Investigation of existing discomfort glare indices using human subject study data

As daylighting becomes an increasingly important component of design for energy savings and views to the outside, it is necessary to take into disadvantages that windows pose including the possibility of glare. It is crucial to understand how current metrics of glare perform this task. Validation studies were performed on five glare indices including Daylight Glare Probability (DGP) and Daylight Glare Index (DGI) that have been developed specifically for daylight glare issues. A parallel human subject study has been performed to collect subjective discomfort glare evaluations of daylit conditions. In addition, high dynamic range imaging was used to capture and analyze the glare scenes that were experienced by those human subjects. More than 450 daylight glare scenes and subjective surveys were collected in a closed office setting. The collected data were processed in Evalglare to obtain glare scores, and the results were compared for statistical analysis of subjective evaluations. The results show that DGP functions best at absence of glare and presence of intolerable glare; but in between it provides disappointingly low predictions. DGI underestimates glare while Visual Comfort Probability and CIE Glare Index overestimate it. This evaluation comparison study supports the findings that the five glare indices have significant inconsistency and inaccuracy issues. The glare indices behaved differently based on the perceived glare categories: imperceptible, perceptible, disturbing, and intolerable. The existing glare score ranges are compared to the newly defined glare score ranges for further improvement.