Quantifying visual comfort of daylight in offices: An experimental study linking illuminance to visual perception factors

Daylight, as an indispensable environmental resource, significantly influences the visual comfort of office workers. Although research on the visual comfort of daylight has been extensively conducted, a comprehensive understanding of how different visual-perception factors—brightness, glare, and uniformity—affect occupants' visual comfort in daylight environments remains lacking. This study aims to analyse and discuss occupants’ daylight requirements under different task types by investigating the effects of these factors and quantifying their relationships with objective daylight illuminance. Eighteen healthy young subjects (9 male, 9 female) participated. Adjustable external blinds created diverse daylight scenarios in a mock office. Subjective evaluations were collected using validated questionnaires, while horizontal and vertical illuminance were simultaneously measured for paper and visual display terminal (VDT) tasks, respectively. The findings reveal that glare was identified as the primary negative factor affecting visual comfort, while perceived brightness in the work plane showed a significant positive correlation. Illuminance uniformity played a secondary role, particularly when illuminance was insufficient (< 500 lux). Subjects consistently preferred a neutral-to-slightly bright daylight environment across both task types. Logistic functions were established to quantify the relationship between illuminance and brightness sensation (R2 = 0.93 & 0.94). Preferred illuminance ranges are derived: 750–1500 lux (horizontal) for paper tasks and 500–2000 lux (vertical) for VDT tasks. These findings provide a quantitative basis for evidence-based daylight design practices and point the way for future revisions of relevant standards and the creation of human-centric luminous environments.