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Glossary
Aabsolute luminance threshold - the luminance threshold for a bright object like a disk on a totally dark background. absorptance - the ratio of the flux absorbed by a medium 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 to draw attention to a part of the field of view. See directional lighting. accommodation - the process by which the eye changes focus from one distance to another. actinic - 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, chromatic adaptation. Note " Adaptation" is also used to refer to the final state of the process, as in "reaching a condition of adaptation to a specific process, as in 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 - Sec 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, run way alignment indicator, runway end identification light, perimeter lights, runway lights, taxi-channel lights and taxiway lights. aeronautical light - any luminous sign or signal 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 navigation light system, anti-collision light, ice detection light, fuselage lights, landing light, 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 characters. Tungsten filaments, gas discharges, light-emitting diodes, liquid crystals, projected or illuminated characters and other principles of operation may he 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. 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 - a unit of wavelength equal to 10_10m (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 l/i- (0.3183) cd/rn2. 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 may 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 - the luminous intensity at a given angle divided by the projected area of the luminaire at that angle. 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. Bback 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. backup lamp - a lighting device mounted on the rear of a vehicle for illuminating the region near the rear of the vehicle while moving or about to move in reverse. It normally can be used only while backing up. bactericidal (germicidal) effectiveness - the capacity of various portions of the ultraviolet ous portions of the ultraviolet (UV) spectrum to destroy bacteria, fungi and viruses. bactericidal (germicidal) efficiency of radiant flux (for a particular wavelength) - the ratio of the bactericidal effectiveness at a particular wavelength to that at wavelength 265.0 nm, which is rated as 1.0. Note Tentative values of the 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 expressed in bactericidal .rW nun/cm2 or bactericidal W mm/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, cited as "bactericidal microwatts." Note Radiated UV power at wavelength 253.7 nm usually is referred to as 'ultraviolet microwatts" or UV watts.” These terms should not be confused with bactericidal microwatts," because they refer to radiation that has not been weighted in accordance with the values given in chapter 5, Nonvisual Effects of Radiant Energy. bactericidal (germicidal) flux density - the bactericidal flux per unit area of the surface being irradiated. It is equal to the incident bactericidal flux divided by the area of the surface when the flux is uniformly distributed. lt usually is measured in .tW/em2 or W/ft2 of bactericidally weighted UV radiation (bactericidal LW/cm2 or bactericidal W/ft2). bactericidal lamp - a UV lamp that radiates a significant portion of its radiative power in the UV-C band (100-280 nm). baffle - a single opaque or translucent clement 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 flux of a fluorescent lamp(s) operated on a ballast as a fraction of the flux when operated on the standard (reference) ballasting specified for rating lamp lumens. Note The lamp(s) are at specified ambient temperature conditions for photometric testing. ballast-lamp photometric - factor the ratio of fluorescent luminaire lumen output with a given ballast and lamp type under photometric conditions to that with the lamp type 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. and vice versa. 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 or four (two-way or four-way), that may be attached to the front of a luminaire (usually a Fresnel spotlight) in order to alter 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). base light - uniform, diffuse, near-shadowless illumination sufficiently intense for a television or film picture of acceptable quality at a desired lens opening. The acceptable base level of unaccented base 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 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 l0o/o 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 may 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 - the 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 he specified; the solid angle need not be a right circular cone. biconical transmittance - the ratio of the 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; the solid angle need not be a right circular cone. bidirectional reflectance - the ratio of the reflected flux collected over an element of solid angle surrounding the given direction to the essentially collimated incident flux. Note The directions of incidence and collection and the size of the solid angle element of collection must he specified. In each case of conical incidence or collection, the solid angle need not be a right circular cone, but may be of any cross section, including a rectangle, a ring, or a combination of two or more solid angles. bidirectional reflectance distribution function (BRDF) - the ratio of the differential luminance of a ray reflected in a given direction to the differential luminous flux density incident from a given direction of incidence which produces it. Note This distribution function is the basic parameter for describing (geometrically) the reflecting properties of an opaque surface element (negligible internal scattering). It may have any positive value and will approach infinity in the specular direction for ideally specular reflectors. The spectral and polarization aspects must he defined for complete specification, since the BRDF as given above only defines the geometric aspects. bidirectional transmittance - The ratio of the incident flux collected over an element of solid angle surrounding the given direction to the essentially collimated incident flux. Note The direction of incidence, direction of collection and size of the solid angle clement must he specified. bidirectional transmittance distribution function (BTDF) - the ratio of the differential luminance of a ray transmitted in a given direction to the differential luminous flux density incident from a given direction of incidence that produces it. Note 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 scatterins-surface reflectance distribution function (BSSRDF). This function may have any positive value and will approach infinity in the direction for regular transmission (possibly with refraction but without scattering). The spectral and polarization aspects must be defined for complete specification, since the BTDF as given above only defines the geometrical aspects. bihemispherical reflectance - The ratio of the reflected flux collected over an entire hemisphere to the incident flux from the entire hemisphere. bihemispherical transmittance - The ratio of the transmitted flux collected over an 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 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 will absorb all the radiant energy that falls upon it. All other temperature radiators are called nonblackbodies. They 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. lt also is called a full radiator, standard radiator, complete radiator or ideal 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 region. Note For engineering purposes the wavelength range 320-400 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 320-400 nm. It is usually measured in milliwatts. Sec fluoren. Note Because of the variahility 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 - the "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 rnilliwatts per unit area of flux. "black light" lamp - An ultraviolet lamp that emits a significant portion of its radiative power in the Uv-A band (315-400 nm). blending lighting - general illumination used to provide smooth transitions between the lighting areas on a stage. blinding glare - glare which is so intense that for an appreciable length of time after it has been removed, no object can be seen. Blondel-Rey law - an expression for the ratio of the thresholds of a squareform fashing light and of a steady light, in point vision conditions at night, as a function of the duration in seconds of the flash. borderlight - a long continuous striplight hung horizontally above a stage and aimed down to provide general diffuse illumination or to light the cyclorarna or a drop usually wired in three or four color circuits. borderline between comfort and discomfort (BCD) - the average luminance of a source in a field of view which 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 account is taken of whether the light comes from a reflecting, transmitting or self-luminous object. See also subjective brightness, luminance, ceiling luminance and brightness of a perceived light source color. brightness contrast threshold - the just-detectable contrast between two patches of color separated by a brightness contrast border, as in the case of a bipartite photometric field or of a disk-shaped object surrounded by its back- ground. 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. Ccandela, cd - the SI unit of luminous intensity, equal to one lumen per steradian (Im/sr). Formerly 'candle." See chapter 2, Measurement of Light and Other Radiant Energy. Note The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 10^12 Hz (wavelength approximately 555 nm) that has a radiant intensity in that direction of (001464) W/sr. The candela so defined is the base unit applicable to photopic, seotopic and mesopic quantities. 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. From 1948 to 1979 the unit of luminous intensity was defined in terms of a complete (blackbody) radiator. I cd was defined as the liminous intensity of 1/600,000 m^2 of projected area of a blackbody radiator operating at the temperature of solidification of platinum. at a pressure of 101,325 Pa. The difference between the candela and the old international candle is so small that only measurements of high accuracy are affected. candlepower - luminous intensity expressed in candelas. candlepower distribution curve - See intensity distribution curve. carbon-arc lamp - an electric-discharge lamp employing an arc discharge between carbon electrodes. One or more of these electrodes may have cores of special chemicals that contribute importantly to the radiation. cavity ratio (CR) - a number indicating cavity proportions calculated from length, width and height. See ceiling cavity ratio, floor cavity ratio and room cavity ratio. 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 indicating ceiling cavity proportions calculated from length, width and height. See chapter 9, Lighting Calculations. 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. lt permits seeing much finer detail than does peripheral vision. central visual field - that region of the visual field corresponding to the fovcal portion of the retina. channel - an enclosure containing the ballast, starter, lamp holders and wiring for a fluorescent lamp, or 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 which expresses the relationship between two variable properties of a light source, such as candlepower and voltage, flux and voltage, etc. 