphere.
Note The direction of collection and the size of the solid angle element must be specified.

hemispherical-directional transmittance
, τ(2π; θt, φt) the ratio of transmitted flux collected over an element of solid angle surrounding the given direction to the incident flux from the entire hemisphere.
Note The direction of collection and size of the solid angle element must be specified.

hemispherical reflectance
the ratio of all of the flux leaving a surface or medium by reflection to the incident flux. The use of this term is deprecated. See hemispherical transmittance.

hemispherical transmittance the ratio of the transmitted flux leaving a surface or medium to the incident flux. The use of this term is deprecated.

high-bay lighting interior lighting where the roof trusses or ceiling height is greater than approximately 7.6 m (25 ft) above the floor.

high-intensity discharge (HID) lamp an electric-discharge lamp in which the light-producing arc is stabilized by bulb wall temperature, and the arc tube has a bulb wall loading in excess of 3 W/cm2. HID lamps include groups of lamps known as mercury, metal halide, and high-pressure sodium.

high-key lighting a type of lighting that, applied to a scene, results in a picture having gradations from middle gray to white with comparatively limited areas of dark gray and black. Also, intense, overall illumination. In motion pictures, high-level accent lighting with strong contrast (dark deep shadows with little or no middle gray). See low-key lighting.

high-mast lighting illumination of a large area by means of a group of luminaires that are designed to be mounted in fixed orientation at the top of a high mast, generally 20 m (65 ft) or higher.

high-pressure sodium (HPS) lamp a high-intensity discharge (HID) lamp in which light is produced by radiation from sodium vapor operating at a partial pressure of about 1.33 × 104 Pa (100 Torr). Includes clear and diffuse-coated lamps.

horizontal exit† an escape route from one building to an area of refuge in another building on approximately the same level. It also is an escape route through or around a fire barrier to an area of refuge on approximately the same level in the same building.

horizontal plane of a searchlight the plane that is perpendicular to the vertical plane through the axis of the searchlight drum and in which the train lies.

hot-cathode lamp an electric-discharge lamp whose mode of operation is that of an arc discharge. The cathodes can be heated by the discharge or by external means.

house lights the general lighting system installed in the audience area (house) of a theatre, film or television studio, or arena.

hue of a perceived color the attribute that determines whether it is red, yellow, green, blue, or the like.

hue of a perceived light-source color† the attribute that determines whether the color is red, yellow, green, blue, or the like. See hue of a perceived color.

hydrargyrum medium-arc-length iodide (HMI) lamp an arc light source utilizing mercury vapor and metal halide additives to produce illumination in the 5000 to 6000 K range. Requires a ballast and ignitor system for operation.

I

ice detection light an inspection light designed to illuminate the leading edge of the aircraft wing to check for ice formation.

ideal radiator† See blackbody.

identification beacon an aeronautical beacon emitting a coded signal by means of which a particular point of reference can be identified.

ignitor a device, either by itself or in association with other components, that generates voltage pulses to start discharge lamps without preheating of electrodes.

illuminance, E = dΦ/dA the areal density of the luminous flux incident at a point on a surface

illuminance (footcandle or lux) meter an instrument for measuring illuminance on a plane. Instruments that accurately respond to more than one spectral distribution are color-corrected, that is, the spectral response is balanced to V(λ) or V’(λ). Instruments that accurately respond to more than one spatial distribution of incident flux are cosine-corrected, that is, the response to a source of unit luminous intensity, illuminating the detector from a fixed distance and from different directions, decreases as the cosine of the angle between the incident direction and the normal to the detector surface. The instrument is comprised of some form of photodetector with or without a filter driving a digital or analog readout through appropriate circuitry.

illumination an alternative but deprecated term for illuminance. It is frequently used because “illuminance” is subject to confusion with “luminance” and “illuminants,” especially when not clearly pronounced.
Note The term illumination also is commonly used in a qualitative or general sense to designate the act of illuminating or the state of being illuminated. Usually, the context indicates which meaning is intended, but occasionally it is desirable to use the expression “level of illumination” to indicate that the quantitative meaning is intended.

incandescence the self-emission of radiant energy in the visible spectrum due to the thermal excitation of atoms or molecules.

incandescent filament lamp a lamp in which light is produced by a filament heated to incandescence by an electric current.
Note Normally, the filament is of coiled or coiled-coil (doubly coiled) tungsten wire. However, it can be uncoiled wire, a flat strip, or of material other than tungsten.

incomplete diffusion (partial diffusion) that in which the diffusing medium partially redirects the incident flux by scattering while the remaining fraction of incident flux is redirected without scattering; that is, a fraction of the incident flux can remain in an image-forming state.

index of sensation (M) (of a source) a number that expresses the effects of source luminance (Ls), solid angle factor (Q), position index (P), and the field luminance (F) on discomfort glare rating. See Chapter 9, Lighting Calculations. See discomfort glare rating (DGR), where Ls and F are expressed in cd/m2.
For an equation defining Q see solid angle factor.
Note A restatement of this formula lends itself more directly to computer applications.

indirect component the portion of the luminous flux from a luminaire that arrives at the workplane after being reflected by room surfaces. See direct component.

indirect lighting lighting involving luminaires that distribute 90 to 100% of the emitted light upward.

infrared lamp a lamp that radiates predominately in the infrared; the visible radiation is not of principal interest.

infrared (IR) radiation for practical purposes any radiant energy within the wavelength range of 770 to 106 nm is considered infrared energy.

inhibition (visual) reduction in magnitude of the sensation aroused by the stimulus (or a reduction in visual sensitivity) caused by some other stimulation that is adjacent spatially or temporally.

initial luminous exitance This term can be used in two different ways. In flux transfer it is the density of luminous flux leaving a surface within an enclosure before interreflections occur. In lighting calculations it is the total exitance at time zero before depreciation (light losses) occur.
Note For light sources this is the luminous exitance as defined in luminous exitance. For nonself-luminous surfaces it is the reflected luminous exitance of the flux received directly from sources within the enclosure or from daylight.

instant-start fluorescent lamp a fluorescent lamp designed for starting by a high voltage without preheating of the electrodes.
Note In the UK, a cold-start lamp.

integrating photometer a photometer that enables geometrically total luminous flux to be determined by a single measurement. The usual type is the Ulbricht sphere with associated photometric equipment for measuring the indirect illuminance of the inner surface of the sphere. (The measuring device is shielded from the source under measurement.)

intensity† a shortening of the terms luminous intensity and radiant intensity. Often misused for level of illumination or illuminance.

intensity (candlepower) distribution curve a curve, often polar, that represents the variation of luminous intensity of a lamp or luminaire in a plane through the light center.
Note A vertical intensity distribution curve is obtained by taking measurements at various angles of elevation about a source in a vertical plane through the light center; unless the plane is specified, the vertical curve is assumed to represent an average such as would be obtained by rotating the lamp or luminaire about its vertical axis. A horizontal intensity distribution curve represents measurements made at various angles of azimuth in a horizontal plane through the light center.

internally illuminated exit sign† a transilluminated exit sign containing its own light source.

interreflected component That portion of the luminous flux from a luminaire that arrives at the workplane after being reflected one or more times from room surfaces, as determined by the flux transfer theory. Also called interflectance. See flux transfer theory.

interreflection The multiple reflection of light by the various room surfaces before it reaches the work plane or other specified surface of a room. Also called interflectance.

interrupted quick-flashing light a quick-flashing light in which the rapid alternations are interrupted by periods of darkness at regular intervals.

inverse square law A law stating that the illuminance E at a point on a surface varies directly with the intensity I of a point source and inversely as the square of the distance d between the source and the point. If the surface at the point is normal to the direction of the incident light, the law is expressed by E = I/d2.
Note For sources of finite size having uniform luminance, this gives results that are accurate within 1% when d is at least 5 times the maximum dimension of the source as viewed from the point on the surface. Even though practical interior luminaires do not have uniform luminance, this distance d is frequently used as the minimum for photometry of such luminaires when the magnitude of the measurement error is not critical.

iris an assembly of flat metal leaves arranged to provide an easily adjustable near-circular opening, placed near the focal point of the beam (as in an ellipsoidal reflector spotlight) or in front of the lens to act as a mechanical dimmer as in older types of carbon arc follow spotlights.

irradiance, E † the density of radiant flux (power) incident on a surface.

isocandela line a line plotted on any appropriate set of coordinates to show directions in space, about a source of light, in which the intensity is the same. A series of such curves, often for equal increments of intensity, is called an isocandela diagram.

isolux (isofootcandle) line a line plotted on any appropriate set of coordinates to show all the points on a surface where the illuminance is the same. A series of such lines for various illuminance values is called an isolux (isofootcandle) diagram.

K

Kelvin† the unit of temperature used to designate the color temperature of a light source. A temperature scale where each degree is the same size as a centigrade degree, but the Kelvin scale has its zero at 273°C.

key light the apparent principal source of directional illumination falling upon a subject or area.

kicker a luminaire used to provide an additional highlight or accent on a subject.

klieg light a high-intensity carbon arc spotlight, typically used in motion picture lighting.

L

laboratory reference standard† the highest-ranking order of standards at each laboratory.

Lambert a lambertian unit of luminance equal to 1/π candela per square centimeter. This term is obsolete, and its use is deprecated.

lambertian surface a surface that emits or reflects light in accordance with Lambert’s cosine law. A lambertian surface has the same luminance regardless of viewing angle.

Lambert’s cosine law, Iθ = I0 cos θ the law stating that the luminous intensity in any direction from an element of a perfectly diffusing surface varies as the cosine of the angle between that direction and the perpendicular to the surface element.

lamp a generic term for a source created to produce optical radiation. By extension, the term is also used to denote sources that radiate in regions of the spectrum adjacent to the visible.
Note Through popular usage, a portable luminaire consisting of a lamp with shade, reflector, enclosing globe, housing, or other accessories is also sometimes called a lamp. In such cases, in order to distinguish between the assembled unit and the light source within it, the latter is often called a bulb or tube, if it is electrically powered. See also luminaire.

lamp burnout factor
the fractional loss of task illuminance due to burned-out lamps left in place for long periods.

lamp lumen depreciation (LLD) factor the fractional loss of lamp lumens at rated operating conditions that progressively occurs during lamp operation.

lamp position factor The ratio of the luminous flux of a lamp at a given operating position to the luminous flux when the lamp is operated in the position at which the lamp lumens are rated.

lamp post a standard support provided with the necessary internal attachments for wiring and the external attachments for the bracket and luminaire.

lamp shielding angle, φ the angle between the plane of the baffles or louver grid and the plane most nearly horizontal that is tangent to both the lamps and the louver blades.
Notes (i) The lamp shielding angle is formed by a sight line tangent to the lowest part of the brightness area to be shielded. H is the vertical distance from the brightness source to the bottom of the shielding element. D is the horizontal distance from the brightness source to the shielding element. Lamp shielding angle φ = arctan (H/D). (ii) The lamp shielding angle frequently is larger than the louver shielding angle, but never smaller. See louver shielding angle.

landing direction indicator a device to indicate visually the direction currently designated for landing and takeoff.

landing light an aircraft aeronautical light designed to illuminate a ground area from the aircraft.

landmark beacon an aeronautical beacon used to indicate the location of a landmark used by pilots as an aid to en route navigation.

laser an acronym for light amplification by stimulated emission of radiation. The laser produces a highly monochromatic and coherent (spatially and temporally) beam of radiation. A steady oscillation of nearly a single electromagnetic mode is maintained in a volume of an active material bounded by highly reflecting surfaces, called a resonator. The frequency of oscillation varies according to the material used and the methods of initially exciting or pumping the material.

