Lumen Method - Method

Method

A step-by-step guide is given in textbooks and the IESNA Lighting Handbook.

The lumen method in brief consists of calculation of the "cavity ratios" of the upper, middle, and lower volumes of the space to be lighted. The lower cavity is from the floor to the working height, the upper cavity is from the lower edge of the luminaires to the ceiling, and the middle cavity is the volume between these planes.

The effective reflectance of ceiling, floor, and walls are estimated from tabular data. A coefficient of utilization, representing the fraction of light that is directed to the working plane, is supplied by manufacturers for each luminaire design for the various calculated room cavity ratios.

Some of the light produced by the lamps is lost due to non-ideal lamp operating conditions, dirt on the luminaires, dirt on the room surfaces. A light loss factor is calculated for all these, based on tabulated empirical factors.

Given the usual lighting problem of obtaining an average lighting level at the working plane, the number of luminaires can be calculated based on the effective amount of useful light that each luminaire has been calculated to emit.

Since the zonal cavity method only gives an average lighting level, manufacturers tabulate recommended spacing to mounting height ratios that must not be exceeded if uniform illumination is desired.

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