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 stale of chromatic adaptation. chromatic color - perceived color possessing a hue. In everyday speech, the term 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 in chromaticness or separated by a contrast border. A contrast border can involve differences contrast border can involve differences both in luminance and in chromaticity between the two sides. chromaticity coordinates (of a color) - X,y,Z the ratios of the three tristimulus values of the color to their sum. chromaticity diagram - a plane diagram formed by plotting 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 may be a difference in hue or saturation, or a combination of the two. chromaticity of a color the dominant or complementary chromaticity of a color - the dominant or complementary wavelength and purity aspects of the color taken together, or the aspects specified by the chromaticity coordinates of the color taken together. chromaticness - the attribute of a visual sensation according to which the to which the (perceived) color of an area appears to he more or less chromatic. CIE (L*,a*,b*) uniform color space (CIELAB) - a transformation of CIE tristimulus values X, Y, Z into three coordinates 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 eases of colorant mixtures. CIL (L*,u*,v*) uniform color space (CIELUV) - a transformation of CIE tristimulus values X, Y, Z into three coordinates 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, such as color television. CIE standard chromaticity diagram - a diagram 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 inadequate. clear sky - a sky that has less than 30% cloud cover. clearance lamp - a lighting device mounted on a vehicle for the purpose of indicating the overall width and height of the vehicle. clerestory - that part of a building rising clear of the roofs or other parts and whose walls contain windows for lighting the interior. cloudy sky - a sky having more than 70% cloud cover. coefficient of attenuation - the decrement in flux per unit distance in a given direction within a medium, defined by the relation where 1 is the flux at any distance x from a reference point having flux 1. More generally, where the coefficient varies from point to point along the path. coefficient of beam utilization (CB) - 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 the luminous flux (lumens) from a luminaire calculated as received on the workplane to the luminous flux emitted by the luminaire's lamps alone. See chapter 9, Lighting Calculations. coffer - a recessed panel or dome in a ceiling. cold-cathode lamp - an electric-discharge lamp whose mode of operation is that of a glow discharge, and having 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 may distinguish between two structure-free patches of light of the same size and shape. See light source color and object color. color difference threshold - the difference in chromaticity or luminance between two colors that makes them just perceptibly different. The difference may be in hue, saturation, brightness (lightness for surface colors) or a combination of the three. color comparison, or color grading (CIL, object color inspection) - the judgment of equality, or of the amount and character of difference, of the color of two objects viewed under identical illumination. color contrast thresholds - 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) - 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 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 subtenses greater than 4°. Tristimulus computational data for CIE standard color sources A and C are given in chapter 5. color preference index (CPI) - 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. 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) - 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 chromatieity 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 (CSI) - an arc source utilizing a mercury vapor arc with metal halide additives to produce illumination in the 5000-6000-K range. Requires a ballast and ignition system for operation. comparison lamp - a light source having a constant, but not necessarily known, luminous intensity with which standard and test lamps are compared successively. complementary wavelength (of a light) - 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 - diffusion 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 - the ratio of the illuminance on a surface at point 1 (due to flux directly received from lambertian surface 2) to the exitance of surface 2. lt 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 to the literature this ratio is also called the angle factor, illumination factor. point configuration factor, and sky factor. conical-directional reflectance - the ratio of the 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 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 - the ratio of the 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 - the ratio of the 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 he 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) - the absolute temperature of a blackbody whose chromaticity most nearly resembles that of the light source. cosine law - the law that the illuminance on any surface varies as the cosine of the angle of incidence. The angle of incidence O 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. See cosine-cubed law and inverse square law. cosine-cubed - law an extension of the cosine law in which the distance d between the source and surface is replaced by h/cos0 (d^2, where h is the perpendicular distance of the source from the plane in which the point is located. It is expressed by E = (1 cos^3 O)/h^2. counter-key light - illumination on a subject from a direction that is opposite to that of the key light. country beam - Sec 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 an " @" For example, a lighting system producing a luminance of 100 cd/m^2 over 60% of the specified area could have its luminance rating expressed as 60%@100 cd/m^2. 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 sight at which the bare source is not visible. Ddark adaptation - the process by which the retina becomes adapted to a luminance less than about 0.034 cd/m^2. daylight availability - the luminous flux from the sun and the 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 that 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. 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 - the process by which incident flux is redirected over a range of angles. diffuse transmission - the 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 workplane or on an object, that is not predominantly incident from any particular direction. diffuser - a device to redirect or scatter the light from a source. primarily by the process of diffuse transmission. diffusing panel - a translucent material covering the lamps in a luminaire to reduce the luminance by distributing the flux over an extended area. diffusing surfaces and media - those that redistribute at least some of the incident flux by scattering in all directions. See complete diffusion, incomplete diffusion, perfect diffusion, narrow-angle 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 which arrives at the workplane without being reflected by any room surfaces. See indirect component. direct glare - glare resulting from high luminances or sufficiently shielded light sources in the field of view. It is usually associated with bright areas, such as luminaires, ceilings and windows, which are outside the visual task or region being viewed. A direct glare source may 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 by luminaires distributing 90-100% 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 reaching the workplane directly to the downward component from the luminaire. directional-conical reflectance - the ratio of reflected flux collected through a conical solid angle to essentially collimated incident flux. Note The direction of incidence must be specified, as must the direction and extent of the cone. directional-conical transmittance - the ratio of transmitted flux collected through a conical solid angle to essentially collimated incident flux. Note The direction of incidence must he specified, as must the direction and extent of the cone. directional-hemispherical reflectance - the ratio of reflected flux collected over the entire hemisphere to essentially collimated incident flux. Note The direction of incidence must he specified. directional-hemispherical transmittance - the 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 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 may 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 definition 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 producing 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 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 luminaires, in a given visual environment for producing discomfort. It is the net effect of the individual values of the index of sensation for all luminous arcas in the field of view. See discomjtht glare Jàctor. See also chapter 9. Lighting Calculations. distal stimuli - any of the points. lines and surfaces and three-dimensional arrays of scattering particles which one can identify in the physical space in front of the eye and which form optical images on the retina. Each clement 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, arid each has a specifiable luminance and chromaticity. Points and lines are specific cases which have to be dealt with in terms of total intensity and intensity per unit length. 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) - 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 which directs the light downward and can be recessed, surface mounted or suspended . downward component - that portion of the luminous flux from a luminaire emitted at angles below the horizontal. See upward component. driving beam - See upper (driving) beams. dual headlighting system - headlighting by means of two double 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 may have two filaments supplying the lower beam and part of the upper beam, respectively. The lower or inner lamps have one filament providing the primary source of light for the upper beam. dustproof luminaire - a luminaire so constructed or protected that dust will not interfere with its successful operation. dustlight luminaire - a luminaire so constructed that dust will not enter the enclosing case. Eeffective ceiling cavity reflectance - a number giving the combined reflectance effect of the wall and ceiling reflectance of the ceiling cavity. Sec ceiling cavity ratio. effective floor cavity reflectance - a number giving the combined reflectance effect of the floor cavity. See floor cavity ratio. effective intensity - a quantity conventionally defined for a flashing light by where the source has constant magnitude I over a time duration . For a time-varying source, The unit is the candela. The times are chosen to maximize the calculated effective intensity. then found that the actual intensity I at time is equal to the calculated equivalent intensity I. 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. EISA (Energy Independence & Security Act of 2007) - The law that in 2007 set further stringent requirements for efficiency for various consumer products including light bulbs. This law has put a requirement into place which will eventually decrease the maximum wattage of general service incandescent light bulbs that will be allowed to be imported or manufactured. This law does not prevent the sale of product which has already been manufactured or imported. http://www1.eere.energy.gov/femp/regulations/eisa.html electric discharge - See are 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 pas- sage of an electric current through a vapor or gas. Sec fluorescent lamp, cold-cathode lamp, hot-cathode lamp, car- bon-arc lamp, glow lamp, fluorescent lamp and high-intensity discharge (HID) lamp. Note Electric-discharge lamps may he named after the fill gas or vapor that is responsible for the major portion of the radiation, as with mercury lamps, sodium lamps, neon lamps and argon lamps. 