lateral width of a light distribution in roadway lighting, the lateral angle between the reference line and the width line, measured in the cone of maximum candlepower. This angular width includes the line of maximum candlepower. See reference line and width line.

lens† a glass or plastic element used in luminaires to change the direction and control the distribution of light rays; also, the part of the eye that allows objects at different distances to be focused onto the retina.

level of illumination† See illuminance.

life performance curve a curve that presents the variation of a particular characteristic of a light source (such as luminous flux, intensity, etc.) throughout the life of the source.
Note Life performance curves sometimes are called maintenance curves, for example, lumen maintenance curves.

life test of lamps a test in which lamps are operated under specified conditions for a specified length of time for the purpose of obtaining information on lamp life. Measurements of photometric and electrical characteristics can be made at specified intervals of time during this test.

light radiant energy that is capable of exciting the retina and producing a visual sensation. The visible portion of the electromagnetic spectrum extends from about 380 to 770 nm.
Note The subjective impression produced by stimulating the retina is sometimes designated as light. Visual sensations are sometimes arbitrarily defined as sensations of light, and in line with this concept, it is sometimes said that light cannot exist until an eye has been stimulated. Electrical stimulation of the retina or the visual cortex is described as producing flashes of light. In illuminating engineering, however, light is a physical entity–radiant energy weighted by the luminous efficiency function. It is a physical stimulus that can be applied to the retina. See spectral luminous efficacy of radiant flux and values of spectral luminous efficiency for photopic vision.

light adaptation the process by which the retina becomes adapted to a luminance greater than about 3.4 cd/m2. See also dark adaptation.

light center (of a lamp) the center of the smallest sphere that would completely contain the light-emitting element of the lamp.

light center length the distance from the light center to a specified reference point on the lamp.

light-emitting diode (LED) a p-n junction solid-state diode whose radiated output is a function of its physical construction, material used, and exciting current. The output can be in the IR or in the visible region.

light loss factor (LLF) Formerly called maintenance factor. The ratio of illuminance (or exitance or luminance) for a given area to the value that would occur if lamps operated at their (initial) rated lumens and if no system variation or depreciation had occurred. Components of this factor can be either initial or maintained.
Note The light loss factor is used in lighting calculations as an allowance for lamp(s) or luminaire(s) operating at other than rated conditions (initial) and for the depreciation of lamps, light control elements, and room surfaces to values below the initial or design conditions, so that a minimum desired level of illuminance can be maintained in service. The light loss factor had formerly been widely interpreted as the ratio of average illuminance in service to initial illuminance.

light meter A common name for an illuminance meter. See illuminance (lux or footcandle) meter.

light source color the color of the light emitted by a source.
Note The color of a point source can be defined by its luminous intensity and chromaticity coordinates; the color of an extended source can be defined by its luminance and chromaticity coordinates. See color temperature, correlated color temperature, and perceived light source color.

lighting effectiveness factor (LEF) the ratio of equivalent sphere illumination to measured or calculated task illuminance.

lightness (of a perceived patch of surface color) the brightness of an area judged relative to the brightness of a similarly illuminated area that appears to be white or highly transmitting.

linear light a luminous signal having a perceptible physical length.

linear polarization the process by which the transverse vibrations of light waves are oriented in or parallel to a specific plane. Polarization can be obtained by using either transmitting or reflecting media.

Linnebach projector a lensless scenic projector using a concentrated source in a black box and a slide or cutout between the source and the projection surface.

liquid crystal display (LCD) a display made of material whose reflectance or transmittance changes when an electric field is applied.

local lighting lighting providing illuminance over a relatively small area or confined space without providing any significant general surrounding lighting.

localized general lighting lighting utilizing luminaires above the visual task and contributing also to the illuminance of the surround.

long-arc lamp an arc lamp in which the distance between the electrodes is large.
Note This type of lamp (such as xenon) generally has an arc tube containing gas at high pressure. The arc fills the discharge tube and is therefore wall stabilized.

longitudinal roadway line (LRL) any line along a roadway that is parallel to the curb line.

louver (or louver grid) a series of baffles used to shield a source from view at certain angles, to absorb or block unwanted light, or to reflect or redirect light. The baffles are usually arranged in a geometric pattern.

louver shielding angle, θ the angle between the horizontal plane of the baffles or louver grid and the plane at which the louver conceals all objects above. See lamp shielding angle.
Note The planes usually are so chosen that their intersection is parallel with the louvered blade.

louvered ceiling a ceiling-area lighting system composed of a wall-to-wall installation of multicell louvers shielding the light sources mounted above it. See luminous ceiling.

low-bay lighting interior lighting where the roof trusses or ceiling height is approximately 7.6 m (25 ft) or less above the floor.

low-key lighting a type of lighting that, applied to a scene, results in a picture having gradations from middle gray to black with comparatively limited areas of light grays and whites. See high-key lighting.

low-pressure mercury lamp a discharge lamp (with or without a phosphor coating) in which the partial pressure of the mercury vapor does not exceed 100 Pa during operation.

low-pressure sodium (LPS) lamp a discharge lamp in which light is produced by radiation from sodium vapor operating at a partial pressure of 0.1 to 1.5 Pa (approximately 10−3 to 10−2 Torr).

lower (passing) beams one or more beams directed low enough on the left to avoid glare in the eyes of oncoming drivers and intended for use in congested areas and on highways when meeting other vehicles within a distance of 300 m (1000 ft). Formerly traffic beam.

lumen, lm SI unit of luminous flux. Radiometrically, it is determined from the radiant power as in luminous flux. Photometrically, it is the luminous flux emitted within a unit solid angle (1 sr) by a point source having a uniform luminous intensity of 1 cd.

lumen depreciation† the decrease in lumen output that occurs as a lamp is operated, until failure.

lumen (or flux) method a lighting design procedure used for predetermining the relation between the number and types of lamps or luminaires, the room characteristics, and the average illuminance on the workplane. It takes into account both direct and reflected flux.

lumen-second (lm × s) a unit of quantity of light, the SI unit of luminous energy (also called a talbot). It is the quantity of light delivered in one second by a luminous flux of 1 lumen.

luminaire (light fixture) a complete lighting unit consisting of a lamp or lamps and ballast(s) (when applicable) together with the parts designed to distribute the light, to position and protect the lamps, and to connect the lamps to the power supply.

luminaire ambient temperature factor the fractional lumen change of a fluorescent luminaire due to internal luminaire temperatures differing from the temperatures at which photometry was performed. This factor takes into consideration a variation in ambient temperature surrounding the luminaire, the means and conditions of mounting the luminaire, and the use of any insulation in conjunction with the application of the luminaire.

luminaire dirt depreciation (LDD) the fractional loss of task illuminance due to luminaire dirt accumulation.

luminaire efficiency the ratio of luminous flux (lumens) emitted by a luminaire to that emitted by the lamp or lamps used therein.

luminaire spacing criterion (SC)† a classification parameter for indoor luminaires relating to the distribution of the direct illuminance component produced on the work plane. The SC of a luminaire is an estimated maximum ratio of spacing to mounting height above the work plane for a regular array of that luminaire such that the work plane illuminance will be acceptably uniform.
Note The SC is not a recommendation for the spacing-to-mounting-height ratio for an installation. It is a characteristic that assists in identifying appropriate luminaires when illuminance uniformity is a design goal. The SC evolved but is distinctly different from an obsolete luminaire parameter called the spacing-to-mounting-height ratio. See Chapter 9, Lighting Calculations, for the SC algorithm.

luminaire surface depreciation factor the fractional loss of task illuminance due to permanent deterioration of luminaire surfaces.

luminance, L = d2φ/(dω dA cos θ) (in a direction and at a point of a real or imaginary surface) the quotient of the luminous flux at an element of the surface surrounding the point, and propagated in directions defined by an elementary cone containing the given direction, by the product of the solid angle of the cone and the area of the orthogonal projection of the element of the surface on a plane perpendicular to the given direction. The luminous flux can be leaving, passing through, and/or arriving at the surface. Formerly, photometric brightness.
By introducing the concept of luminous intensity, luminance can be expressed as L = dI/(dA cos θ). Here, luminance at a point on a surface in a direction is interpreted as the quotient of luminous intensity in the given direction, produced by an element of the surface surrounding the point, by the area of the orthogonal projection of the element of surface on a plane, perpendicular to the given direction. Luminance can be measured at a receiving surface by using

This value can be less than the luminance of the emitting surface due to the attenuation of the transmitting media.
Note In common usage the term brightness usually refers to the strength of sensation that results from viewing surfaces or spaces from which light comes to the eye. This sensation is determined in part by the definitely measurable luminance defined above and in part by conditions of observation such as the state of adaptation of the eye. In much of the literature, brightness, when used alone, refers to both luminance and sensation. The context usually indicates which meaning is intended. Previous usage not withstanding, neither the term brightness nor the term photometric brightness should be used to denote the concept of luminance.

luminance coefficient (LC) The ratio of average wall or ceiling cavity luminance to the total lamp flux (lumens) divided by the floor area. This requires the luminance in the nonstandard units of footlamberts, where floor area is in square feet. See exitance coefficient for current terminology.

luminance contrast the relationship between the luminances of an object and its immediate background. It is equal to (L1 − L2)/L1 or (L2 − L1)/L1 = |ΔL/L1|, where L1 and L2 are the luminances of the background and object, respectively. The form of the equation must be specified. The ratio ΔL/L1 is known as Weber’s fraction.
Note See note under luminance. Because of the relationship among luminance, illuminance, and reflectance, contrast often is expressed in terms of reflectance when only reflecting surfaces are involved. Thus, contrast is equal to (ρ1 − ρ2)/ρ1, or (ρ2 − ρ1)/ρ1, where ρ1 and ρ2 are the reflectances of the background and object, respectively. This method of computing contrast holds only for perfectly diffusing surfaces; for other surfaces it is only an approximation unless the angles of incidence and view are taken into consideration. See reflectance.

luminance difference the difference in luminance between two areas, such as the detail of a visual task and its immediate background, in which case it is quantitatively equal to the numerator in the formula for luminance contrast. See note under luminance.

luminance factor, β the ratio of the luminance of a surface or medium under specified conditions of incidence, observation, and light source, to the luminance of a completely reflecting or transmitting, perfectly diffusing surface or medium under the same conditions.
Note The reflectance or transmittance cannot exceed 1, but luminance factor can have any value from 0 to values approaching infinity.

luminance ratio the ratio between the luminances of any two areas in the visual field.

luminance threshold the minimum perceptible difference in luminance for a given state of adaptation of the eye.

luminescence any emission of light not ascribable directly to incandescence. See electroluminescence, fluorescence, and phosphorescence.

luminous ceiling a ceiling area lighting system comprising a continuous surface of transmitting material of a diffusing or light-controlling character with light sources mounted above it. See louvered ceiling.

luminous density, w = dQ/dV quantity of light (luminous energy) per unit volume.

luminous efficacy of radiant flux the quotient of the total luminous flux by the total radiant flux. It is expressed in lumens per watt.

luminous efficacy of a source of light the quotient of the total luminous flux emitted the total lamp power input. It is expressed in lumens per watt.
Note The term luminous efficiency has in the past been extensively used for this concept.

luminous efficiency
† See spectral luminous efficiency of radiant flux.

luminous energy
† See quantity of light.

