2017 - Daylight Symposium
"Temporal variability on discomfort glare from daylight" by Michael Kent
Researcher and architectural engineer
Lecture from the 7th VELUX Daylight Symposium “Healthy & climate-friendly architecture– from knowledge to practice” that took place in Berlin on 3-4 May 2017. For more information visit http://thedaylightsite.com
“Temporal variability on discomfort glare from daylight” by Michael Kent. Discomfort glare is generally considered an annoyance or distraction caused by sources of non-uniform or high luminance within the field of view of an observer. The conditions that give rise to this phenomenon have not yet been fully characterized, this being particularly evident in the presence of large sources of luminance such as daylight from windows. When subjective ratings of glare sensation are correlated with calculated glare indices, in fact, a large scatter in responses can often be observed, this suggesting that discomfort glare may be dependent on other variables than the physical and photometric parameters typically embedded in glare formulae.
Previous experiments conducted by the authors under a highly controlled laboratory setting detected statistical and substantive evidence of increased tolerance to luminance increase from an artificial light source as the day progressed. Further laboratory studies by the authors also revealed a statistically significant and practically relevant influence of several temporal variables – e.g., fatigue, hunger, caffeine intake, mood, prior (day)light exposure, sky condition – on the glare sensation reported by test subjects. Having established the existence of a temporal effect on glare response from artificial light, a semi-controlled study was set up in a test-room with direct access to daylight and to an external view.
Under a repeated-measures design, 40 participants were instructed to provide glare sensation votes (GSVs) at 3 test sessions evenly distributed at 3-hour intervals, randomised over consecutive days, whilst performing a series of 3 visual tasks under 2 shading conditions. Selfassessments of temporal variables were provided by test subjects along with their glare assessment, and their ratings of view importance. Photometric measurements of the observer’s field of view were recorded during the subjective assessments, instantaneously capturing the luminous environment with three photometric instruments: a Charged Coupled Device (CCD) camera equipped with a fish-eye lens, an illuminance chromameter, and a series of sensors measuring desk level and window sill horizontal illuminance.
High Dynamic Range Images were created by the software Photosphere and evaluated by the Evalglare tool. In the analysis of the data, a mixed-effects multilevel model – partitioning sources of variability associated with variables that were experimentally manipulated (fixed-effects) from that of temporal variables that were subject-dependent (randomeffects) – detected a statistically significant and practically relevant effect of time of the day on the reported levels of glare sensation. The findings from this experiment showed a tendency towards an increased tolerance to the discomfort given by the glare source as the day progresses. Rigorous statistical analysis also revealed that the variability (variance components) associated with the temporal variables partially confounded the effect of time of the day on glare response. Finally, statistically significant and practically relevant evidence of an influence of view importance on glare response was also detected.
The results from this test-room study support the findings previously derived under controlled laboratory conditions, suggesting that the physical and photometric parameters typically found in glare indices and formulae might not be sufficient to accurately describe and rigorously predict the subjective evaluation of glare sensation from natural and artificial light.
Michael Kent is a researcher and architectural engineer working in the field of lighting and daylighting, indoor environmental quality and human factors in building design. During his doctorate, he explored the relationship between glare response and the time of the day. Currently, his focus lies on human (psycho) physiological responses to luminous stimuli and the advancements in experimental methodology and statistical techniques in built environment research.