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. For angles above the horizontal, elevation is positive; for angles below, negative. 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 may 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 would compromise 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 may include any or all of the following: 1. utility power failure. 2. utility power voltage reduction (brownout) below the minimum required to support the arc in fluorescent or HID lamps. 3. 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 safety of life and property in the event of failure of the normal supply. Emergency lighting may inelude any or all of the following: 1. llumination of the means of egress of the premises during an emergency. 2. llumination of the signs which mark the means of egress. 3. Safety lighting necessary to enhance the safety of occupants remaining on the premises during an emergency. 4. Standby lighting to provide the illumination necessary to enable normal activities to continue during an emergency. emissivity - 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 - (1)The ratio of the 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 hlackbody radiator at the same temperature. (2) See exilance. Use of the term with this meaning is deprecated. enclosed and gasketed - See vapor-tight. EPAct (Energy Policy Act of 1992) - This law set a number of requirements for various consumer products including light bulbs. The positive effect of this law on efficiency was essentially nil. Examples of the changes effected by this law were that standard 40 watt T12 four foot bulbs were required to be lowered to 34 watts and 75 watt R30 shaped bulbs were required to change to 65 watts and to a BR30 shape. http://www1.eere.energy.gov/femp/regulations/epact1992.html equal interval (isophase) light - a rhythmic light in which the light and dark periods are equal. equipment operating factor - the flux of a high-intensity discharge (HID) lamp-ballast-luminaire combination in a given operating position as a fraction of the flux of the lamp-luminaire combination (1) operated in the position for rating lamp lumens and (2) using the reference ballasting specified for rating lamp lumens. Note If the given lamp operating position is not the same as the lamp rating position, the ratio of the flux for the operating ballast to that for the 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 - a 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 evalvator. equivalent contrast, actual - the equivalent contrast in a real luminous environment with nondiffuse illumination. This quantity may he less than the equivalent contrast defined above, on account of veiling reflection. We have CRF. See contrast rendition.factor. equivalent luminous intensity (of an extended source at a specified distance) - the intensity of a point source which would produce the same illuminance at that distance. Formerly, the apparent luminous intensity of an extended source. equivalent sphere illumination (ESI) - the level of sphere illumination which 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 aetinie UV radiation. UV-induced erythema is due to actinïc 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 1.0. Note This quantity formerly was called the "relative erythemal factor." The ervthemal efficiency of radiant flux of various wavelengths for producing a minimum perceptible erythema (MPE) is given in chapter 5, Nonvisual Effects of Radiant Energy. erythemal exposure - the product of erythemal flux density on a surface and time. It usually is measured in erythemal.Note Fur average untanned skin a minimum perceptible crythema requires about 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 he in erythemal microwatts. See erythemal efficiency of radiant flux and ervthemal unit. erythemal flux density - the erythemal flux per unit area of the surface being irradiated. It is equal to the incident erythemal flux divided by the area of the surface when the flux is uniformly distributed. It usually is measured in 1iW/cm2 of erythemally weighted UV radiation (erythemal sW/cm). See finsen. erythemal threshold - See minimal perceptible erythema. erythemal unit (Eu) - a unit of erythemal flux that is equal to the amount of radiant flux that will produce the same erythemal effect as 10 rW 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 - the ratio of the average initial (time zero) wall or ceiling cavity exitance to the lamp flux per unit floor area. Note Exitance is measured in lumens per unit area, where the units of area agree with those of the floor area. Average wall or ceiling cavity luminances can be determined by noting the underlying assumption of lambertian room surfaces where L = M/; here L is in candelas per unit area, where the units of area agree with those of M. The exitance coefficients are numerically identical to what were formerly called luminance coefficients. excitation purity (of a light) - the ratio of the distance on the CIE x, y 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 which is completely enclosed and capable of withstanding an explosion of a gas or vapor that may occur within it, and preventing the ignition of a 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 will not be 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 provide a specular reflection from the eyes, teeth. or jewelry without significantly increasing the total illumination of the subject. Ffar (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 measured in a plane through the nominal beam centerline. For beams that do not possess rotational symmetry, the beam angle is 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 the beam angle. fill light - supplementary illumination to reduce shadow 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. 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 of fluorescent materials exposed to "black light': where E is in fluorens per square meter, f1 is the glow factor, and f2is the 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 flashes of light of extremely short duration, flattery index (of a light source) - a measure appraising a light source for appreciative viewing of colored objects, or 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 (elf) or critical flicker frequency (eff). 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. The beam spread of floodlights may range from narrow field angles (10°) to wide ones (more than 100°). See heavy-dutv floodlight, generalpurpose (GP) floodlight, ground-area open floodlight and ground-area open floodlight with reflector insert. In theatre lighting, a floodlight (or flood) is a luminaire consisting of a lamp and spread reflector with resultant field angle greater than 100°. Fixed and variable beams types are available. floodlighting - a system designed for lighting a scene or object to a luminance greater than its surroundings. It may 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 indicating floor cavity proportions calculated from length. width and height. 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 320-400 nm. fluorescence - the emission of light (luminescence) as the result of, and only during, the absorption of radiation of shorter wavelengths. fluorescent lamp - a low-pressure mercury electric-discharge lamp in which a fluorescing coating (phosphor) transforms sonic 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 which 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 which may 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 - any instrument operated so as to follow the movement of an actor. Follow spots are usually high-intensity controlled-beam luminaires. footcandle - a unit of illuminance, equal to 1 lm/ft or 10.76 lx. footcandle meter - See illuminance (lux orfoorcandle) meter. footlambert - a lambertian unit of luminance equal to 1/ir (0.3183) ed/ft2. 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 east by overhead luminaires and to add general toning lighting from below. form factor - the ratio of the flux directly received by surface 2 (and due to lambertian surface i) to the total flux emitted by surface 1.It is used in flux transfer theory. Also, the ratio of the average illuminanee on surface i to the causative exitance of lamhertian 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 nf the retina, subtending about 2°, containing cones but no rods and forming the site of most distinct vision. foveal vision - See central (foceal) iislon. Fresnel spotlight - a luminaire containing a lamp and a Fresnel lens (stepped "flat" lens with a textured hack) which has variable field and beam angles obtained by changing the spacing between lamp and lens (flooding and spotting). The Fresnel spotlight produces a beam of light with a smooth, soft edge. fuselage lights - aircraft aeronautical lights, mounted on the top and bottom of the fuselage, used to supplement the navigation light. Ggas-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 - a measure of the average shift of eight standardized colors chosen to he of intermediate saturation and spread throughout the range of hues. If a color rendering index is not qualified as to the color samples used. R is assumed. general diffuse lighting - lighting involving luminaires which distrihute 40-60% of the emitted light downward and the batanee upward, sometimes with a strong component at 90° (horizontal). See direct -indirect lighting. general lighting - lighting designed to provide a substantially uniform level of illumination throughout an area, exclusive of any provision for special local requirements. See direct lighting, semidirect lighting, general diffuse lighting, direct- indirect lighting, semi-indirect lighting, indirect lighting, ceiling area lighting and localized general lighting. general-purpose (GP) floodlight - 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 luminance within the visual field that is sufficiently greater than the luminance to which the eyes arc adapted to cause annoyance, discomfort or loss in visual performance and visibility. Sec blinding glare, direct glare, disability glare and discomfort glare. Note The magnitude of the sensation of glare depends upon 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. glossmeter - an instrument for measuring gloss in terms of the directionally selective reflecting properties of a material at 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 jiA/mm2) at low cathode temperature, and a high voltage drop (typically 50 V or more). The 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 - times the luminance in cd/m2 produced on the material divided by the incident "black light" flux density in mW/m2. It may be measured in 1m/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 - a unit providing a weatherproof enclosure for the lamp socket and housing. No cover glass is required. ground-area open floodlight with reflector insert - 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. Hhard 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 may 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 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 a luminaire. heavy-duty floodlight (HD) - a weatherproof unit having a a weatherproof unit having a substantially constructed metal housing in which is placed a separate and removable reflector. A weatherproof hinged door with cover glass encloses the assembly but providesan unobstructed light opening at least equal to the effective diameter of the reflector. hemispherical-conical reflectance - the ratio of reflected flux collected over a conical solid angle to the incident flux from the entire hemisphere. reflected flux collected over a conical solid angle to the incident flux from the entire hemisphere. Note The direct ion and extent of the cone must