luminous exitance
, M = dφ/dA the areal density of luminous flux leaving a surface at a point. Formerly luminous emittance (deprecated).
Note This is the total luminous flux emitted, reflected, and transmitted from the surface and is independent of direction.

luminous flux
, Φ radiant flux (radiant power); the time rate of flow of radiant energy, evaluated in terms of a standardized visual response:

where
Φv = lumens
Φe, λ = watts per nanometer
λ = nanometers
V(λ) = the spectral luminous efficiency
Km = the maximum spectral luminous efficacy in lumens per watt
Unless otherwise indicated, the luminous flux is defined for photopic vision. For scotopic vision, the corresponding spectral luminous efficiency V(λ) and the corresponding maximum spectral luminous efficacy Km are substituted in the above equation. Km and K’m are derived from the basic SI definition of luminous intensity and have the values 683 lm/W and 1754 lm/W, respectively.

luminous flux density at a surface
, dΦ/dA the luminous flux per unit area at a point on a surface.
Note This need not be a physical surface; it can also be a mathematical plane. See also illuminance and luminous exitance.

luminous intensity
, I = dΦ/dω (of a point source of light in a given direction) the luminous flux per unit solid angle in the direction in question. Hence, it is the luminous flux on a small surface centered on and normal to that direction divided by the solid angle (in steradians) that the surface subtends at the source. Luminous intensity can be expressed in candelas or in lumens per steradian (lm/sr).
Note Mathematically a solid angle must have a point as its apex; the definition of luminous intensity, therefore, applies strictly only to a point source. In practice, however, light emanating from a source whose dimensions are negligible in comparison with the distance from which it is observed can be considered as coming from a point. Specifically, this implies that with change of distance (1) the variation in solid angle subtended by the source at the receiving point approaches 1/distance2, and that (2) the average luminance of the projected source area as seen from the receiving point does not vary appreciably. For extended sources see equivalent luminous intensity of an extended source at a specified distance. The word intensity as defined above is used to designate luminous intensity (or candlepower). It is also widely used in other ways, either formally or informally, in other disciplines. Stimulus intensity can be used to designate the retinal illuminance of a proximal stimulus (see proximal stimuli) or the luminance of a distal stimulus (see distal stimuli). Intensity is used in the same sense with respect to other modulates such as audition. Intensity has been used to designate the level of illuminance on a surface or the flux density in the cross section of a beam of light. In physical optics, “intensity” usually refers to the square of the wave amplitude.
luminous intensity distribution curve† See intensity distribution curve.

luminous reflectance
any of the geometric aspects of reflectance in which both the incident and the reflected flux are weighted by the spectral luminous efficiency of radiant flux, V(λ).
Note Unless otherwise qualified, the term reflectance means luminous reflectance.

luminous transmittance
any of the geometric aspects of transmittance in which the incident and transmitted flux are weighted by the luminous efficiency of radiant flux, V(λ).
Note Unless otherwise qualified, the term transmittance means luminous transmittance.

lux, lx
the SI unit of illuminance. One lux is one lumen per square meter (lm/m2). See the Appendix for conversion values.

lux meter
† See illuminance (lux or footcandle) meter.

M

maintenance factor (MF)† a factor formerly used to denote the ratio of the illuminance on a given area after a period of time to the initial illuminance on the same area. This term is obsolete and is no longer valid. See light loss factor.

matte surface
a surface from which the reflection is predominantly diffuse, with or without a negligible specular component. See diffuse reflection.

mean horizontal intensity
(candlepower) the average intensity (in candelas) of a lamp in a plane perpendicular to the axis of the lamp that passes through the luminous center of the lamp.

mean spherical luminous intensity
the average value of the luminous intensity in all directions for a source. Also, the quotient of the total emitted luminous flux of the source by 4π.

mean zonal candlepower the average intensity (candelas) of a symmetrical luminaire or lamp at an angle to the luminaire or lamp axis that is in the middle of the zone under consideration.

means of egress
† An unobstructed and continuous way of exit from any point in a building or structure to a public way. It consists of three distinct parts: the exit access, the exit, and the exit discharge. A means of egress consists of the vertical and horizontal travel ways including intervening room spaces, doorways, hallways, corridors, passageways, ramps, stairs, lobbies, horizontal exits, escalators, enclosures, courts, balconies, and yards.

mercury lamp
a high-intensity discharge (HID) lamp in which the major portion of the light is produced by radiation from mercury operating at a partial pressure in excess of 105 Pa (approximately 1 atm). Includes clear, phosphor-coated (mercury-fluorescent), and self-ballasted lamps.

mercury-fluorescent lamp
(phosphor mercury lamp) an electric-discharge lamp having a high-pressure mercury arc in an arc tube and an outer envelope coated with a fluorescing substance (phosphor) that transforms some of the ultraviolet energy generated by the arc into light.

mesopic vision
vision with fully adapted eyes at luminance conditions between those of photopic and scotopic vision, that is, between about 3.4 and 0.034 cd/m2.

metal halide lamp
a high-intensity discharge (HID) lamp in which the major portion of the light is produced by radiation of metal halides and their products of dissociation–possibly in combination with
metallic vapors such as mercury. Includes clear and phosphor-coated lamps.

metamers
lights of the same color but of different spectral power distribution.
Note The term “metamers” is also used to denote objects that, when illuminated by a given source and viewed by a given observer, produce metameric lights.

middle ultraviolet
† a portion of the electromagnetic spectrum in the range of 200 to 300 nm.

minimal perceptible erythema
, MPE the erythemal threshold.

mired
† See reciprocal color temperature.

modeling light
illumination that reveals the depth, shape, and texture of a subject; key light, cross lighting, counter-key light, side light, back light, and eye light are types of modeling light.

modulation threshold
in the case of sinisoidal wave gratings, manipulation of luminance differences can be specified in terms of modulation and the threshold can be called the modulation threshold. Periodic patterns that are not sinusoidal can be similarly specified in terms of the modulation of the fundamental sine wave component. The number of periods or cycles per degree of visual angle represents the spatial frequency.

monocular visual field the field for a single eye. See binocular portion of the visual field.

mounting height
(roadway)† the vertical distance between the roadway surface and the center of the apparent light source of a luminaire.

mounting height above the floor
(MHf)† the distance from the floor to the light center of the luminaire, or to the plane of the ceiling for recessed equipment.

mounting height above the workplane
(MHwp)† the distance from the work plane to the light center of the luminaire, or to the plane of the ceiling for recessed equipment.

multiple-beam headlamp
a headlamp so designed as to permit the driver of a vehicle to use any one of two or more distributions of light on the road.

Munsell chroma
, C an index of perceived chroma of the object color defined in terms of the luminance factor (Y) and chromaticity coordinates (x, y) for CIE Standard Illuminant C and the CIE 1931 Standard Observer.

Munsell color system
a system of surface-color specification based on perceptually uniform color scales for the three variables: Munsell hue, Munsell value, and Munsell chroma. For an observer of normal color vision, adapted to daylight and viewing a specimen when illuminated by daylight and surrounded with a middle-gray to white background, the Munsell hue, value, and chroma of the color correlate well with the hue, lightness, and perceived chroma.

Munsell hue
, H the index of the hue of the perceived object color defined in terms of the luminance factor (Y) and coordinates (x, y) for CIE Standard Illuminant C and the CIE 1931 Standard Observer.

Munsell value
, V the index of the lightness of the perceived object color defined in terms of the luminance factor Y for CIE Standard Illuminant C and the CIE 1931 Standard Observer.
Note The exact definition gives Y as a fifth-power function of V, so that tabular or iterative methods are needed to find V as a function of Y. However, V can be estimated within ± 0.1 by V = 11.6(Y/100)1/3 − 1.6 or within ± 0.6 by V = Y1/2, where Y is the luminance factor expressed in percent.

N

nanometer, nm† a unit of wavelength equal to 10−9 m. See the Appendix for conversion values.

narrow-angle diffusion
that in which flux is scattered at angles near the direction that the flux would take by regular reflection or transmission. See wide-angle diffusion.

narrow-angle luminaire
a luminaire that concentrates the light within a cone of a comparatively small solid angle. See wide-angle luminaire.

national standard of light
† a primary standard of light that has been adopted as a national standard. See primary standards of light.

navigation lights
† an alternative term for position lights.

navigation light system
a set of aircraft aeronautical lights provided to indicate the position and direction of motion of an aircraft to pilots of other aircraft or to ground observers.

near infrared
† the region of the electromagnetic spectrum from 770 to 1400 nm.

near ultraviolet
† the region of the electromagnetic spectrum from 300 to 380 nm.

night
the hours between the end of evening civil twilight and the beginning of morning civil twilight.
Note Civil twilight ends in the evening when the center of the sun’s disk is 6° below the horizon, and begins in the morning when the center of the sun’s disk is 6° below the horizon.

nit, nt
† a unit of luminance equal to 1 cd/m2.
Note the candela per square meter (cd/m2) is the SI unit of luminance.

nonrecoverable light loss factors
(initial or maintained) factors that give the fractional light loss that cannot be recovered by cleaning or lamp replacement. Comprised of those components that account for the lamps operating at other than their rated luminous value. This factor is applied to lighting calculations irrespective of the age of the lighting system.

normal ac power
† power supplied to a facility during non-emergency situations. It is usually supplied by a local electric utility.

normal lighting
† permanently installed task and corridor electric lighting normally for use when the premises are occupied.

numerical display
(digital display)† an electrically operated display of digits. Tungsten filaments, gas discharges, light-emitting diodes, liquid crystals, projected numerals, illuminated numbers, and other principles of operation can be used.

O

object color† the color of the light reflected or transmitted by an object when illuminated by a standard light source, such as CIE source A, B, C, or D65. See standard source and perceived object color.

obstruction beacon
† See hazard or obstruction beacon.

obstruction lights
aeronautical ground lights provided to indicate obstructions.

occulting light
a rhythmic light in which the periods of light are clearly longer than the periods of darkness.

opaque
† impenetrable to light; not able to transmit, or not transmitting light.

orientation
the relation of a building with respect to compass directions.

Ostwald color system
a system of describing colors in terms of color content, white content, and black content. It is usually exemplified by color charts in triangular form with full color, white, and black mixtures at the apices providing a gray scale of white and black mixtures, and parallel scales of constant white content as these grays are mixed with varying proportions of the full color. Each chart represents a constant dominant wavelength (called hue), and the colors lying on a line parallel to the gray scale represent constant purity (called shadow series).

overcast sky
one that has 100% cloud cover; the sun is not visible.

overhang
the distance between a vertical line passing through a specified point (often the photometric center) of a luminaire and the curb or edge of a roadway.

ozone-producing radiation
UV energy of wavelength shorter than 220 nm that decomposes oxygen, O2, thereby producing ozone, O3. Some UV sources generate energy at 184.9 nm, which is particularly effective in producing ozone.

P

panel (open) face exit sign† a transilluminated sign where both the exit legend and background are translucent.

