Glossary - C

candela, cd the SI unit of luminous intensity, equal to one lumen per steradian (lm/sr). Formerly *candle*. See Chapter 2, Measurement of Light and Other Radiant Energy.

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

candlepower (cp), I = d。/dm luminous intensity expressed in candelas.

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

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

room cavity ratio.

For cavities of irregular shape:

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

ceiling area lighting a general lighting system in which the entire ceiling is, in effect, one large luminaire.

Note Ceiling area lighting includes luminous ceilings and louvered ceilings.

ceiling cavity the cavity formed by the ceiling, the plane of the luminaires, and the wall surfaces between these two planes. ceiling cavity ratio (CCR) a number computed by using the distance from the plane of the luminaire to the ceiling (hc) as Heightcavity in the equations given for *cavity ratio.* ceiling projector a device designed to produce a well-defined illuminated spot on the lower portion of a cloud for the purpose of providing a reference mark for the determination of the height of that part of the cloud.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

coffer a recessed panel or dome in the ceiling.

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

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

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

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

color contrast threshold† See c*hromaticity difference threshold*.

color correction (of a photograph or printed picture) the adjustment of a color reproduction process to improve the perceived-color conformity of the reproduction to the original.
color discrimination the perception of differences between two or more colors.

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

color-matching functions (spectral tristimulus values), ¯x(λ) = Xλ/φeλ,¯ y(λ) = Yλ/φeλ, ¯ z(λ)

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

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

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

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

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

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

colorfulness† See *chromaticness*.

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

colorimetric purity (of a light), pc the ratio *L*1/*L*2, where *L*1 is the luminance of the single-frequency component that must be mixed with a reference standard to match the color of the light, and *L*2 is the luminance of the light. See *excitation purity.*

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

colorimetry the measurement of color.

compact-arc lamp† See short-arc lamp.

compact source iodide lamp (CSI) an arc source utilizing a mercury vapor arc with metal halide additives to produce illumination typically in the 5000 to 6000 K range. Requires a ballast and ignitor- system for opera- tion.
comparison lamp a light source having a constant, but not necessarily known, luminous intensity with which standard and test lamps are successively compared.

complementary wavelength (of a light), .1c the wavelength of radiant energy of a single frequency that, when combined in suitable proportion with the light, matches the color of a reference standard. See dominant wavelength.

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

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

configuration factor, *C*1→2 the ratio of the illuminance on a surface at point 1 (due to the flux directly received from lambertian surface 2) to the exitance of surface 2. It is used in flux transfer theory.

Also, the ratio of the differential flux directly received by surface 2 (and due to element 1) to the total differential flux emitted by differential lambertian surface element 1:

Note In the literature this ratio is also called the angle factor, illumination factor, point configuration factor, and sky factor.

conical-directional reflectance, p(mi; er, φr) ratio of reflected flux collected over an element of solid angle surrounding the given direction to the incident flux limited to a conical solid angle.

Note The direction and the extent of the cone must be specified, and the direction of collection and size of the solid angle "element" must be specified.

conical-directional transmittance, τ(mi; et, φt) ratio of transmitted flux, collected over an element of solid angle surrounding the direction to the incident flux limited to a conical solid angle.

Note The direction and extent of the cone must be specified, and the direction of collection and size of the solid angle "element" must be specified.

conical-hemispherical reflectance, p(mi; 2n) ratio of reflected flux collected over the entire hemisphere to the incident flux limited to a conical solid angle.

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

conical-hemispherical transmittance, τ(mt; 2n) ratio of transmitted flux collected over the entire hemisphere to the incident flux limited to a conical solid angle.

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

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

contrast† See luminance contrast.

contrast rendition factor (CRF) the ratio of the visual task contrast with a given lighting environment to the contrast with sphere illumination. Also known as the contrast rendering factor.
contrast sensitivity the ability to detect the presence of luminance differences. Quantitatively, it is equal to the reciprocal of the brightness contrast threshold.

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

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

cosine law a law stating that the illuminance on any surface varies as the cosine of the angle of incidence. The angle of incidence is the angle between the normal to the surface and the direction of the incident light. The inverse square law and the cosine law can be combined as *E* = (*I* cos e)/*d*2. See *cosine- cubed law* and *inverse square law.*

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

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

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

country beam† See upper (driving) beams.

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

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

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

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

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

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

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

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