Influence of indoor temperature and daylight illuminance on visual perception

This paper details the results of an experimental study investigating the combined effect of indoor temperature (19 ℃, 23 ℃ and 27 ℃) and horizontal illuminances (∼140 lx, ∼610 lx and ∼1440 lx) on visual perception, with daylight as the only source of light. The goal is to evaluate the cross-modal effect of temperature and the unimodal effect of daylight illuminances on the visual perception of daylight. A 3 × 3 mixed-design experiment was conducted in an office-like test room with 84 participants. Results show that cross-modal effects of indoor temperature on visual perception occurred: Temperature influenced the perceived warmth of daylight, and temperature interacted with daylight illuminance as the high illuminance was considered more pleasant, was better evaluated and was more often chosen than the low one only in a thermally comfortable condition. In terms of unimodal effects, differences in visual comfort evaluations were substantial only between the low and the other two illuminances, suggesting that visual comfort did not increase with illuminance above an already comfortable threshold. Moreover, perceived brightness under the high daylight illuminance was lower than that reported in past studies under a comparable electric light illuminance, leading to the hypothesis that high illuminances might be more tolerated in a naturally-lit environment compared to a space illuminated with electric light. Findings advance the understanding of occupant perceptions of daylit environments and suggest that, together with visual factors, thermal conditions should be considered in visual perception investigations.