Perception of discomfort glare in relation to the CIE Unified Glare Rating scale (UGR) and the impact of ambient light on both UGR and Subjective Glare Indexes (SGIs)

The Unified Glare Rating (UGR) method defined by the International Commission on Illumination is practically the only one used for evaluation of discomfort glare in the lighting design process. However, it is unclear if this method aligns with subjective discomfort glare measures using subjective glare indexes (SGIs). We compared calculated UGR values with commonly used subjective discomfort glare scales across five common office lighting systems. In a controlled laboratory setting with three studies, fifty-nine participants evaluated the degree of perceived discomfort from the selected lighting systems. The findings indicate that classical SGIs were not consistent with UGR values and individual factors (age, sex and emotional state) can influence the discomfort sensitivity. Furthermore, the findings provide new insights into how ambient light levels and viewing direction affect both calculated discomfort glare and perceived discomfort glare. Collectively, these findings deepen the understanding of discomfort glare and underscore the need to refine glare evaluation methods, with direct implications for office lighting design and standards. • Calculated discomfort glare (UGR) was compared with subjective glare indexes • UGR values showed limited consistency with subjective discomfort glare evaluations • Ambient light and viewing direction affect perceived and calculated discomfort glare • Personal factors (age, sex, emotional state) influence discomfort glare sensitivity • Findings emphasize the need to improve discomfort glare evaluation methods