Challenges in Daytime Outdoor Glare Evaluation Within the Built Environment: A Review

Direct sunlight and reflections from buildings and ground surfaces create multi-source highlights in high-density built environments, potentially affecting outdoor visual safety and spatial use. This integrative review synthesizes daytime outdoor glare research in terms of physical characteristics, observer effects, and assessment methods. Solar movement, surface reflection, spatial obstruction, and observer movement jointly alter the intensity, direction, scale, and number of bright regions. Outdoor glare is therefore high-luminance, multi-source, non-uniform, and time-varying, occurring as concentrated, continuous-area, and discrete-patch highlights. Evidence indicates that outdoor glare causes visual discomfort, impairs target recognition, and may induce immediate behavioral adjustments. However, studies focus mainly on static viewpoints, short-term perception, and driving tasks, with limited direct observation of spatial-use outcomes such as dwell time, detouring, and location choice. Current assessments identify potential exposure through geometric-path analysis or quantify glare stimuli using established indices and extended models. Some metrics support relative comparisons within specific settings but cannot simultaneously characterize spatial reachability, stimulus intensity, dynamic exposure, and observer responses. Future research should develop hierarchical, type-specific dynamic assessment methods based on glare source, spatial scale, and visual task to inform building-envelope design, material selection, and outdoor glare-risk control.