Discomfort glare from small light sources

Abstract

Glare from very small sources is grossly overrated by the UGR (Unified Glare Rating) formula. Experiments were carried out on glare from small sources at interior lighting distances. Results varied widely but were of adequate statistical accuracy. They showed that a small glare source is characterised by a constant projected area of 0.005 m2, not a constant solid angle; that glare from a source below 0.005 m2 is determined by its intensity, not by its luminance; and that the UGR formula is applicable to small sources if it is suitable modified (L2ω becomes 200I 2/R2).