be specified. hemispherical-conical transmittance – 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 transmittance - 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. Note If “reflectance” is not preceded by an adjective descriptive of the angles of view. hemispherical reflectance is implied. hemispherical transmittance - the ratio of the transmitted flux leaving a surface or medium to the incident flux. The use of this term is deprecated. Note If 'transmittance" is not preceded by an adjective descriptive of the angles of view. hemispherical reflectance is implied. high-bay lighting - interior lighting where the roof truss or ceiling height is greater that 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 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 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 which are designed to he 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 X l0 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 which 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 may be heated by the discharge or by external means. house lights - the general lighting system installed in the audience area (house) nf a theatre, film or television studio or arena. hue of a perceived color - the attribute that determines whether the color 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 for an approximation of daylight (5000-6000-K) illumination. Requires a ballast and ignition system for operation. Iice detection light - an inspection light designed to illuminate the leading edge of an aircraft wing to check for icc formation. ideal radiator - See blackbodv. identification beacon - an aeronautical beacon emitting a coded signal by means of which a particular point of reference can he 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 - the areal density of the luminous flux incident at a point on a surface. illuminance (lux or footcandle) meter - an instrument for measuring illuminance on a plane. Instruments which accurately respond to more than one spectral distribution are color corrected, that is, the spectral response is balanced to V(À ) or V'(À ). Instruments which 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 comprises 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 will indicate 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 filantent is of coiled or coiled-coil (doubly coiled) tungsten wire. however, it may he 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 which expresses the effects of source luminance, solid angle factor, position index and field luminance on discomfort glare rating. See chapter 9. Lighting Calculations. indirect component the portion of the luminous flux from a luminaire arriving at the workplane after being reflected by room surfaces. See direct component. indirect lighting - lighting by luminaires distributing 90- l00% of the emitted light upward. infrared lamp - a lamp that radiates predominately in the infrared region of the spectrum; the visible radiation is not of principal interest. infrared (IR) radiation - for practical purposes any radiant energy within the wavelength range of 770-10 nm. This radiation is arbitrarily divided as follows: Near (short-wavelength) IR 770-1400 nm, Intermediate IR 1400-5000 nm, Far (long-wavelength) IR 5000-1,000,000 nm Note In general, unlike UV energy, IR energy is not evaluated on a wavelength basis hut rather in terms of all of such energy incident upon a surface. Examples of these applications are industrial heating. drying, baking and photo reproduction. However, some applications. such as IR viewing devices. involve detectors sensitive to a restricted range of wavelengths; in such cases the spectral characteristics of the source and receiver are of importance. inhibition (visual) - reduction in magnitude of the sensation aroused by the stimulus (or a reduction in visual sensitivity) caused by some other situation that is adjacent spatially or temporally. initial lumens - lamp light output, measured while operated on a reference ballast in a laboratory at 77 degrees F, after being operated at the same conditions in a laboratory for 100 hours of operation after manufacture. initial luminous exitance - the density of luminous flux leaving a surface within an enclosure before interreflections occur. Note For light sources this is the luminous exitance defined as the luminous flux density at a surface. For non-self-luminous surfaces it is the reflected luminous exitance of the flux received directly from sources within the enclosure or from daylight. Input Watts/ANSI Watts (ballasts) - Input watts published by ballast manufacturers are the total watts consumed by both the ballast and the lamps it operates. ANSI watts are the rating given for a ballast measured under the strict testing procedures specified by ANSI standards and are the only dependable measure of this performance. Energy savings can be determined by comparing the input watts of different lighting systems. Input watts may be affected by tolerance build-up from the ballast, lamp, input voltage and ambient temperature. Input watts published in a catalog are for nominal conditions only. instant-start fluorescent lamp - a fluorescent lamp designed for starting by a high voltage without preheating of the electrodes. Note Also known as a cold-start lamp in some countries. integrating photometer - a photometer that enables total luminous flux to be determined by a single measurement. The usual type is the Ulbrieht sphere with associated photometric equipment for measuring the indirect luminance of the inner surface of the sphere. (The measuring device is shielded from the source under measurement.) intensity - a shortening of the terms luminous infensity and radiant intensity'. Often misused for level of illumination or illuminance. intensity 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 of a luminaire is obtained b taking measurements at various angles of elevation in a vertical plane through the light center: unless the plane is specified, the curve is assumed to represent an average suds as would he 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 horïzontal plane through the light center. internally illuminated exit sign - a transilluminated exit sign containing its own light source. interreflected component - the portion of the luminous flux from a luminaire arriving at the workplane after being reflected one or more times from room surfaces, as determined by the flux transfer theory. interreflection - the multiple reflection of light by the various room surfaces before it reaches the workplane or other specified surface of a room. 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 - the 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 = l/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 are follow spotlights). Irradiance - the density of radiant flux (power) incident on a surface. isocandela line - a curve plotted on any appropriate set of coordinates to show the distances in various directions in space, about a source of light, at 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 (isofootcan-die) diagram. Kkey 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. Llaboratory reference standard - the highest-ranking order of standards at each laboratory. lambert – a lambertian unit of luminance equal to 1/ir (0.3183) cd/cm2. 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 - 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 an artificial source of light. By extension, the term is also used to denote sources that radiate in regions of the spectrum adjacent to the visible. Note A lighting Unit 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 factor (LLD) - the fractional loss of lamp lumens at rated operating conditions that progressively occurs during lamp operation. lamp position factor - The ratio of the flux of a high-intensity discharge (HID) lamp at a given operating position to the 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 - the angle between the plane of the baffles or louver grid and (he plane most nearly horizontal that is tangent to both the lamps and the louver blades. Note The lamp shielding angle is formed by a straight line tangent to the lowest part of the brïghtness area to be shielded. If H is the vertical distance from the brightness source to the bottom of the shielding element, and D is the horizontal distance from the brightness source to the shielding element, then the lamp shielding angle 'b = arctan(H/D). The lamp shielding angle frequently is larger than the louver shielding angle. hut never smaller. See louver shielding angle. Fig. 1. 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. Lample shielding element th arctan H/D. 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 hounded 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 candlcpower. See reference line and width Iine. lens - a glass or plastic element used in luminaires to change the direction and control the distribution of light rays; also, that part of the eye which allows objects at different distances to be focused onto the retina. level of illumination - See luminance. life performance curve - a curve which represents the variation of a particular characteristic of a light source (such as luminous flux or intensity) throughout the life of the source. Note Life performance curves sometimes are called maintenance curves, as, 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 may 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 also sometimes called light. Visual sensations arc 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. lt is a physical stimulus which can be applied to the retina. (See spectral luminous efficacy of radiant flux and calues of spectral luinmous efficiency for photopic vision.) light adaptation - the process by which the retina becomes adapted to a luminance greater than about 3.4 cd/ni2. 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 (of a lamp) - 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 that emits light depending on its physical construction, material used and exciting current. The output may be in the IR or in the visible region of the spectrum. 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 lumen Output and if no system variation or depreciation had occurred. Components of this factor may 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 may be maintained in service. However, this phrase was formerly often defined as the ratio of average illuminance in service to initial illuminance. light meter - See illurninance (lux or footcandle) meter, light source color - the color of the light emitted by a source. Note The color of a point source may be defined by its luminous intensity and chromaticitv coordinates; the color of an extended source may he defined by its Iuminance and chromaticity coordinates. See perceived light source color, color temperature and correlated color temperature. lighting effectiveness factor (LEFV) - the ratio of equivalent sphere illumination to ordinary measured or calculated illumination. 