PAR
lamp See pressed reflector lamp.

parking lamp
a lighting device placed on a vehicle to indicate its presence when parked.

partial diffusion
† See incomplete diffusion.

partly cloudy sky
a sky that has 30 to 70% cloud cover.

passing beams
† See lower (passing) beams.

pendant luminaire
† See suspended (pendant) luminaire.

perceived light source color
the color perceived to belong to a light source.

perceived object color
† the color perceived to belong to an object resulting from characteristics of the object, of the incident light, and of the surround, the viewing direction, and observer adaptation. See object color.

percent flicker
a relative measure of the cyclic variation in output of a light source (percent modulation). It is given by the expression where A is the maximum and B is the minimum output during a single cycle. See Chapter 6, Light Sources.

perfect diffusion that in which flux is uniformly scattered in accord with Lambert’s cosine law.

perimeter lights
aeronautical ground lights provided to indicate the perimeter of a landing pad for helicopters.

period life
the time interval until lamps are replaced or luminaires are cleaned.

peripheral vision
the seeing of objects displaced from the primary line of sight and outside the central visual field.

peripheral visual field
that portion of the visual field that falls outside the region corresponding to the foveal portion of the retina.

phosphor mercury lamp
† see mercury-fluorescent lamp.

phosphorescence
the emission of light as the result of the absorption of radiation, and continuing for a noticeable length of time after excitation (longer than approximately 10−8 s).

phot, ph
a unit of illuminance equal to one lumen per square centimeter. The use of this unit is deprecated.

photobiology
a branch of biology that deals with the effects of optical radiation on living systems.

photochemical radiation
energy in the ultraviolet, visible, and infrared regions capable of producing chemical changes in materials.
Note Examples of photochemical processes are accelerated fading tests, photography, photoreproduction, and chemical manufacturing. In many such applications a specific spectral region is of importance.

photoelectric receiver
† a device that reacts electrically in a measurable manner in response to incident radiant energy.

photoflash lamp
a lamp in which combustible metal or other solid material is burned in an oxidizing atmosphere to produce light of high intensity and short duration for photographic purposes.

photoflood lamp
an incandescent filament lamp of high color temperature for lighting objects for photography or videography.

photometer
an instrument for measuring photometric quantities such as luminance, luminous intensity, luminous flux, or illuminance. See densitomer, goniophotometer, illuminance (lux or footcandle) meter, integrating photometer, reflectometer, spectrophotometer, and transmissometer.

photometry
the measurement of quantities associated with light.
Note Photometry can be either visual, in which the eye is used to make a comparison, or physical, in which measurements are made by means of physical receptors.

photometric brightness
† a term formerly used for luminance.

photoperiod
the environmental light/dark cycle to which living organisms may be exposed; for example, the natural cycle at the earth’s equator of light (L) for 12 hours and darkness (D) for 12 hours. This is expressed as LD 12:12.

photopic vision
vision mediated essentially or exclusively by the cones. It is generally associated with adaptation to a luminance of at least 3.4 cd/m2. See scotopic vision.

photosynthetic irradiance
irradiance within the wavelength band 400 to 700 nm. Unit: watts per square meter.

photosynthetic photon flux density
(PPFD) the number of photons per unit time and per unit area in the wavelength band 400 to 700 nm. Unit: micromoles per second and per square meter.
Note (1) There are 6.0222 × 1023 photons in one mole; (2) this unit was formerly known as microeinsteins per second and per square meter.

photosynthetically active radiation
(PAR) photon flux in the wavelength band 400 to 700 nm.

phototherapy
the treatment of disease involving the use of optical radiation.

physical photometer
an instrument containing a physical receptor and associated filters that is calibrated so as to read photometric quantities directly. See visual photometer.

pilot house control
a mechanical means for controlling the elevation and train of a searchlight from a position on the other side of the bulkhead or deck on which it is mounted.

Planck radiation law an expression representing the spectral radiance of a blackbody as a function of the wavelength and temperature. This law commonly is expressed by the formula

where
Lλ = the spectral radiance
dIλ = the spectral radiant intensity
dA’ = the projected area (dA cos θ) of the aperture of the blackbody
e = the base of natural logarithms (2.71828)
T = absolute temperature
c1L and c2 = constants designated as the first and second radiation constants.
Note The symbol c1L is used to indicate that the equation in the form given here refers to the radiance L, or to the intensity I per unit projected area A′, of the source. Numeric values are commonly given not for c1L but for c1, which applies to the total flux radiated from a blackbody aperture, that is, in a hemisphere (2π sr), so that, with the Lambert cosine law taken into account, c1 = π c1L. The currently recommended value of c1 is 3.741832 × 10−16 W × m2, or 3.741832 × 10−12 W × cm2. Then c1L is 1.191062 × 10−16 W × m2 × sr−1, or 1.191062 × 10−12 W × cm2 × sr−1. If, as is more convenient, wavelengths are expressed in micrometers and area in square centimeters, then c1L = 1.191062 × 104 W × μm4 × cm−2 × sr−1, Lλ being given in W × cm−2 × sr−1 × μm−1. The currently recommended value of c2 is 1.438786 × 10−2 m × K.

The Planck law in the following form gives the energy radiated from the blackbody in a given wavelength interval (λ1­λ2):
If A is the area of the radiation aperture or surface in square centimeters, t is the time in seconds, λ is the wavelength in micrometers, and c1 = 3.741832 × 104 W × μm4 × cm−2, then Q is the total energy in watt-seconds (joules), emitted from this area (that is, in the solid angle 2π) in time t within the wavelength interval (λ1­λ2).

planckian locus
† See blackbody (planckian) locus.

plano-convex spotlight
a spotlight embodying a plano-convex lens and a lamp movable within the housing in relation to the lens in order to vary beam and field angles.

point of fixation
a point or object in the visual field at which the eyes look and upon which they are focused.

point of observation
for most purposes it can be assumed that the distribution of luminance in the field of view can be described as if there were a single point of observation located at the midpoint of
the baseline connecting the centers of the entrance pupils of the two eyes. For many problems it is necessary, however, to regard the centers of the entrance pupils as separate points of observation for the two eyes.

point-by-point method
† a method of lighting calculation, now called the point method.

point method
a lighting design procedure for predetermining the illuminance at various locations in lighting installations by use of luminaire photometric data. The direct component of illuminance due to the luminaires and the interreflected component of illuminance due to the room surfaces are calculated separately. The sum is the total illuminance at a point.

point source
a source of radiation whose dimensions are sufficiently small, compared with the distance between the source and the irradiated surface, that these dimensions can be neglected in calculations and measurements.

point vision
the mode of vision of a small source of light such that the sensation is determined by its intensity rather than by its size. Point vision occurs with sources so small that their form or shape is not perceived and that they appear as points of light; this generally means less than 1-minute angular subtense.

polarization
† the process by which unpolarized radiation is polarized. It can be accomplished by either a reflection process or a transmission process.

polarized radiation
† radiation whose electromagnetic field, which is transverse, is oriented in defined directions. The polarization can be rectilinear, elliptic, or circular.

pole
(roadway lighting) a standard support generally used where overhead lighting distribution circuits are employed.

portable lighting
lighting involving equipment designed for manual portability.

portable luminaire
a lighting unit that is not permanently fixed in place. See table lamp and floor lamp.

portable traffic control light
a signaling light designed for manual portability that produces a controllable distinctive signal for purposes of directing aircraft operations in the vicinity of an aerodrome.

position index, P a factor that represents the relative average luminance for a sensation at the borderline between comfort and discomfort (BCD) for a source located anywhere within the visual field.

position lights
aircraft aeronautical lights forming the basic, internationally recognized navigation light sys tem.
Note The system is composed of a red light showing from dead ahead to 110° to the left, a green light showing from dead ahead to 110° to the right, and a white light showing to the rear through 140°.

Position lights are also called navigation lights.

prefocus lamp
a lamp in which, during manufacture, the luminous element is accurately adjusted to a specified position with respect to the physical location element (usually the base).

preheat
(switch start) fluorescent lamp a fluorescent lamp designed for operation in a circuit requiring a manual or automatic starting switch to preheat the electrodes in order to start the arc.

pressed reflector lamp
an incandescent filament or electric-discharge lamp in which the outer bulb is formed of two pressed parts that are fused or sealed together; namely, a reflectorized bowl and a cover, which can be clear or patterned for optical control.
Note Often called a projector or PAR lamp.

primary
(light) any one of three lights in terms of which a color is specified by giving the amount of each required to match it by additive combination.

primary line of sight
the line connecting the point of observation and the point of fixation. For a single eye, it is the line containing the point of fixation and the center of the entrance pupil.

primary standards of light
a light source by which the unit of light is established and from which the values of other standards are derived. This order of standard also is designated as the national standard. See national standard of light.
Note A satisfactory primary (national) standard must be reproducible from specifications (see candela). Primary (national) standards usually are found in national physical laboratories such as the National Institute of Standards and Technology (NIST) in the United States.

projection lamp
a lamp with physical and luminous characteristics suited for projection systems (e.g., motion picture projectors, slide projectors, and microfilm viewers).

projector
a lighting unit that, by means of mirrors and lenses, concentrates the light to a limited solid angle so as to obtain a high value of luminous intensity. See floodlight, searchlight, and signaling light.

protective lighting
a system intended to facilitate the nighttime policing of industrial and other properties.

proximal stimuli
the distribution of illuminance on the retina constitutes the proximal stimulus.

public way
† any road, alley, or other similar parcel of land essentially open to the outside air, permanently appropriated for public use, and having a clear height and width of not less than 3 m (10 ft).

pupil
(pupillary aperture) the opening of the iris that admits light into the eye. See artificial pupil.

Purkinje phenomenon
the reduction in subjective brightness of a red light relative to that of a blue light when the luminances are reduced in the same proportion without changing the respective spectral distributions. In passing from photopic to scotopic vision, the curve of spectral luminous efficiency changes, the wavelength of maximum efficiency being displaced toward the shorter wavelengths.

purple boundary
the straight line drawn between the ends of the spectrum locus on a chromaticity diagram.

Q

quality of lighting pertains to the distribution of luminance in a visual environment. The term is used in a positive sense and implies that all luminances contribute favorably to visual performance, visual comfort, ease of seeing, safety, and aesthetics for the specific visual tasks involved.

quantity of light
(luminous energy), Q = ∫Φdt the product of the luminous flux by the time it is maintained. It is the time integral of luminous flux.

quartz-iodine lamp
† an obsolete term for the tungsten halogen lamp.

quick-flashing light
a single flashing light at a frequency equal to or greater than 1 Hz. There is no agreed verbal differentiation between lights that flash at 1 Hz and those that flash more rapidly (a quick-flashing light can be a sequence of single flashes or a sequence of multiflick flashes, at 1-s intervals; there is no restriction on the ratio of the durations of the light to the dark periods).

