The influence of daytime on discomfort glare: Results of an inter-laboratory comparative study

Modern façade systems often include sophisticated controls to balance users’ comfort and energy requirements. However, the influence of the time of day on comfort perception is neither well understood nor implemented in any control algorithms to date. A better understanding of how the time of day affects glare sensation would expand knowledge on glare perception and also enable the development of shading device control schemes better suited to users’ needs. This study examines the impact of time of day (morning vs. afternoon) on glare perception, using data from three independent studies conducted in Argentina, Denmark and Germany. A common feature of these studies is their implementation in controlled, office-like test rooms under daylight conditions. Participants were exposed to different daylight scenarios, each involving different viewing directions relative to the window and different shading elements. Their evaluations were conducted over two separate days, with sessions held either in the morning or afternoon, randomly assigned to minimize bias. The data was repeatedly filtered randomly with the rule to keep only datasets that are “similar” for the two daytimes. After data preparation, each of the 2000 randomly sampled filtered datasets for each location were analysed with Bayesian cumulative ordinal model to evaluate the effect of the daytime on the subjective glare perception. A consistent trend emerged across all three locations: a higher fraction of participants reported discomforting glare in the afternoon compared to the morning despite the similar glare stimuli in both daytimes. However, the strength of this effect varied considerably by location.