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 may 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 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 illumination 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) is generally of 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 - a series of baffles used to shield a source from view at certain angles, to absorb or block unwanted light or to reflect or direct light. The baffles usually are arranged in a geometric pattern. louver shielding angle - the angle between the horizontal plane of the baffles or louver grid and the plane at which the louver conceals all objects above. See figure 2 and 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 comprising 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 truss 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 small arcas of light grays and whites. Sec high-key lighting. low-pressure mercury lamp - a discharge lamp (with or with- out a phosphor coating) in which the partial pressure of the mercury vapor does not exceed 100 Pa during operation. low-pressure sodium lamp - a discharge lamp in which light is produced by radiation from sodium vapor operating at a partial pressure of 0.1-1.5 Pa (approximately 10 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 usc in congested areas and on highways when meeting other vehicles within a distance of 300 m (1000 ft). Formerly "traffic beam." lumen, Im - the SI unit of luminous flux. Radiometrically, it is determined from the radiant power. Photometrically, it is the luminous flux emitted within a unit solid angle (1 sr) by a point source having a uniform luminous intensity of t cd. lumen depreciation - see lumen maintenance lumen maintenance - A measure of how well a lamp maintains its light output over time. It may be expressed numerically or as a graph of light output vs. time. The “mean lumens” of a lamp is the lumens at 40% of rated life (50% for HPS lamp). 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 illuminanee on the workplane. It takes into account both direct and reflected flux. lumen-second, Im - 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 one lumen. luminaire (light fixture) - a complete lighting unit consisting of a lamp or lamps and ballasting (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 considcration a variation in ambient temperature surrounding the luminaire, the means and the conditions of mounting the luminaire, and the use of anyinsulation in conjunction with the application of the luminaire. luminaire dirt depreciation factor (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 workplane. The SC of a luminaire is an estimated maximum ratio of spacing to mounting height above the workplane for a regular array of that luminaire such that the workplane 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-mountingheight 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 (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 may he leaving, passing through or arriving at the surface. Formerly, "photometric brightness." By introducing the concept of luminous intensity, luminance may be expressed as L: dl/(dA cos0). Here, the luminance at a point of 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 may be measured at a receiving surface by using L = dE/(dA eos O). This value may he 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 magnitude of sensation which 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,' 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 - a coefficient similar to the coefficient of utilization used to determine wall and ceiling luminances. An obsolete term; see exitance coefficient. luminance contrast - the relationship between the luminances of an object and its immediate background. It is equal to IL1 - L/L1 = L/L1, where L1 and L, arc equal to lLt the luminances of the background and object, respectively. It must be specified which of L and L2 is greater. The ratio SL/LI is known as Weber's fraction. Note See note under luminance. Because of the relationship among luminance, illuminance and reflectance, contrast often s expressed in terms of reflectance when only reflecting surfaces are involved. Thus, the contrast is equal to P2 Pi - where are the reflectances of the background and object, respectively, This method of computing contrast holds only for perfectly diffusing (matte) 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. It usually is applied to contiguous areas, such as the detail of a visual task atid its immediate background, in which case it is quantitatively eqital to the numerator in the formula for luminance contrast. Note See note under luminance, luminance factor - 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 I. but the luminance factor may have any value greater than O. 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 Iouvered ceiling. luminous density - 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 total luminous flux emitted by a lamp divided by the total lamp power input. lt 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 - 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 - radiant flux (radiant power); the time rate of flow of radiant energy, evaluated in terms of a standardized visual response: where iF t. = KrnftiV(À) dÀ is in lumens, is in watts per nanometer, À is in nanometers, y(A) is the spectral luminous efficiency. and Km 5 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 J/'(À) and the corresponding maximum spectral luminous efficacy K are substituted into the above equation. K,1 and K1 are derived from the basic SI definition of luminous intensity and have the values 683 and 1754 lm/W, respectively. luminous flux density at a surface – the luminous flux per unit area at a point on a surface. Note This need not he a physical surface: it may also be a mathematical plane. See also illuminance and luminous exitance. luminous intensity (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) which the surface subtends at the source. Luminous intensity may be expressed in candelas or in lumens per steradian (Ini/sr). Note Mathematically a solid angle must have a point a its apex: the definition of luminous intensity, therefore. applies strictly only to a point source. In practice, however. light eminating from a source whose dimensions are negligible in comparison with the distance from which it is observed may 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/distancel, and (2) the average luminance of the projected source area as seen from the receiving point does not vary appreciably. The bare word "intensity" is sometimes used to designate luminous intensity. However, t is also widely used in other ways, either formally or informally, in other disciplines. "Stimulus intensity" may mean either the retinal illuminance of a proximal stimulus or the luminance of a distal stimulus." Intensity" is used in the same sense with respect to other modulates such as audition (sound). The same word 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 aniplitude. luminous intensity distribution curve - See intensity distribution curie. 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, y(A). 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 spectral luminous efficiency of radiant flux, Y(A). Note Unless otherwise qualified, the term 'transmittance' means luminous transmittance. lux, lx - the SI unit of illuminancc. One lux is one lumen per square meter (lm/m2). (See the Appendix for conversion values.) lux meter - Sec i/luminance (lux or footcandle) meter, Mmaintenance 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 pre-dominantly 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 and which passes through the luminous center of the lamp. mean lumens (nominal mean lumens) - the nominal light output of the lamp, measured on a reference ballast under laboratory conditions at 77 degrees F, after being operated on a reference ballast under laboratory conditions at 77 degrees F for 40% rated life (50% rated life for HPS bulbs). mean spherical luminous intensity - the average value of the luminous intensity in all directions for a source. Also, the total emitted luminous flux of the source divided by 4rr. mean zonal candlepower - the average intensity (candelas) of a symmetrical luminaire or lamp at an angle to the luminaire or lamp axis which 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 . 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 of 10 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) which 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 photopie and scotopie vision, that is, between about 3.4 and 0.034 cd/rn2. 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 is also used to denote objects which, when illuminated by a given source and viewed by a given observer. produce nietameric lights. middle ultraviolet - a portion of the electromagnetic spectrum in the range of 200-300 nm. minimal perceptible erythema (MPE) - the erythemal thresh old. mired - See recprocal color temperature. modeling light - that illumination which reveals the depth, shape and texture of a subject. Key light, cross light, counter-key light, side light, back light and eye light are types of modeling light. modulation threshold - threshold value of the quantity in terms of which manipulation of luminance differences can he specified in the case of a sine wave grating. 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 visual field of 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 (MH1) - 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 (MH\V1) - the distance from the workplane to the light center of the luminaire, or to the plane of the ceiling for recessed equipment. multiple-beam headlamp - a headlamp designed to permit the driver of a vehicle to use any one of two or more distributions of light on the road. Munsell chroma - an index of perceived chroma of the object color, defined in terms of the luminance factor Y and chromaticity coordinates x, y for CTE 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 - an index of the hue of the perceived object color defined in terms of the luminance factor Y and chromaticity coordinates x, y for CTE Standard Illuminant C and the CIE 1931 Standard Observer. Munsell value, V - an index of the lightness of the perceived object color defined in terms of the luminance factor Y for CIE Standard Illuminant C and the CJE 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 =1l.6(Y/1O0)'3 1.6 or within ±0.6 by V = Y'2, where Y is the luminance factor expressed in percent. Nnanometer, nm - a unit of wavelength equal to 10 m. See the Appendix for conversion values. narrow-angle diffusion - that in which flux is scattered at angles near the direction that the Ilux would take by regular reflection or transmission. See wide-angle diffusion. narrow-angle luminaire - a luminaire that concentrates the light within a cone of small solid angle. See wide-angle lumin aire. national standard of light - a primary standard of light which has been adopted as a national standard. See primaly standard of light. navigation lights - an alternative terni 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 suns disk is 6° below the horizon. nit, nt - a unit of luminance equal to I cd/m2. Note The candela per square meter (cd/m) is the International Standard (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, consisting of those components that arc due to the lamps operating at other than their rated luminous value. These factors are 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 may be used. Oobject color - the color of the light reflected or transmitted by an object when illuminated by a standard light source, such as CJE source A, 13, C or D5. See standard source and perceited object color. obstruction beacon - See hazard 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. 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 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; that is, no sun is visible. overhang - the distance between a vertical line passing through a luminaire and the curb or edge of a roadway. ozone-producing radiation - UV energy of wavelength shorter than about 220 nm that decomposes oxygen thereby producing ozone (03). Some UV sources generate energy at 184.9 nm that is particularly effective in producing ozone. Ppanel (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 - one that has 30-70% 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 AB '00A + B 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 (such that the luminance or radiance is the same in all directions). perimeter lights - aeronautical ground lights provided to indicate the perimeter of a landing pad for helicopters. period - life the time interval until lamps arc 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 s). phot, ph - the unit of illuminance when the centimeter is taken as the unit of length; it is equal to one lumen per square centimeter. 