Luminaire Dirt Depreciation for an Indirect Lighting System

Abstract

Sustainable design practices for lighting systems require that “just enough” luminaires are installed (and thus “just enough” electric power connected) to meet the visual and environmental needs of a space. Determining the proper quantity of luminaires requires estimating the expected light losses due to dirt accumulation. If the estimates employed in the lighting design process overstate the actual loss of light due to this accumulation, more luminaires will be specified than are actually needed, and thus more energy will be employed for lighting.

The study reported in this paper evaluated the effect on illuminances that resulted from cleaning the lighting equipment in a portion of a library. The study was a relatively simple “before and after” field study in which the illuminances were measured, then the lighting equipment cleaned, then the illuminances measured again. Results showed a mean reduction in illuminance due to luminaire dirt of 29 percent, less than the reduction that would have been predicted using common dirt depreciation factors. The results of this study of a single lighting installation are consistent with recently reported results from a much larger study, and extend those results to another luminaire type and to another type of environmental condition.