Glossary - C

C 

candela, cd the SI unit of luminous intensity, equal to one lumen per steradian (lm/sr).

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. 

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. 

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. 

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

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

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 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 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. 

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

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

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

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

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.