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, photo reproduction 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 densitometer, goniophotometer, illuminance (lux or footcandle) meter, integrating photometer, ref lectometer, spectrophotometer and fransmissometer. photometry - the measurement of quantities associated with light. Note Photometry maybe visual, in which the eye LS 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 (natural or artificial) light-dark cycle to which living organisms may be exposed, as for example the natural cycle at the earth's equator of light (L) for 12 h and darkness (D) for 12 h, 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 t'ision. photosynthetic irradiance - irradiance within the wavelength band 400-700 nm, measured in W/m2. photosynthetic photon flux density (PPFD) the number of photons per unit time and per unit area in the wavelength band 400-700 nm. It is measured in units of micromoles (formerly microeinsteins) per second per square meter [mol/(s . m2)I. One mole contains 6.022 X 10 photons. photosynthetically active radiation (PAR) - photon flux in the wavelength band 400-700 nm. phototherapy - the treatment of disease involving the use of optical radiation. physical photometer - an instrument containing a physical receptor (such as a photoemissive cell, barrier-layer cell or thermopile) 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 dl3 - L3 = = c11À in which L3 is the spectral radiance, dl3 is the spectral radiant intensity, dA' is the projected area (dA cos () of the aperture of the blackbody, e is the base of natural logarithms (2.71828), T is the absolute temperature in kelvins, and CIL and c, are constants called the first and second radiation constants. Note The symbol c1 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 arca A'. of the source. Numerical values are commonly given not for CIL hut for c, which applies to the total flux radiated from a blackbody aperture, that is, in a hemisphere (2r sr), so that, with the Lambert cosine law taken into account, l0 (, c11. The currently recommended value of c is 3.741832 X W m, or 3.741832 X l.t9t0621 X l0' m2 sr lO_I2 I, W cm2. Then c1 is or 1.191(1621 X 10_12 sr If, as is more convenient, wavelengths are expressed in micrometers and arca in squarc centimeters, then c11 = 1.1910621 x io W stn4 cm2 sr cm2 sr I LA w cm being given in W . .sm L The currently recommended value of c 15 1.438786 X 10-2 ni K. The Planck law in the following form gives the energy radiated from the blackbody in a given wavelength interval (À1, À7): Q = fQ3dÀ =AtcifÀ2À(e (PPFD) If A is the area of the radiation aperture or surface in square centimeters, 1 is the time in seconds, A is thesquare centimeters, t is the time in seconds, À is the wavelength in micrometers, and rm4 cm2. then Q = 3.7418 X 1O W wavelength in micrometers, and ct:3.7418 x 104 W' pma ' cm then Q is the total energy in watt-seconds, or joules (J), emitted from this area (that is, in the solid angle 2v-), in time t, within the wavelength interval (Ai, A2). planckian locus – See biackbody (planckian) locus. planoconvex spotlight - a spotlight embodying a planocon- vex lens and a lamp movable within the housing in relation to the lens in order to vary beam and Ield 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 - a point at which luminance distributions are calculated. For most purposes it may 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 al 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 installa- tions, by use of luminaire photometric data. Note The direct component of illuminauce 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 the point. point source - a source of radiation the dimensions of which arc small enough, compared with the distance between the source and the irradiated surface, for them to be neglected in calculations and measurements. point vision - the niode 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' angular subtense. polarization - the process by which unpolarized radiation is polarized. It can he accomplished by either a reflection 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 which 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 system. 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 I 1O to the left. a green light showing from dead ahead ti) I 1400 fl2 to the right. and a white light showing to the rear through 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 locating 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 of which the outer bulb is formed of two pressed parts that are fused or sealed together; namely, a reflectorized bowl and a cover, which may 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. 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 standard of light - a light source by which the unit of light is established and from which the values of other standards are derived. See national standard of light. Note A satisfactory primary (national) standard must he reproducible from specifications (see candela). Primary (national) standards usually are found in national physical laboratories such as the National Institute of Standards and Technology in the United States. projection lamp - a lamp with physical and luminous characteristics suited for projection systems such as 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 stimulus - the distribution of illuminance on the retina 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 in 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 photopie 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 - favorable distribution of luminance in a visual environment, with regard to visual performance, visual comfort, ease of seeing, safety and esthetics or the specific visual tasks involved. quantity of light (luminous energy) - Q = f c1 dt the product of the luminous flux by the time it is maintained. It is the time integral of luminous flux (compare light and 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 may 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). RRadiance, L =d2T'/(dw dA coso) =dI/(dA coso) (in a direction, at a point of 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 O is the angle between the normal to the element of the source and the given direction. radiant energy density - = dO/dV radiant energy per unit volume; typical units, joules per cubic meter (J/m3). radiant energy - Q energy traveling in the form of electromagnetic waves. It is measured in units of energy such as joules, ergs or kilowatthours. See spectral radiant energy. 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) - =dQ/dt the time rate of flow of radiant energy. lt is expressed preferably in watts (W), that is, joules per second. 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 =db/dw the radiant flux proceeding from a source per unit solid angle in the direction considered, often measured in 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 ohseied may 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 proiccted 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 may be used; for example, the life may be based on the average time until the lamp type produces no more than a given fraction of the 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 luminlaire. reciprocal color temperature - color temperature pressed on a reciprocal scale (l/Tk). An important use stems from the fact that a given small increment in reciprocal color temperature is approximately equally perceptible regardless of the color temperature itself. 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). The reciprocal color temperature expressed in MK has the numerical value of 105/Tk when Tk Tk ex- 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 which give the fractional light loss that can be recovered by cleaning or lamp replacement. redirecting surfaces and media - those surfaces and media that change the direction of the flux without scattering the redirected flux. reference ballast - a ballast specially constructed to have certain prescribed characteristics for use in testing electric-discharge lamps and other ballasts. reference line (roadway lighting) - either of two radial lines where the surface of the cone of maximum candlepower 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 - p : Q,/Qi the ratio of the rcflccted flux to the incident flux. Reflectance is a function of: l. Geometry:1. Geometry: a. of the incident flux; b. of collection for the retlecteci 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 nf the reflected flux are stated, t should be considered that the incident flux is unpolarized and that the total reflected flux (including all polarization) is evaluated. Spectral reflectance depends only on the beam geometry sud 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 arc always hiconical in nature. Direetional refiectances cannot exist. since one component would he finite while the other was infinitcsimal here the reflectance distribution function is required. T-lowever. 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 2n- conical angle would increase complexity without appreciably affecting the immediate results. In each case of conical incidence or collection, the solid angle need not he a right cone, hut may he 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 incident 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 he reflected into the same reflected-beam geometry by an ideal (glossless), perfectly diffuse (lambertian) standard surface irradiated in exactly the same way as the sample. Note the analogies to reflectance in the fact that nitic canonical forms are possible. that "spectral" may be applied as a modifier, and that one may have a luminous or radiant reflectance factor, for example. Note also that reflectance cannot exceed I. but the reflectance factor may have any nonnegative value. 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 ieiling reflection. reflection - a general term for the process by which the incident flux leaves a 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 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 may 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 may 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 luminous 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:Vacuum ultraviolet: Extreme ultraviolet 10-100 nm Far ultraviolet 100-200 nm Middle Ultraviolet 200-300 nm Near Ultraviolet 300-380 nm Visible 380-770 nm Near (short-wavelength) infrared 770-1400 nm Intermediate infrared 1400-5000 nm far (long-wavelength) infrared 5000-1,000,000 nm Note The 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 may differ according to the phenomena of interest. Another division of the UV spectrum often used by photobiologists is given by the CIE: UV-A 3 15-400 nm CV-B 280-315 nm UV-C 100-280 nm regressed luminaire - a luminaire mounted above the ceiling with its opening above the ceiling line. See flush-mounted, surface-mounted, suspended and troffer. 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 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 flux Icaving 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 lumitious 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. It is 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 uisual 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, in minutes of arc, between their centers when they can just be distinguished as two objects. resultant color shift - the difference between the perceived color of an object illuminated by a test source and that of the same object illuminated by the reference source, taking account of the 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, 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 which 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 workplane, 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. Sec zonal-cacity interreflectatce 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 "intert]ectancc." lt is based on the nux emitted by a complete luminaire, whereas the coefficient of utilization is based on the flux generated by the bare lamps in a luminaire. room surface dirt depreciation (RSDD) - the fractional loss of task illuminance due to dirt on the room surface. 