Discomfort glare perception of non-uniform light sources in an office setting

For indoor lighting the glare assessment by Unified Glare Rating (UGR) is widely adopted. However, disagreement exists on how to evaluate glare of non-uniform sources. This recently gained attention with the introduction of small and bright LEDs in general lighting. Earlier studies in a laboratory set-up showed that with equivalent average luminance, discomfort glare from a non-uniform stimulus seems to be greater than that of a uniform stimulus. We investigate the relation between discomfort glare perception of office employees and a set of parameters that is typical for LED luminaires, in a representative office environment. The offered light settings varied in luminance pattern, beam shape and illuminance on the work plane. In agreement with earlier work it can be concluded that point array LED luminaires provoke more discomfort glare in open plan offices than uniform sources and that the currently used UGR is not a good predictor in these cases. The luminance characteristics in the exit window (peak luminance, luminance contrasts, and spatial luminance distribution) play an important role in the design of comfortable (LED) office luminaires. A better understanding of these characteristics and their effects on glare perception, as well as a redefinition of a glare index are essential for a reliable prediction of glare perception.