Luminance and vertical eye illuminance thresholds for occupants’ visual comfort in daylit office environments

Abstract

This paper investigates field measured visual parameters inside a daylit office space to define occupants' visual comfort thresholds, particularly for a view direction parallel to windows. Twelve human subjects performed a computer-based reading and typing task and provided subjective responses on various luminous environments, which were created by adjusting interior venetian blinds and electric lighting fixtures. A total of 120 different lighting conditions were tested and documented for both quantitative and qualitative analysis. High dynamic range photography was conducted to capture the luminance distributions of interior surfaces in addition to horizontal and vertical illuminance measurements by sensors. Statistical analysis of the collected data defined occupants’ visual comfort thresholds: a luminance of 2420 cd/m2 and luminance contrast ratio of 11.7. These new thresholds lie perfectly within the existing visual discomfort threshold ranges and confirm that a higher glare source luminance is tolerant in a view direction parallel to windows compared to view facing windows. A vertical eye illuminance threshold was also defined, but there are large discrepancies among the existing and newly defined thresholds. The collected subjective survey data also revealed 53%–60% evaluation accuracy rates of the existing glare indices. Daylight Glare Index and Unified Glare Rating metrics show the highest evaluation accuracy when daylight is the only light source. The CIE Glare Index metric shows the highest accuracy when both natural and electric lighting are introduced. Full understanding of visual comfort parameters and validation of the existing glare indices would help to achieve successful daylight harvesting in buildings.