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 - lights installed in the surface of the runway along the centerline, indicating the location and direction of the 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 light - 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 to 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 with respect to 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, representing the horizontal distance a pilot will see 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. Ssaturation 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 sonic 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/m. See photopic vision. 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 (such as automotive headlamps or aircraft landing lights) to distinguish such lamps from similar devices in which the light source is replaceable within the reflector-lens unit. sealed beam headlamp - an integral optical assembly de- signed for headlighting purposes, identified by the name "Sealed Beam" branded on the lens. searchlight - a projector designed to produce an approximately parallel beam of light. and having an optical system with an aperture of 20 cm (8 in.) or more. 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, Iaboratory secondary (reference) standards are maintained at other photometric laboratories. A self-calibrated detector may be used as a secondary standard. self-ballasted lamp - any arc discharge lamp of which the current-limiting device is an integral part. Note Known as a "blended lamp' in some countries. 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. semidirect lighting - lighting by luminaires distributing 60-90% of their emitted light downward and the balance upward. semi-indirect lighting - lighting by luminaires distributing 60-90% of their emitted light upward and the balance downward. service period - the number of hours per day for which daylighting provides a specified illuminance. It often is stated as a monthly average. set light - in theatrical lighting, the separate illumination of the background or scenic elements. shade - a screen made of opaque or diffusing material de- signed 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 1-10 mm). Note This type of lamp (often 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 camera. 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 may also indicate its length. signal shutter - a device that modulates a beam of light by mechanical means for the purpose of transmitting intelligence. signaling light - a projector used for directing light signals toward a designated target zone. size threshold - the minimum perceptible size of an object. It also is defined as the size that can he detected some specific percentage of the times it is presented to an observer, usually 50%. It usually is measured in minutes of arc. See risual 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 - diffuse illumination that produces soft-edged. poorly defined shadows on the background when an object is placed in its path. Also, a Itiniinaire 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 X l011 m (92.9 X 10 6 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 characteristics similar to those of the sun outside the earth's atmosphere. solid angle - a measure of that portion of space about a point bounded by a conic surface whose vertex is at the point. lt 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. Note Solid angle is a convenient way of expressing the arca of light sources and luminaires for computations of discomfort glare factors. It combines into a single number the projected area A5 of the luminaire and the distance D between the luminaire and the eye. lt usually is computed by means of the approximate formula in which A5 and D2 are expressed in the same units. This formula is satisfactory when the distance D is greater than about three times the maximum linear dimension of the projected area of the source. Larger projected areas should be subdivided into several elements. solid angle factor Q - a function of the solid angle w subtended by a source at a viewing location, given by Q=20.4w + 1.52w°2 -- 0.075. 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 - 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 - a measure of the 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, e(O, ó, À, T) (of an element of surface of a temperature radiator at any 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, (À, T) (of an element of surface of a temperature radiator) – 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, the word " total" is ambiguous and should be avoided, since it may also refer to the spectral total (all wavelengths) or to the directional total (all directions). spectral (spectroscopic) lamp - a discharge lamp that radiates a significant portion of its radiative power in a line spectrum and that, in combination with filters, may be used to obtain monochromatic radiation. spectral luminous efficacy of radiant flux - the quotient of the luminous flux at a given wavelength by the radiant flux at that wavelength. lt is expressed in lm/W. 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 sonietimes called the "mechanical equivalent of light." The most probahle value is 0.00146 W/Im, corresponding to 683 lm/W as the maximum possible luminous efficacy. For scotopic vision values the maximum luminous efficacy is 1754 lni/W. spectral luminous efficiency for photopic vision - V(I) See values of spectral luminous efficiency for photopic vision. spectral luminous efficiency for scotopic vision - y(A) 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 - radiant energy per unit wavelength interval at wavelength A; typical units are J/nm. spectral radiant exitance - See radiant flux density. spectral radiant flux - radiant flux per unit wavelength interval at wavelength A; typical units are W/nm. spectral radiant intensity - radiant intensity per unit wavelength interval; typical units are W/(sr ' nm). spectral reflectance of a surface or medium - the ratio of the reflected flux to the incident flux at a particular wavelength, A, or within a small band of wavelengths, A, about A. Note The various geometrical aspects of reflectance may each he considered restricted to a specific region of the spectrum and may be so designated by the use of the adjective "spectral." spectral-total directional emissivity (of an element of surface of a temperature radiator in a given direction) - the ratio of the radiance of the thermal radiator at temperature T, at a point (x, y) and in the given direction (8, 4), to that of a blackbody at the same temperature T where O and are directional angles and T is temperature. spectral-total hemispherical emissivity (of an element of surface of a temperature radiator) - the ratio of its radiant exitance to that of a blackbody at the same temperature. spectral transmittance of a medium - the ratio of the transmitted flux to the incident flux at a particular wavelength A or within a small hand of wavelengths, SA, about A. Note The various geometrical aspects of transmittance may each he considered restricted to a specific region of the spectrum and may he so designated by the use of the adjective "spectral." spectral tristimulus values - See color-matching functions. spectrophotometer - an instrument for measuring the transmittanee 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 loctis of points representing the colors of the visible spectrum in a chromaticity diagram. specular angle - the angle between the perpendicular to a surface and the reflected ray; it is numerically equal to the angle of incidence. and it lies in the same plane as the incident ray and the perpendicular but on the opposite side of the perpendicular to the surface. 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 X l0 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 required for something to be seen. sphere illumination - illumination on a task from a source providing equal luminance in all directions about thai task, such as an illuminated sphere with the task located at the center. spherical reduction factor - an obsolete term: the ratio of the mean spherical luminous intensity io the nican horizontal intensity. 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. 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. 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 - in colorimetry, an artificial source that has a specified spectral distribution and is used as a standard. standard source A - a tungsten filament lamp operated at a color temperature of 2856 K, and approximating a blackbody operating at that temperature. standard source B - an approximation of noon sunlight bay standard source B an approximation of noon sunlight having a correlated color temperature of approximately 4874 K. It is obtained by a combination of source A and a special filter. standard source C - an approximation of daylight provided by a combination of direct sunlight and clear sky, having a correlated color temperature of approximately 6774 K. lt is obtained by a combination of source A plus 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 of a blackbody is proportional to the fourth power of its absolute temperature. Note The currently recommended value of the Stefan-Boltzmann constant is 5.67032x 1 0 - 8W 'm 2 'K- 1, or . o r 5 . 6 7 0 3x2 lo -l : w'cm l .K 1. 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 arca 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/cm. The tise 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, retractor 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 they are used: a light activated switch may he mounted on the luminaire. street lighting unit - the assembly of a pole or lamp post with 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 horderlights, footlights or cyclorama lighting from above or below. 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. sunlamp - an ultraviolet lamp that radiates a significant portion of its radiative power in the UV-B hand (280-315 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 he 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 mounted directly on a ceiling. suspended (pendant) luminaire - a luminaire hung from a ceiling by supports. switch start fluorescent lamp - See preheat (switch start) fluorescent lamp. Ttable lamp - a portable luminaire with a short stand, suitable for standing on furniture. tail lamp - a lighting device used to identify the rear of a vehicle by a warning light. talbot, T - a unit of light, equal to crnc lumen-second. tanning lamp - an ultraviolet lamp that radiates a significant portion of its radiative power in the Uy-A or UV-B band. task lighting - lighting directed to a specific surface or area that provides illumination for visual tasks. 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. taxi-channel lights - aeronautical ground lights arranged along a taxi channel of a water aerodrorne to indicate the route to be followed by taxiing aircraft. taxi light - an aircraft aeronautical light designed to provide necessary illumination for taxiing. 