Absorption: The process by which incident flux is dissipated within a medium.

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 of the eye to light.

Brightness: The luminous intensity on any surface 1 in a given direction viewed in that direction.

Brightness ratio: See Luminance Ratio.

Candela, cd: The unit of luminous intensity; one candela is defined as the luminous intensity of 1/600,000 square meter of projected area of a blackbody radiator operating at the temperature of solidification of platinum. (Can be substituted for candlepower).

Candlepower, cp: Luminous intensity expressed in candela. (Can be substituted for candela.)

Cavity ratio: A number indicating cavity proportions calculated from length, width, and height. Ceiling cavity: the cavity formed by the ceiling, the plane of the luminaires, and the wall surfaces between these two planes.

Effective floor cavity reflectance (PFC): The effective reflectance of all the area below the work  plane as seen from the room cavity.

Effective ceiling cavity reflectance (PCC): The effective reflectance of all the area below the workplane as seen from the room cavity.

Floor cavity: The cavity formed by the work plane, the floor, and the wall surfaces between these two planes.

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

Coefficient of utilization (CU): Ratio of luminous flux (lumens) received on the work plane to the rated lumens emitted by the lamps.

Cutoff angle (of a luminaires): The angle, measured up from nadir, between the vertical axis and the first line of sight at which the bare source is not visible. Diffuser: a device to redirect or scatter the light from a source, primarily by the process of diffuse transmission.

CU: See Coefficient of Utilization

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

FC: See footcandle.

FL: See footlambert

Footcandle: (fc): The unit of illumination when the foot is the unit of length; the illumination on a surface one square foot in area on which there is a uniformly distributed flux of one lumen. It equals one lumen per square foot.

Footlambert: (fl): A unit of luminaire (photometric brightness) equal to 1/1T candela per square foot, or to the uniform luminance of a perfectly diffusing surface emitting or reflecting light at the rate of one lumen per square foot, or to the average luminance of any surface emitting or reflecting light at that rate. See units of luminance.

Note: The average luminance of any reflecting surface in footlamberts is, therefore, the product of the illumination in footcandles by the luminous reflectance of the surface.

Glare: The sensation produced by luminances within the visual field that are sufficiently greater than the luminance to which the eyes are adapted to cause annoyance, discomfort, or loss in visual performance and visibility.

Direct glare: Glare resulting from high luminance or insufficiently shielded light sources in the field of view or from reflecting areas of high brightness.

Disability glare: Glare which reduces visual performance and visibility and which is often accompanied by discomfort.

Discomfort glare: Glare which produces discomfort. It does not necessarily interfere with visual performance or visibility.

Reflected glare: Gare resulting from specular reflections of high brightnesses in polished or glossy surfaces in the field of view. It is usually associated with reflections from within a visual task or areas in close proximity to the region being viewed.

Hectolux: Equal to 100 lux.

High-intensity Discharge Lamps (HID): High- pressure discharge lamps notably of the mercury, metal halide, and HPS types.

Illumination (illuminance), E: The density of luminous flux incident on a surface; the quotient of the flux divided by the area of the surface, when the flux is uniformly distributed.

Inter-reflectance: The portion of the lumens reaching the work plane that has been reflected one or more times as determined by the Flux Transfer Theory.

Lamp: A generic term for a man-made source of light. By extension, the term is also used to denote sources that radiate in regions of the spectrum adjacent to the visible. (A lighting unit consisting of a lamp with shade, reflector, enclosing globe, housing, or other accessories is also 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.)

LL: See Lamp; Lumen

LDD: See Luminaire Dirt Depreciation Factor under Maintenance Factor.

LLD: See Lamp Lumen Depreciation Factor under Maintenance Factor.

Lumen, (1m): The unit of luminous flux; equal to the flux in a unit solid angle (one steradian from a uniform point source of one candela.)

Luminance (brightness) ratio: The ratio between the luminances of any two areas in the visual field.

Luminaire: A complete lighting unit consisting of a lamp or lamps 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 efficiency: The ratio of the luminous flux emitted by a luminaire to that emitted by the lamp or lamps used therein.

Lux, Ix: The unit of illumination when the meter is the unit of length; equal to one lumen per square meter.

Maintenance Factor (MF): The product of the lamp lumen depreciation factor and the luminaire dirt depreciation factor (MF = LLD x LDD). For indoor calculations, MF should also include the effect of reflectance depreciation on walls, ceiling and floors.

Luminaire dirt depreciation factor (LDD): The multiplier to be used in illumination calculations to relate the initial illumination provided by clean, new luminaires to the reduced illumina