R

radiance, L = d2Φ/[dω (dA cos θ)]V =dI(dA cos θ) (in a direction, at a point on the surface of a source, of a receiver, or of any other real or virtual surface) the quotient of the radiant flux leaving, passing through, or arriving at an element of the surface surrounding the point, and propagated in directions defined by an elementary cone containing the given direction, by the product of the solid angle of the cone, and the area of the orthogonal projection of the element of the surface on a plane perpendicular to the given direction.
Note In the defining equation, θ is the angle between the normal to the element of the source and the given direction.

radiant energy
, Q energy traveling in the form of electromagnetic waves. It is measured in units of energy such as joules or kilowatt hours. See spectral radiant energy.

radiant energy density
, w = dQ/dV radiant energy per unit volume, for example, joules per cubic meter.

radiant exitance
, M† the density of radiant flux leaving a surface. It is expressed in watts per unit area of the surface.

radiant flux
(radiant power), Φ = dQ/dt the time rate of flow of radiant energy. It is expressed preferably in watts. See spectral radiant flux.

radiant flux density at a surface
† the quotient of radiant flux of an element of surface to the area of that element, in units such as W/m2. When referring to radiant flux emitted from a surface, this has been called radiant emittance (deprecated); the preferred term is radiant exitance: The radiant exitance per unit wavelength interval is called spectral radiant exitance. The radiant flux density incident on a surface is called irradiance (E).

radiant intensity
, I = dΦ/dω (in a given direction) the radiant flux proceeding from a source per unit solid angle in a given direction, for example, W/sr. See spectral radiant intensity.
Note Mathematically, a solid angle must have a point at its apex; the definition of radiant intensity therefore applies strictly only to a point source. In practice, however, radiant energy emanating from a source whose dimensions are negligible in comparison with the distance from which it is observed can be considered as coming from a point. Specifically, this implies that with change of distance (1) the variation in solid angle subtended by the source at the receiving point approaches 1/distance2, and that (2) the average radiance of the projected source area as seen from the receiving point does not vary appreciably.

radiator
an emitter of radiant energy.

radiometry
the measurement of quantities associated with radiant energy and power.

range lights
groups of color-coded boundary lights provided to indicate the direction and limits of a preferred landing path (normally) on an aerodrome without runways, but exceptionally on an aerodrome with runways.

rapid-start fluorescent lamp
a fluorescent lamp designed for operation with a ballast that provides a low-voltage winding for preheating the electrodes and initiating the arc without a starting switch or the application of high voltage.

rated lamp life
the life value assigned to a particular type lamp. This is commonly a statistically determined estimate of average or of median operational life. For certain lamp types other criteria than failure to light can be used; for example, the life can be based on the average time until the lamp type produces a given fraction of initial luminous flux.

reaction time
the interval between the beginning of a stimulus and the beginning of the response of an observer.

recessed luminaire
† See flush-mounted or recessed luminaire.

reciprocal color temperature
color temperature (Tc) expressed on a reciprocal scale (1/Tc). An important use stems from the fact that a given small increment in reciprocal color temperature is approximately equally perceptible regardless of color temperature. Also, color temperature conversion filters for sources approximating graybody sources change the reciprocal color temperature by nearly the same amount anywhere on the color temperature scale.
Note The unit is the reciprocal megakelvin (MK−1). The reciprocal color temperature expressed in this unit has the numerical value of 106/Tc when Tc is expressed in kelvins. The acronym mirek (for micro-reciprocal-kelvin) occasionally has been used in the literature. The acronym mired (for micro-reciprocal-degree) is now considered obsolete as the name for this unit.

recoverable light loss factors
factors that give the fractional light loss that can be recovered by cleaning or lamp replacement. Comprised of those components that account for depreciation of luminous quantities in a lighting system. This factor is a function of lighting system age and maintenance processes and is applied to lighting calculations for systems after time zero.

redirecting surfaces and media
those that change the direction of the flux without scattering the redirected flux.

reference ballast
a ballast that is specially constructed, having certain prescribed characteristics and that is used for testing electric-discharge lamps and other ballasts.

reference line
(roadway lighting) either of two radial lines where the surface of the cone of maximum intensity is intersected by a vertical plane parallel to the curb line and passing through the light center of the luminaire.

reference standard
† an alternative term for secondary standard.

reflectance of a surface or medium
, ρ = Φr/Φi the ratio of the reflected flux to the incident flux. Reflectance is a function of:
1. Geometry
a. of the incident flux
b. of collection for the reflected flux
2. Spectral distribution
a. characteristic of the incident flux
b. weighting function for the collected flux
3. Polarization
a. of the incident flux
b. component defined for the collected flux.

Note Unless the state of polarization for the incident flux and the polarized component of the reflected flux are stated, it should be considered that the incident flux is unpolarized

and that the total reflected flux (including all polarization) is evaluated. Spectral reflectance depends on only the beam geometry and the character of the reflecting surface (and on polarization). Luminous reflectance also is a function of the spectral distribution of the incident flux. If no qualifying geometric adjective is used, the reflectance for hemispherical collection is meant. Certain of the reflectance terms are theoretically imperfect and are recognized only as practical concepts to be used when applicable. Physical measurements of the incident and reflected flux are always biconical in nature. Directional reflectances cannot exist, since one component would be finite while the other was infinitesimal; here the reflectance distribution function is required. However, the concepts of directional and hemispherical reflectance have practical application in instrumentation, measurements, and calculations when including the effect of the nearly zero or nearly 2π conical angle would increase complexity without appreciably affecting the immediate results. In each case of conical incidence or collection, the solid angle need not be a right cone but can be of any cross section, including a rectangle, a ring, or a combination of two or more solid angles. For many geometrically specified reflectance properties it is assumed that the radiance (luminance) is isotropic over the specified solid angle of incidence. Otherwise, the property is a function of the directional distribution of the radiance (luminance) as well as the beam geometry and the character of the reflecting surface.

reflectance factor
, R the ratio of the flux actually reflected by a sample surface to that which would be reflected into the same reflected-beam geometry by an ideal (glossless), perfectly diffuse (lambertian), completely reflecting standard surface irradiated in exactly the same way as the sample. Note the analogies to reflectance in the fact that nine canonical forms are possible that “spectral” can be applied as a modifier, that it can be luminous or radiant reflectance factor, and so on. Note that reflectance cannot exceed unity, but reflectance factor can have any value from zero to values approaching infinity.

reflected glare
glare resulting from reflections of high luminances in polished or glossy surfaces in the field of view. It usually is associated with reflections from within a visual task or areas in close proximity to the region being viewed. See veiling reflection.

reflection
a general term for the process by which the incident flux leaves a (stationary) surface or medium from the incident side without change in frequency.
Note Reflection is usually a combination of regular and diffuse reflection. See regular (specular) reflection, diffuse reflection, and veiling reflection.

reflectivity
† reflectance of a layer of a material of such a thickness that there is no change of reflectance with increase in thickness.

reflectometer
a photometer for measuring reflectance.
Note Reflectometers can be visual or physical instruments.

reflector
a device used to redirect the flux from a source by the process of reflection. See retro-reflector.

reflector lamp
an incandescent filament or electric-discharge lamp in which the outer blown glass bulb is coated with a reflecting material so as to direct the light (such as R- or ER-type lamps). The light-transmitting region can be clear, frosted, patterned, or phosphor coated.

reflex reflector
† See retro-reflector.

refraction
† the process by which the direction of a ray of light changes as it passes obliquely from one medium to another in which its speed is different.

refractor
a device used to redirect the flux from a source, primarily by the process of refraction.

regions of the electromagnetic spectrum
for convenience of reference, the electromagnetic spectrum is arbitrarily divided as follows:
NoteThe spectral limits indicated above have been chosen as a matter of practical convenience. There is a gradual transition from region to region without sharp delineation. Also, the division of the spectrum is not unique. In various fields of science the classifications can differ due to the phenomena of interest.
Another division of the UV spectrum often used by photobiologists is given by the Commission Internationale de l’Eclairage (CIE):

regressed luminaire a luminaire mounted above the ceiling with its opening above the ceiling line. See flush-mounted, surface-mounted, suspended, and troffer luminaires.

regular
(specular) reflectance the ratio of the flux leaving a surface or medium by regular (specular) reflection to the incident flux. See regular (specular) reflection.

regular
(specular) reflection that process by which incident flux is redirected at the specular angle. See bihemispherical reflectance and specular angle.

regular transmission
that process by which incident flux passes through a surface or medium without scattering. See regular transmittance.

regular transmittance
the ratio of the regularly transmitted (straight through) flux leaving a surface or medium to the incident flux.

relative contrast sensitivity
(RCS) the relation between the reciprocal of the luminous contrast of a task at visibility threshold and the background luminance expressed as a percentage of the value obtained under a very high level of diffuse task illumination.

relative erythemal factor
† See erythemal efficiency of radiant flux.

relative luminosity
† an obsolete term for the spectral luminous efficiency of radiant flux.

relative luminosity factor
† an obsolete term for the spectral luminous efficiency of radiant flux.

resolving power
the ability of the eye to perceive the individual elements of a grating or any other periodic pattern with parallel elements measured by the number of cycles per degree that can be resolved. The resolution threshold is the period of the pattern that can be just resolved. The visual acuity, in such a case, is the reciprocal of one-half the period expressed in minutes. The resolution threshold for a pair of points or lines is the distance between their centers when they can be distinguished as two, not one, expressed in minutes of arc.

resultant color shift
the difference between the perceived color of an object illuminated by a test source and of the same object illuminated by the reference source, taking account of the state of chromatic adaptation (see state of chromatic adaptation) in each case; that is, the resultant of colorimetric shift and adaptive color shift.

retina
a membrane lining the posterior part of the inside of the eye. It comprises photoreceptors (cones and rods) that are sensitive to light and nerve cells that transmit to the optic nerve the responses of the receptor elements.

retro-reflector
(reflex reflector) a device designed to reflect light in a direction close to that at which it is incident, whatever the angle of incidence.

rhythmic light
a light that when observed from a fixed point has a luminous intensity that changes periodically. See equal interval (isophase) light, flashing light, group flashing light, interrupted quick-flashing light, and occulting light.

ribbon filament lamp
an incandescent lamp in which the luminous element is a tungsten ribbon.
Note This type of lamp is often used as a standard in pyrometry and radiometry.

rods
retinal receptors that respond at low levels of luminance even below the threshold for cones. At these levels there is no basis for perceiving differences in hue and saturation. No rods are found near the center of the fovea.

room cavity
the cavity formed by the plane of the luminaires, the work plane, and the wall surfaces between these two planes.

room cavity ratio
(RCR) a number indicating room cavity proportions, calculated from the length, width, and height. See zonal-cavity interreflectance method.

room utilization factor
(utilance) the ratio of the luminous flux (lumens) received on the workplane to that emitted by the luminaire.
Note This ratio sometimes is called interflectance. Room utilization factor is based on the flux emitted by a complete luminaire, whereas coefficient of utilization is based on the total flux generated by the lamps in a luminaire.

room surface dirt depreciation
(RSDD) the fractional loss of task illuminance due to dirt on the room surfaces.

runway alignment indicator
a group of aeronautical ground lights arranged and located to provide early direction and roll guidance on the approach to a runway.

runway centerline lights
runway lights installed in the surface of the runway along the centerline indicating the location and direction of the runway centerline; of particular value in conditions of very
poor visibility.

runway edge lights
lights installed along the edges of a runway marking its lateral limits and indicating its direction.

runway end identification lights
a pair of flashing aeronautical ground lights symmetrically disposed on each side of the runway at the threshold to provide additional threshold conspicuity.

runway exit lights
lights placed on the surface of a runway to indicate a path of the taxiway centerline.

runway lights
aeronautical ground lights arranged along or on a runway. See runway centerline lights, runway edge lights, runway end identification light, and runway exit lights.

runway threshold
† the beginning of the part of the runway usable for landing.

runway visibility
the meteorological visibility along an identified runway. Where a transmissometer is used for measurement, the instrument is calibrated in terms of a human observer; for example, the sighting of dark objects against the horizon sky during daylight and the sighting of moderately intense unfocused lights of the order of 25 candelas at night. See visibility (meteorological).

runway visual range
(RVR) in the United States, an instrumentally derived value based on standard calibrations that represents the horizontal distance a pilot sees down the runway from the approach end; it is based either on the sighting of high-intensity runway lights or on the visual contrast of other targets, whichever yields the greater visual range.