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. taxiway centerline lights - taxiway lights placed along the centerline of a taxiway except on curves or corners having fillets, where they 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. temperature radiator - an ideal radiator whose radiant flux density (radiant exitance) is determined by its temperature and the material and charactcr 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 associated with 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, that is, accuracy 50% of the time. However, the threshold can also be defined as the value of the physical variable that permits the object to be just barely seen. The threshold may he determined by merely detecting the presence of an object, or it may 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 placed 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 sending all or nearly all of its light upward. tormentor light - a luminaire mounted directly behind the sides of the stage arch. total emissivity - See spectral-total directional emissi'ity and spectral-total hemispherical emissiL'ity. 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 when the searchlight is in a position designated as having zero train. transient adaptation factor (TAF) - a factor which 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. 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 regular transmission, diffuse transmission and transmittance. transmissometer - a photometer for measuring transmittance. Note Transmissometers may be visual or physical instruments. Transmittance - 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 tise surface as well as absorption within the material operate to reduce tise transnìittance. Trans- mittance 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 Ilux; 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 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. Spectral transmittance depends on the beam geometry and the character of the transmitting surfaces and media (and on polarization). In addition, luminous transmittance is a function of the spectral distribution of the incident flux. If no qualifying geometric adjective is used, transmittance for hemispherical collection is meant. In each case of conical incidence or collection, the solid angle need not be a right cone, but may he of any cross section, including a rectangle, a ring, or a combination of two or more solid angles. These concepts must be applied with care if the area of the transmitting element is not large compared to its thickness, in view of internal transmission across the boundary of the area. For many geometrically specified transmittance 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 incident radiance (luminance) as well as the beam geometry and the character of the transmitting surfaces or media. Most transmittance quantities are 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 or rays. [he 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). 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 recessed lighting unit, usually long and installed with the opening flush with the ceiling. The term is derived from 'trough" and 'coffer." troland - a unit of retinal illuminance which is based upon the fact that retinal illuminance is proportional to the product of the luminance of the distal stimulus and the area of entrance pupil. One troland is the relinal illuminance produced when the luminance of the distal stimulus is i cd/rn2 and the area of the pupil is 1 mm2. Note The troland makes no allowance for interocular attenuation or for the Stiles-Crawford effect. tube - See lamp. tungsten-halogen lamp - a gas-fïlled tungsten 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. turn signal operating unit - that part of a signal system by which the operator of a vehicle indicates the direction a turn will he made, usually by a flashing light. Uultraviolet lamp - a lamp which radiates 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 lO-3S0 nni. 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: Ozone-producing 180-220 nm Bactericidal (germicidal) 220-300 nm ErythemaI 280-320 nm Black light" 320- 400 nm There are no sharp demarcations between these hands, the indi. cated effects usually being produced to a lesser extent hs longer and shorter wavelengths. For engineering purposes. the black light" region extends slightly into the visible portion of the spectrum. Another division of the ultraviolet spectrum often used hs photobiologists is given by the CJE: UV-A 315-400 nm UV-B 280-315 nm UV-C tOO-280 nm units of luminance - the luminance of a surface in a specified direction may be expressed as luminous intensity per unit of projected arca of surface or as luminous flux per unit of solid angle and per unit of projected surface area. Note Typical units are the cd/m2 [lm/(sr . m2)] and the cd/ft2 [lm/(sr ft2 )l. 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 mf the luminance in all directions withi,m the lmenmisp/mere 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 (fi..), equal to 1 lm/ft2. This method of specifying luminance is equivalent to staling the number of lumens that would leave the surface if the surface were replaced by a perfectly diffusing surface with a luminance in all drectmoims seit/min the henmisphere equal to time luminance of time actual surface mn 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 amid for use on the open highway when not meeting other vehicles. 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 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(l) values for spectral luminous efficiency at 10-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 on 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 The standard values of spectral luminous efficiency were determined by observations with a 2° photometric field having a moderately high luminance. and photometric evaluations based upon them consequently do not apply exactly lo other conditions of observation. Power in watts weighted in accord with these standard values is often referred to as being measured in "light watts." values of spectral luminous efficiency for scotopic vision - V'(À) values of spectral luminous efficiency at I0-nm intervals (see chapter 1, Light and Optics) were provisionally adopted by the CJE in 1951. Note These values of spectral luminous efficiency were determined by observation hs' young dark-adapted observers using extrafoveal vision at near-threshold lumïnance. vapor-tight luminaire - a luminaire designed and approved for installation in damp or wet locations. lt also is described as "enclosed and gasketed." VASIS (Visual Approach Slope Indicator System) - the systern 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 upon his or her position with respect to the glide path. veiling luminance - a luminance superimposed on the retinal image which reduces its contrast. It is this veiling effect produced by bright sources or arcas 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 and that partially or totally obscure the details to be seen by reducing the contrast. This sometimes is called reflected glare. Another kind of veiling reflections occurs when one looks through a plate of glass. A reflected image of a bright element or surface may he seen superimposed on what is viewed through the glass plate. vertical plane of a searchlight - the plane through the axis of the searchlight drum which 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 view conditions, having the same threshold as the given object. See risibility (meteorological). visibility (meteorological) - the greatest distance, expressed in kilometers or miles, that selected objects (visibility markers) or lights of moderate intensity (25 cd) can be seen and identified under specified conditions of observation. 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 4' disk exposed for 0.2 s. vision - See central (foneal) 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 subtended by an object or detail 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 percentage 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. It 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. The field may be monocular or binocular. See monocular visual field, binocular portion of the visual field, central 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; 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 - a photometer in which the equality of brightness of two surfaces is established visually. See physical photometer. Note The two surfaces usually arc viewed simultaneously side by side. This method is used in portable visual luminance meters. lt is satisfactory when the color difference between the test source and comparison source is small. However, when there is a substantial color difference, a flicker photometer provides more precise measurements. tn 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 it can he seen and identified. visual surround - all portions of the visual field except the visual display used in performing a task. visual task - conventionally, those details and objects that must be seen for the performance of a given activity, including their immediate background. Note This term is a misnomer in that it refers to the visual display itself and not the task of extracting information from it. That task in turn 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 visibility level (VL). voltage-to-luminaire factor - the fractional loss of illuminance due to improper voltage at the luminaire. Wwavelength - the distance between two successive points of a periodic wave, in the direction of propagation, at which the oscillation has the same phase. The use of the terms micron and angstrom is deprecated. Weber's fraction - See luminance contrast. wide-angle diffusion - diffusion 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 a cone of large solid angle. See also 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 half-maximum candlepower on the lateral candlepower distribution curve plotted on the surface of the cone of maximum candlepower. 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: L5 dl - = dA' = c11Af(AT) The two principal corollaries of this law are AmT L T5 = b = b' which show how the maximum spectral radiance the wavelength Am at which it occurs arc related to the absolute temperature T. See Wien radiation law. Note The currently recommended value of h is 2.897$ X l0 m K or 2.8978 X 10-' cm K. From the Planck radiation law. and with the use of thc values of h, c1 and c,as givcn abus>. h i. found to he 4.0956>< l04W em2 srt . im K. Wien radiation law - an expression representing approximately the spectral radiance of a blackbody as a function of its wavelength and temperature. lt commonly is expressed by the formula I dl5 dA' -5 -2/ÀT c11A e This formula is accurate to 1% or better for values of AT less than 3000 tm K. wing clearance lights - aircraft lights provided at the wing tips to indicate the extent of the wing span when the navigation lights are located an appreciable distance in-board of the wing tips. workplane - the plane at which work usually is done, and on which the illurninance is specified and measured. Unless otherwise indicated, this is assumed to be a horizontal plane 0.76 m (30 in.) above the floor. working standard - a standardized light source for regular use in photometry. Zzonal-cavity interreflectance method - a procedure for calculating coefficients of utilization, wall exitance coefficients and ceiling cavity exïtance 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 emit- ted by a source of light in a given angular zone is multiplied to obtain the lumens in the zone. Sec chapter 2, Measurement of Light and Other Radiant Energy. zonal-factor interreflection method - a formerly used procedure for calculating coefficients of utilization, based on integral equations. zonal-factor method - a procedure for predetermining, from typical luminaire photometric data in discrete angular zones, the proportion of luminaire output which 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° 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 o! the room cavity ratio and are used to determine the direct ratio.
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