Light - Distribution

Light - Distribution

from Ching Viz Dictionary of Architecture

Beam spread = the angle of a light beam that intersects the candlepower distribution curve at points where the luminous intensity equals a stated percent of a maximum reference intensity.

beam = a group of nearly parallel rays of light.

throw = the effective length of a beam of light.

Spacing criteria = A formula for determining how far apart luminaires may be installed for uniform lighting of a surface or area, based on mounting height.

spacing criteria(sc) = spacing(s) / mounting height(mh)

point method = a procedure for calculating the illumination produced on a surface by a point source from any angle, based on the inverse square and cosine laws.

lumen method = a procedure for determining the number and types of lamps, luminaires, or windows required to provide a uniform level of illumination on a work plane, taking into account both direct and reflected luminous flux. Also called Zonal cavity method.

work plane = the horizontal plane at which work is done and on which illumination is specified and measured, usually assumed to be 30 inches above the floor

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

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

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

room cavity ratio = a single number derived from the dimensions of a room cavity for use in determining the coefficient of utilization.

coefficient of utilization (CU)= the ratio of the luminous flux reaching a specified work plane to the total lumen output of a luminaire, taking into account the proportions of a room and the reflectance of its surfaces.

light loss factor = any of several factors used in calculating the effective illumination provided by a lighting system after a given period of time and under given conditions. formerly called maintenance factor.

recoverable light loss factor (RLLF)= a light loss factor that may be recovered by relamping or maintenance, as lamp lumen depreciation, luminaire dirt depreciation and room surface dirt depreciation.

lamp lumen depreciation = a light loss factor representing the decrease in luminous output of a lamp during its operating life, expressed as a percentage of initial lamp lumens.

Luminaire dirt depreciation = a light loss factor representing the decrease in luminous output of a luminaire resulting from the accumulation of dirt on its surfaces, expressed as a percentage of the illumination from the luminaire when new or clean.

Room surface dirt depreciation = a light loss factor representing the decrease in reflected light resulting from the accumulation of dirt on a room's surfaces, expressed as a percentage of the light reflected from the surfaces when clean.

nonrecoverable light loss factor (NRLLF) = any of several permanent light loss factors that take in to account the effects of temp., voltage drops or surges, ballast variations, and partition heights.

Average maintained illuminance = (initial lamp lumens x CU x RLLF x NRLLF) / work area

initial lamp lumens = lumens per lamp X lamps per muminaire