S

saturation of a perceived color the attribute according to which it appears to exhibit more or less chromatic color judged in proportion to its brightness. In a given set of viewing conditions, and at luminance levels that result in photopic vision, a stimulus of a given chromaticity exhibits approximately constant saturation for all luminances.

scoop
a floodlight consisting of a lamp in an ellipsoidal or paraboloidal matte reflector, usually in a fixed relationship, though some types permit adjustment of the beam shape.

scotopic vision
vision mediated essentially or exclusively by the rods. It is generally associated with adaptation to a luminance below about 0.034 cd/m2. See photopic vision.

sealed-beam headlamp
an integral optical assembly designed for headlighting purposes, identified by the name “Sealed Beam” branded on the lens.

sealed-beam lamp
A pressed-glass reflector lamp (PAR) that provides a closely controlled beam of light.
Note This term is generally applied in transportation lighting (for instance, automotive headlamps and aircraft landing lights) to distinguish sealed-beam lamps from similar devices in which the light source is replaceable within the reflector-lens unit.

searchlight
a projector designed to produce an approximately parallel beam of light.
Note The optical system of a searchlight has an aperture of greater than 20 cm (8 in).

secondary standard source
† a constant and reproducible light source calibrated directly or indirectly by comparison with a primary standard. This order of standard is also called a reference standard
Note National secondary (reference) standards are maintained at national physical laboratories; laboratory secondary (reference) standards are maintained at other photometric laboratories. A self-calibrated detector can be used as a secondary standard.
self-ballasted lamps any arc discharge lamp of which the current-limiting device is an integral part.

self-luminous exit sign
† an exit sign consisting of phosphor-coated glass tubes filled with a radioactive tritium gas. When the radioactive gas bombards the phosphor, the tube emits light (luminescence) and illuminates the exit legend, typically between 0.2 and 0.7 cd/m2.

semi-direct lighting
lighting involving luminaires that distribute 60 to 90% of the emitted light downward and the balance upward.

semi-indirect lighting
lighting involving luminaires that distribute 60 to 90% of the emitted light upward and the balance downward.

service period
the number of hours per day for which daylighting provides a specified illuminance level. It often is stated as a monthly average.

set light
in theatrical lighting, the separate illumination of background or scenic elements.

shade
a screen made of opaque or diffusing material that is designed to prevent a light source from being directly visible at normal angles of view.

shielding angle
(of a luminaire) the angle between a horizontal line through the light center and the line of sight at which the bare source first becomes visible. See cutoff angle (of a luminaire).

short-arc lamp
an arc lamp in which the distance between the electrodes is small (on the order of 1 to 10 mm).
Note This type of lamp (e.g., xenon or mercury) generally has an arc tube containing gas at very high pressure.

side-back light
illumination from behind the subject in a direction not parallel to a vertical plane through the optical axis of the cameras. See back light.

side light
lighting from the side to enhance subject modeling and place the subject in depth, apparently separated from the background.

side marker lamps
lamps indicating the presence of a vehicle when seen from the front and sometimes serving to indicate its width. When seen from the side they can also indicate its length.

signaling light
a projector used for directing light signals toward a designated target zone.

signal shutter
a device that modulates a beam of light by mechanical means for the purpose of transmitting intelligence.

size threshold
the minimum perceptible size of an object. It also is defined as the size that can be detected some specific fraction of the time it is presented to an observer, usually 50%. It usually is measured in minutes of arc. See visual acuity.

sky factor
the ratio of the illuminance on a horizontal plane at a given point inside a building due to the light received directly from the sky, to the illuminance due to an unobstructed hemisphere of sky of uniform luminance equal to that of the visible sky.

sky light
† visible radiation from the sun redirected by the atmosphere.

sky luminance distribution function
for a specified sky condition, the luminance of each direction of the sky relative to the zenith luminance.

soft light
(1) diffuse illumination that produces soft-edged, poorly defined shadows on the background when an object is placed in its path; (2) a luminaire designed to produce such illumination.

solar efficacy
the ratio of the solar illuminance constant to the solar irradiance constant. The current accepted value is 94.2 lm/W.

solar illuminance constant
the solar illuminance at normal incidence on a surface in free space at the earth’s mean distance from the sun. The currently accepted value is 127.5 klx (11,850 fc).

solar (irradiance) constant
† the irradiance, averaging 1353 W/m2 (125.7 W/ft2), from the sun at its mean distance from the earth, 1.5 × 1011 m (92.9 × 106 mi), before modification by the earth’s atmosphere.

solar radiation simulator
a device designed to produce a beam of collimated radiation having a spectrum, flux density, and geometric characteristic similar to those of the sun outside the earth’s atmosphere.

solid angle
, ω a measure of that portion of space about a point bounded by a conic surface whose vertex is at the point. It is defined as the ratio of intercepted surface area of a sphere centered on that point to the square of the sphere’s radius. It is expressed in steradians.

solid angle factor
, Q a function of the solid angle ω, given in steradians, subtended by a source and is given by Q = 20.4ω + 1.52ω0.2 − 0.75. See index of sensation.

spacing
† for roadway lighting, the distance between successive lighting units, measured along the centerline of the street. For interior applications see Chapter 9, Lighting Calculations.

spacing-to-mounting-height ratio, S/MHwp
† the ratio of the actual distance between luminaire centers to the mounting height above the workplane. Also, an obsolete term that described a characteristic of interior luminaires. See luminaire spacing criterion.

special color rendering index
, Ri measure of color shift of various standardized special colors, including saturated colors, typical foliage, and Caucasian skin. It also can be defined for other color samples when the spectral reflectance distributions are known.

spectral-directional emissivity
, ε (λ,θ,φ,T) (of an element of surface of a temperature radiator at a given wavelength and in a given direction) the ratio of its spectral radiance at that wavelength and in the given direction to that of a blackbody at the same temperature and wavelength:

spectral hemispherical emissivity, ε(λ, T) (of an element of surface of an opaque temperature radiator that has an optical smooth surface) the ratio of its spectral radiant exitance to that of a blackbody at the same temperature.
Note Hemispherical emissivity is frequently called “total” emissivity. However, “total” is ambiguous and should be avoided, since it can also refer to the spectral total (all wavelengths) as well as directional total (all directions). See spectral-total hemispherical emissivity.

spectral
(spectroscopic) lamp a discharge lamp that emits a significant portion of its radiative power in a line spectrum and that, in combination with filters, can be used to obtain monochromatic radiation.

spectral luminous efficacy of radiant flux
, K) = Φvλ/Φeλ the quotient of the luminous flux at a given wavelength by the radiant flux at that wavelength. It is expressed in lm/W. See also radiant flux and spectral radiant flux.
Note This quantity formerly was called the luminosity factor. The reciprocal of the maximum luminous efficacy of radiant flux, that is, the ratio between radiant and luminous flux at the wavelength of maximum luminous efficacy, is sometimes called the mechanical equivalent of light; that is, the ratio between radiant and luminous flux at the wavelength of maximum luminous efficacy. The most probable value is 0.00146 W/lm, corresponding to 683 lm/W as the maximum possible luminous efficacy. For scotopic vision values the maximum luminous efficacy is 1754 scotopic lm/W.

spectral luminous efficiency for photopic vision
, V(λ)† See values of spectral luminous efficiency for photopic vision.

spectral luminous efficiency for scotopic vision
, V(λ)† See values of spectral luminous efficiency for scotopic vision.

spectral luminous efficiency of radiant flux
† the ratio of the luminous efficacy for a given wavelength to the value for the wavelength of maximum luminous efficacy. It is dimensionless.
Note This term replaces the previously used terms relative luminosity and relative luminosity factor.

spectral radiant energy
, Qλ = dQ/dλ radiant energy per unit wavelength interval, for example, joules per nanometer. Qλ (λ) = dQ/dλ at wavelength λ.

spectral radiant exitance, Mλ
, and irradiance, Eλ spectral concentration of radiant exitance, Mλ = dM/dλ, and spectral concentration of irradiance, Eλ = dE/dλ. See radiant flux density at a surface.

spectral radiant flux
, Φλ = dΦ/dλ radiant flux per unit wavelength interval at wavelength λ, for example, W/nm.

spectral radiant intensity
, Iλ = dI/dλ radiant intensity per unit wavelength interval, for example, W/sr × nm.

spectral reflectance of a surface or medium
, ω (λ) = Φrλ/Φiλ the ratio of the reflected flux to the incident flux at a particular wavelength, λ, or within a small band of wavelengths, Δλ, about λ.
Note The various geometrical aspects of reflectance can each be considered restricted to a specific region of the spectrum and can be so designated by the use of the adjective “spectral.”

spectral-total directional emissivity
, ε (θ,φ,T) (at a point on the surface of a thermal radiator and in a given direction) the ratio of the radiance of the thermal radiator at temperature T at the point and in the given direction, to that of a blackbody at the same temperature, T.

where
x, y are the coordinates of the point,
θ, φ define the direction.

spectral-total hemispherical emissivity
, ε (x,y,2π,T) (at a point on the surface of a thermal radiator)† the ratio of the radiant exitance of the thermal radiator at temperature T, at the given point on the surface, to that of a blackbody at the same temperature T:

spectral transmittance of a medium, τ(λ) = Φtλ/Φiλ the ratio of the transmitted flux to the incident flux at a particular wavelength, λ, or within a small band of wavelengths, Δλ, about λ.
Note The various geometrical aspects of transmittance can each be considered restricted to a specific region of the spectrum and can be so designated by the addition of the adjective “spectral.”

spectral tristimulus values
† See color-matching functions.

spectrophotometer
an instrument for measuring the transmittance and reflectance of surfaces and media as a function of wavelength.

spectroradiometer
an instrument for measuring radiant flux as a function of wavelength.

spectrum locus
the locus of points representing the colors of the visible spectrum in a chromaticity diagram.

specular angle
that angle between the perpendicular to the surface and the reflected ray that is numerically equal to the angle of incidence, and that lies in the same plane as the incident ray and the perpendicular, but on the opposite side.

specular reflectance
† See regular (specular) reflectance.

specular reflection
† See regular (specular) reflection.

specular surface
one from which the reflection is predominantly regular. See regular (specular) reflection.

speed of light
† the speed of all radiant energy, including light, is 2.9979258 × 108 m/s in vacuum (approximately 186,000 mi/s). In all material media the speed is less and varies with the material’s index of refraction, which itself varies with wavelength.

speed of vision
the reciprocal of the duration of the exposure time required for something to be seen.

sphere illumination
illumination on a task from a source providing equal luminance in all directions about that task, such as an illuminated sphere with the task located at the center.

spherical reduction factor
the ratio of the mean spherical luminous intensity to the mean horizontal intensity. Retained for reference or literature search.

spotlight
any of several different types of luminaires with narrow beam angle designed to illuminate a well-defined area. In motion pictures, generic for Fresnel lens luminaires. Also, a form of floodlight, usually equipped with lenses and reflectors to give a fixed or adjustable narrow beam.

standard illuminant
A a blackbody at a temperature of 2856 K. It is defined by its relative spectral power distribution over the range from 300 to 830 nm.

standard illuminant B
a representation of noon sunlight with a correlated color temperature of approximately 4900 K. It is defined by its relative spectral power distribution over the range from 320 to 770 nm.
Note It is anticipated that at some future date, that is yet to be decided, illuminant B will be dropped from the list of recommended standard illuminants.

standard illuminant C
a representation of daylight having a correlated color temperature of approximately 6800 K. It is defined by its relative spectral power distribution over the range from 320 to 770 nm.
Note It is anticipated that at some future date, that is yet to be decided, illuminant C will be dropped from the list of recommended standard illuminants.

standard illuminant D65
a representation of daylight at a correlated color temperature of approximately 6500 K. It is defined by its relative spectral power distribution over the range from 300 to 830 nm.
Note At present, no artificial source for matching this illuminant has been recommended.

standard source
an electric light source having the same spectral power distribution as a specified standard illuminant.

standard source A
a tungsten filament lamp operated at a color temperature of 2856 K (International Practical Temperature Scale, 1968) and approximating the relative spectral power distribution of standard illuminant A.

standard source B
an approximation of standard illuminant B obtained by a combination of source A and a special filter.

standard source C
an approximation of standard illuminant C obtained by a combination of source A and a special filter.

starter
a device used in conjunction with a ballast for the purpose of starting an electric-discharge lamp.

state of chromatic adaptation
the condition of the chromatic properties of the visual system at a specified moment as a result of exposure to the totality of colors of the visual field currently and in the past.

