Comparison of objective and subjective visual comfort and associations with non-visual functions in young subjects

Visual comfort is a key element at work places and at home. Besides task illuminance and color rendering it is the indoor light distribution, which strongly impacts visual comfort. We aimed to show possible relations between subjective and objective visual comfort and nonvisual functions such as mood, alertness, and wellbeing. Twenty-two healthy young volunteers were recruited to spend six hours in the afternoon in a testing room at our laboratory. They were exposed to daylight in the range between 1000 and 2000 lx. Artificial lighting was added when daylight illuminance decreased below this target range. In order to determine objective visual comfort, luminance ratios of the room were extracted from high dynamic range (HDR) images, taken three times during the afternoon. From these HDR images we calculated glare indexes by using specialized software (Evalglare v0.9, Fraunhofer ISE). Those glare indexes served as objective visual comfort assessments. At several occasions we asked subjects to assess their subjective visual comfort and glare as well as alertness, mood and well-being on visual analogue scales. We correlated objective and subjective glare assessments with those from alertness, mood and wellbeing. We found that objective visual comfort exhibited a time of day-dependent positive or negative association with subjective physical wellbeing, visual comfort and alertness. Higher objective glare indexes at the beginning of the afternoon slightly correlated with greater physical wellbeing (p<0.1); whereas in the middle of the afternoon, higher objective glare indexes were significantly correlated with lower subjective visual comfort and lower alertness. Towards the end of the afternoon, we found that higher objective glare was associated with significantly lower physical wellbeing (for all significant results: p<0.05). Taken together, our results show that objective glare indexes were associated not only with subjective visual comfort but also with non-visual functions such as subjective physical wellbeing and alertness. However, these assessments varied with the time of day, reflecting the effects of dynamic light conditions on subjective homeostatic and circadian functions. This might have implications on the lighting environment at many work places and homes in the future.