Photometry from the right distance
Abstract
It may be misleading to measure the intensity distribution of an interior luminaire at the standard distance of five times length if most of the light from the luminaire will only travel half that distance in service. Figures are given for the results of short-range photometry which can reduce the errors inherent in simple point-to-point calculation of illuminance. It is suggested that the 'working intensity' distribution of an interior luminaire be defined as that obtained by measurement at two metres radius.