Stefan-Boltzmann
law the statement that the radiant exitance or radiance of a blackbody radiator is proportional to the fourth power of its absolute temperature; that is,
Note The currently recommended value of σ is 5.67032 × 10−8 (W × m−2 × K−4) and that of σL is 1.80492 × 10−8 (W × m−2 × sr−1 × K−4)

stencil face exit sign† a transilluminated sign where either the exit legend or the background are opaque. Usually the exit legend is translucent and the background is die cut from an opaque medium such as plastic or metal.

steradian, sr
(unit of solid angle)† the solid angle subtended at the center of a sphere by an area on the surface of the sphere equal to the square of the sphere radius.

stilb
a cgs (cm-gram-second) unit of luminance. One stilb equals 1 cd/cm2. The use of this term is deprecated.

Stiles-Crawford effect
the reduced luminous efficiency of rays entering the peripheral portion of the pupil of the eye. This effect applies only to cones and not to rods. Hence, there is no Stiles-Crawford effect in scotopic vision.

stop lamp
a device giving a steady warning light to the rear of a vehicle or train of vehicles, to indicate the intention of the operator to diminish speed or to stop.

stray light
(in the eye) light from a source that is scattered onto parts of the retina lying outside the retinal image of the source.

street lighting luminaire
a complete lighting device consisting of a light source and ballast, where appropriate, together with its direct appurtenances such as globe, reflector, refractor, housing, and such support as is integral with the housing. The pole, post, or bracket is not considered part of the luminaire.
Note Modern street lighting luminaires contain the ballasts for high-intensity discharge lamps where such lamps are used; a photocontrol can be mounted on the luminaire.

street lighting unit
the assembly of a pole or lamp post with a bracket and a luminaire.

striplight
(theatrical) once an open trough reflector containing a series of lamps; now usually a compartmentalized luminaire with each compartment containing a lamp, reflector, and color frame holder, wired in rotation in three or four circuits and used as borderlights, footlights, or cyclorama lighting from above or below. Often in short 0.9- to 2.4-m [3- to 8-ft] portable sections.

stroboscopic lamp
(strobe light) a flash tube designed for repetitive flashing.

subjective brightness
the subjective attribute of any light sensation giving rise to the perception of luminous magnitude, including the whole scale of qualities of being bright, light, brilliant, dim, or dark. See saturation of a perceived color.
Note The term brightness often is used when referring to the measurable luminance. While the context usually makes it clear as to which meaning is intended, the term luminance should be used for the photometric quantity, thus reserving brightness for the subjective sensation.

sun bearing
the angle measured in the plane of the horizon between a vertical plane at a right angle to the window wall and the position of this plane after it has been rotated to contain the sun.

sunburn
inflammation with reddening (erythema) of the skin, of variable degree, caused by exposure to direct or diffuse solar radiation or artificial optical radiation.

sun lamp
an ultraviolet lamp that radiates a significant portion of its radiative power in the UV-B band (280 to 315 nm).

sunlight
direct visible radiation from the sun.

suntan
a darkening of the skin due to an increase of melanin pigmentation above constitutive level and induced by UV radiation.

supplementary lighting
lighting used to provide an additional quantity and quality of illumination that cannot readily be obtained by a general lighting system and that supplements the general lighting level, usually for specific work requirements.

supplementary standard illuminant D55
a representation of a phase of daylight at a correlated color temperature of approximately 5500 K.

supplementary standard illuminant D75
a representation of a phase of daylight at a correlated color temperature of approximately 7500 K.

surface-mounted luminaire
a luminaire that is mounted directly on a ceiling.

suspended (pendant) luminaire
a luminaire that is hung from a ceiling by supports.

switch start fluorescent lamp
† See preheat (switch start) fluorescent lamp.

Systeme Internationale (SI)
† a measurement system used throughout the world, commonly referred to as the metric system. Public Law 100-418 designated the metric system as the preferred system of weights and measures for the United States.

T

table lamp a portable luminaire with a short stand, suitable for standing on furniture.

tail lamp
a lighting device used to designate the rear of a vehicle by a warning light.

talbot, T†
a unit of light equal to one lumen-second.

tanning lamp
an ultraviolet lamp that emits a significant portion of its radiative power in the UV-A band (315 to 400 nm) or UV-B band (280 to 315 nm).

task ambient lighting
a combination of task lighting and ambient lighting within an area such that the general level of ambient lighting is lower than and complementary to the task lighting.

task lighting
lighting directed to a specific surface or area that provides illumination for visual tasks.

taxi-channel lights
aeronautical ground lights arranged along a taxi channel of a water airdrome to indicate the route to be followed by taxiing aircraft.

taxi light
an aircraft aeronautical light designed to provide necessary illumination for taxiing.

taxiway centerline lights
taxiway lights placed along the centerline of a taxiway except on curves or corners having fillets. These lights are placed a distance equal to half the normal width of the taxiway from the outside edge of the curve or corner.

taxiway edge lights
taxiway lights placed along or near the edges of a taxiway.

taxiway holding-post light
a light or group of lights installed at the edge of a taxiway near an entrance to a runway, or to another taxiway, to indicate the position at which the aircraft should stop and obtain clearance to proceed.

taxiway lights
aeronautical ground lights provided to indicate the route to be followed by taxiing aircraft. See taxiway centerline lights, taxiway edge lights, and taxiway holding-post light.

temperature radiator
an ideal radiator whose radiant flux density (radiant exitance) is determined by its temperature and the material and character of its surface and is independent of its previous history. See blackbody and graybody.

thermopile
† a thermal radiation detector consisting of a number of thermocouples interconnected in order to increase the sensitivity to incident radiant flux.

threshold
the value of a variable of a physical stimulus (such as size, luminance, contrast, or time) that permits the stimulus to be seen a specific percentage of the time or at a specific accuracy level. In many psychophysical experiments, thresholds are presented in terms of 50% accuracy, or accurately 50% of the time. However, the threshold also is expressed as the value of the physical variable that permits the object to be just barely seen. The threshold can be determined by merely detecting the presence of an object, or it can be determined by discriminating certain details of the object. See absolute luminance threshold, brightness contrast threshold, luminance threshold, and modulation threshold.

threshold lights
runway lights so placed as to indicate the longitudinal limits of that portion of a runway, channel, or landing path usable for landing.

top light
illumination of a subject directly from above, employed to outline the upper margin or edge of the subject.

torchère
an indirect floor lamp that sends all or nearly all of its light upward.

tormentor lighting
luminaires mounted directly behind the sides of the stage arch.

total emissivity
† See spectral-total directional emissivity and spectral-total hemispherical emissivity.

total internal reflectance
(TIR)† total reflection of a light ray at a surface of a transmitting medium occurs when the angle of incidence exceeds a certain value whose sine equals n2/n1, the ratio of indices of refraction, or when sin r = 1, where r equals the angle of reflection.

touchdown zone lights
barrettes of runway lights installed in the surface of the runway between the runway edge lights and the runway centerline lights to provide additional guidance during the touchdown phase of a landing in conditions of very poor visibility.

traffic beam
† See lower (passing) beams.

train
the angle between the vertical plane through the axis of a searchlight drum and the corresponding plane in which this plane lies when the searchlight is in a position designated as having zero train.

transient adaptation factor
(TAF) a factor that reduces the equivalent contrast due to readaptation from one luminous background to another.

transition lighting
in roadway lighting, lighting gauged to compensate for visual adaptation between regions of high and low light level, as when entering tunnels.

translucent
† transmitting light diffusely or imperfectly.

transmission
a general term for the process by which incident flux leaves a surface or medium on a side other than the incident side, without change in frequency.
Note Transmission through a medium is often a combination of regular and diffuse transmission. See diffuse transmission, regular transmission, and transmittance.

transmissometer
a photometer for measuring transmittance.
Note Transmissometers can be visual or physical instruments.

transmittance
, τ = Φti the ratio of the transmitted flux to the incident flux. It should be noted that transmittance refers to the ratio of flux emerging to flux incident; therefore, reflections at the surface as well as absorption within the material operate to reduce the transmittance. Transmittance is a function of
1. Geometry
a. of the incident flux
b. of collection for the transmitted flux
2. Spectral distribution
a. characteristic of the incident flux
b. weighting function for the collected flux
3. Polarization
a. of the incident flux
b. component defined for the collected flux.
Notes (i) Unless the state of polarization for the incident flux and the polarized component of the transmitted flux are stated, it should be considered that the incident flux is unpolarized and that the total transmitted flux (including all polarization) is evaluated. (ii) Spectral transmittance depends on the beam geometry and the character of the transmitting surfaces and media (and polarization). In addition, luminous transmittance is a function of the spectral distribution of the incident beam. (iii) If no qualifying geometric adjective is used, transmittance for hemispherical collection is meant. (iv) In each case of conical incidence or collection, the solid angle is not restricted to a right circular cone but can be of any cross section, including a rectangle, a ring, or a combination of two or more solid angles. (v) These concepts must be applied with care if the area of the transmitting element is not large compared to its thickness, due to internal transmission across the boundary of the area. (vi) For all of the following geometrical quantitites–biconical transmittance, bidirectional transmittance, bihemispherical transmittance, conical-directional transmittance, conical-hemispherical transmittance, directional-conical transmittance, directional-hemispherical transmittance, hemispherical-conical transmittance, and hemispherical-directional transmittance–it is assumed that the radiance (luminance) is isotropic over the specified solid angle of incidence. Otherwise, the property is a function of the directional distribution of incident radiance (luminance) as well as the beam geometry and the character of the transmitting surfaces and/or media. The following breakdown of transmittance quantities is applicable only to the transmittance of thin films with negligible internal scattering, so that the transmitted radiation emerges from a point that is not significantly separated from the point of incidence of the incident ray that produces the transmitted ray(s). The governing considerations are similar to those for application of the bidirectional reflectance distribution function (BRDF), rather than the bidirectional scattering-surface reflectance distribution function (BSSRDF).

transparent
† having the property of transmitting rays of light through its substance so that bodies situated beyond or behind can be distinctly seen (opposed to opaque and usually distinguished from translucent).

transverse roadway line
(TRL) any line across a roadway that is perpendicular to the curb line.

tristimulus values of a light, X, Y, Z
the amounts of each of three specific primaries required to match the color of the light.

troffer
a long recessed lighting unit usually installed with the opening flush with the ceiling. The term is derived from “trough” and “coffer”

troland
a unit of retinal illuminance that is defined as the product of object luminance (candela per square meter) and pupillary aperture area (square millimeters), that is, one troland is the retinal illuminance produced when the luminance of the distal stimulus is 1 cd/m2 and the area of the pupil is 1 mm2. The troland can be photopic or scotopic.
Note The troland makes no allowance for interocular attenuation or for the Stiles-Crawford effect.

tube
† See lamp.

tungsten-halogen lamp
a gas-filled tungsten filament incandescent lamp containing a certain proportion of halogens in an inert gas whose pressure exceeds 3 atm.
Note The tungsten-iodine lamp (U.K.) and quartz iodine lamp (U.S.) belong to this category. Obsolete U.S. term.

Turn-signal operating unit
that part of a signal system by which the operator of a vehicle indicates the direction a turn will be made, usually by a flashing light.

U

ultraviolet lamp a lamp that emits a significant portion of its radiative power in the ultraviolet (UV) part of the spectrum; the visible radiation is not of principal interest.

ultraviolet radiation
for practical purposes, any radiant energy within the wavelength range 100 to 400 nm is considered ultraviolet radiation. See regions of the electromagnetic spectrum.
Note On the basis of practical applications and the effect obtained, the ultraviolet region often is divided into the following bands:

There are no sharp demarcations between these bands, the indicated effects usually being produced to a lesser extent by longer and shorter wavelengths. For engineering purposes, the black light region extends slightly into the visible portion of the spectrum.

units of luminance
† the luminance of a surface in a specified direction can be expressed as luminous intensity per unit of projected area of surface or as luminous flux per unit of solid angle and per unit of projected surface area.
Note Typical units are cd/m2 [lm/(sr × m2)] and cd/ft2 [lm/(sr × ft2)]. The luminance of a surface in a specified direction is also expressed (incorrectly) in lambertian units as the number of lumens per unit area that would leave the surface if the luminance in all directions within the hemisphere on the side of the surface being considered were the same as the luminance in the specified direction. A typical unit in this system is the footlambert (fL), equal to 1 lm/ft2. This method of specifying luminance is equivalent to stating the number of lumens that would leave the surface if the surface were replaced by a perfectly diffusing surface with a luminance in all directions within the hemisphere equal to the luminance of the actual surface in the direction specified. In practice no surface follows exactly the cosine formula of emission or reflection; hence the luminance is not uniform but varies with the angle from which it is viewed. For this reason, this practice is denigrated.

unrecoverable light loss factors
† See nonrecoverable light loss factors.

upper (driving) beams
† one or more beams intended for distant illumination and for use on the open highway when not meeting other vehicles. Often referred to as high beams. Formerly country beams. See lower (passing) beams.

upward component
that portion of the luminous flux from a luminaire emitted at angles above the horizontal. See downward component.

utilance
† See room utilization factor.

V

vacuum lamp an incandescent lamp in which the filament operates in an evacuated bulb.

valance
a longitudinal shielding member mounted across the top of a window or along a wall (and is usually parallel to the wall) to conceal light sources, giving both upward and downward distributions.

valance lighting
lighting comprising light sources shielded by a panel parallel to the wall at the top of a window.

values of spectral luminous efficiency for photopic vision
, V(λ) values at 5-nm intervals (see Chapter 1, Light and Optics) were provisionally adopted by the CIE in 1924 and were adopted in 1933 by the International Committee for Weights and Measures as a basis for the establishment of photometric standards of types of sources differing from the primary standard in spectral distribution of radiant flux.
Note These standard values of spectral luminous efficiency were determined by observations with a 2° photometric field having a moderately high luminance. Photometric evaluations based upon them consequently do not apply exactly to other conditions of observation. Watts weighted in accord with these standard values are often referred to as light watts.

values of spectral luminous efficiency for scotopic vision
, V’(λ) values at 10-nm intervals (see Chapter 1, Light and Optics) were provisionally adopted by the CIE in 1951.
Note These values of spectral luminous efficiency were determined by observation by young dark-adapted observers using extra-foveal vision at near-threshold luminance.

vapor-tight luminaire
a luminaire designed and approved for installation in damp or wet locations. It also is described as enclosed and gasketed.

VASIS
(Visual Approach Slope Indicator System) the system of angle-of-approach lights accepted as a standard by the International Civil Aviation Organization, comprising two bars of lights located at each side of the runway near the threshold and showing red or white or a combination of both (pink) to the approaching pilot, depending on his or her position with respect to the glide path.

veiling brightness
a brightness superimposed on the retinal image that reduces its contrast. It is this veiling effect produced by bright sources or areas in the visual field that results in decreased visual performance and visibility.

veiling reflection
regular reflections that are superimposed upon diffuse reflections from an object that partially or totally obscure the details to be seen by reducing the contrast. This sometimes is called reflected glare. Another kind of veiling reflection occurs when one looks through a plate of glass. A reflected image of a bright element or surface can be seen superimposed on what is viewed through the glass plate.

vertical plane of a searchlight
the plane through the axis of the searchlight drum that contains the elevation angle. See horizontal plane of a searchlight.

visibility
the quality or state of being perceivable by the eye. In many outdoor applications, visibility is defined in terms of the distance at which an object can be just perceived by the eye. In indoor applications it usually is defined in terms of the contrast or size of a standard test object, observed under standardized viewing conditions, having the same threshold as the given object. See visibility (meteorological).

visibility
(meteorological) a term than denotes the greatest distance that selected objects (visibility markers) or lights of moderate intensity on the order of 25 candles (25 cd) can be observed and identified under specified conditions of observation. The distance can be expressed in kilometers or miles in the United States until the metric system becomes more widely used.

visibility level
(VL) a contrast multiplier to be applied to the visibility reference function to provide the luminance contrast required at different levels of task background luminance to achieve visibility for specified conditions relating to the task and observer.

visibility performance criteria function
(VL8) a function representing the luminance contrast required to achieve 99% visual certainty for the same task used for the visibility reference function, including the effects of dynamic presentation and uncertainty in task location.

visibility reference function
(VL1) a function representing the luminance contrast required at different levels of task background luminance to achieve visibility threshold for the visibility reference task consisting of a disk that subtends 4 minutes of arc exposed for 0.2 s.

vision
† See central (foveal) vision, mesopic vision, peripheral vision, photopic vision, and scotopic vision.

visual acuity
a measure of the ability to distinguish fine details, measured with a set of optotypes (test types for determining visual acuity) of different sizes. Quantitatively, it is the reciprocal of the minimum angular size in minutes of the critical detail of an object that can just be seen.

visual angle
the angle that an object or detail subtends at the point of observation. It usually is measured in minutes of arc.

visual approach slope indicator system
† See VASIS.

visual comfort probability
(VCP) the rating of a lighting system expressed as a percent of people who, when viewing from a specified location and in a specified direction, will be expected to find it acceptable in terms of discomfort glare. Visual comfort probability is related to the discomfort glare rating (DGR).

visual field
the locus of objects or points in space that can be perceived when the head and eyes are kept fixed. Separate monocular fields for the two eyes can be specified or the combination of the two. See binocular portion of the visual field, central visual field, monocular visual field, and peripheral visual field.

visual perception
the interpretation of impressions transmitted from the retina to the brain in terms of information about a physical world displayed before the eye.
Note Visual perception involves any one or more of the following recognizing the presence of something (object, aperture, or medium); identifying it; locating it in space; noting its relation to other things; and identifying its movement, color, brightness, or form.

visual performance
the quantitative assessment of the performance of a visual task, taking into consideration speed and accuracy.

visual photometer
one in which the equality of brightness of two surfaces is established visually. See physical photometer.
Note The two surfaces usually are viewed simultaneously side by side. This is satisfactory when the color difference between the test source and comparison source is small. However, when there is a color difference, a flicker photometer provides more precise measurements. In this type of photometer the two surfaces are viewed alternately at such a rate that the color sensations either nearly or completely blend, and the flicker due to brightness difference is balanced by adjusting the comparison source.

visual range
(of a light or object) the maximum distance at which that particular light (or object) can be seen and identified.

visual surround
includes all portions of the visual field except the visual display used in performing a task.

visual task
conventionally designates those details and objects that must be seen for the performance of a given activity, and includes the immediate background of the details or objects.
Note The term visual task as used is a misnomer because it refers to the visual display itself and not the task of extracting information from it. The task of extracting information also has to be differentiated from the overall task performed by the observer.

visual task evaluator
(VTE) a form of visibility meter that measures the level of contrast of a given visual display above the threshold of visibility. The ratio of the contrast of a display to its threshold contrast represents its level of visibility. Used to evaluate the visibility level (VL).

vitrine
† a transparent enclosure of glass or acrylic around an artifact, usually the top of a display case.

volt
† the difference in electrical potential between two points in a circuit. It is also called the electromotive force. The symbol often used in equations is “E” (from the latter term), although “V” is also acceptable.

voltage-to-luminaire factor
the fractional loss of illuminance due to improper voltage at the luminaire.

W

watt† the unit of power (rate of doing work). In electrical calculations, one watt is the power produced by a current of one ampere across a potential difference of one volt. The symbol often used in equations is “P,” although “W” is also acceptable.

wavelength
† the distance between two successive points of a periodic wave, in the direction of propagation, at which the oscillation has the same phase. The three commonly used units are listed in the following table:
The use of the terms micron and angstrom is deprecated.

Weber’s fraction† See luminance contrast.

wide-angle diffusion
that in which flux is scattered at angles far from the direction that it would take by regular reflection or transmission. See narrow-angle diffusion.

wide-angle luminaire
a luminaire that concentrates the light within the cone of a comparatively large solid angle. See narrow-angle luminaire.

width line
(roadway lighting) the radial line (the one that makes the larger angle with the reference line) that passes through the point of one-half maximum intensity on the lateral intensity distribution curve plotted on the surface of the cone of maximum intensity.

Wien displacement law
an expression representing, in a functional form, the spectral radiance of a blackbody as a function of the wavelength and the temperature:

The two principal corollaries of this law are

which show how the maximum spectral radiance Lm and the wavelength λm at which it occurs are related to the absolute temperature T. See Wien radiation law.
Note The currently recommended value of b is 2.8978 × 10−3 m × K or 2.8978 × 10−1 cm × K. From the Planck radiation law, b′ is found to be 4.0956 × 10−14 (W × cm−2 × sr−1 × μm−1 × K−5).

Wien radiation law
an expression representing approximately the spectral radiance of a blackbody as a function of its wavelength and temperature. It commonly is expressed by the formula
This formula is accurate to 1% or better for values of λT less than 3000 μm × K.

wing clearance lights a pair of aircraft lights provided at the wing tips to indicate the extent of the wing span when the navigation lights are located an appreciable distance inboard of the wing tips.

working standard
a standardized light source for regular use in photometry.

workplane
the plane on which a visual task is usually done, and on which the illuminance is specified and measured. Unless otherwise indicated, this is assumed to be a horizontal plane 0.76 m (30 in.) above the floor.

Z

zonal-cavity interreflectance method a procedure for calculating coefficients of utilization, wall exitance coefficients, and ceiling cavity exitance coefficients, taking into consideration the luminaire intensity distribution, room size and shape (cavity ratio concepts), and room reflectances. It is based on flux transfer theory.

zonal constant
a factor by which the mean intensity emitted by a source of light in a given angular zone is multiplied to obtain the lumens in the zone. See Chapter 2, Measurement of Light and Other Radiant Energy.

zonal factor interreflection method
a procedure used for calculating coefficients of utilization, based on integral equations, that takes into consideration the ultimate disposition of luminous flux from every 10° zone from luminaires.

zonal factor method
a procedure for predetermining, from typical luminaire photometric data in discrete angular zones, the proportion of luminaire output that would be incident initially (without interreflections) on the workplane, ceiling, walls, and floor of a room.

zonal multipliers
multipliers for the flux in each 10-degree conical zone from 0° (nadir) to 90° (horizontal) from a luminaire, expressing the fraction of that zonal flux that is directly incident on the floor of a room cavity. These multipliers are a function of the room cavity ratio and are used to determine the direct ratio.