A field study investigation on the influence of light level on subjective thermal perception in different seasons

This paper evaluates the influence of light levels (i.e., illuminance) on subjective thermal perception of people, distinguishing between thermal sensation and thermal evaluation. The goal is to investigate whether reported effects found by other studies in controlled environments can be observed in real-life contexts and to understand if results are influenced by the season. By means of a post-occupancy evaluation conducted in four buildings in Switzerland, instantaneous air temperature and illuminance measurements were collected together with occupant’s thermal perception votes during daytime in both summer and winter. Findings show that illuminance has a significant effect on the thermal perception of people, but only in terms of thermal evaluation and not of thermal sensation. In particular, results indicate that, at high temperature (above 25 °C), a less satisfying thermal evaluation is reported by people exposed to dim light (lower than 300 lux) compared to people exposed to brighter environments. We assume that this finding can be explained by thermal expectations induced by light intensity. The evaluation of data across summer and winter indicates that results are independent of the season (no interactions between illuminance, temperature and time of the year) and that the illuminance effect is accentuated depending on the season, which tends to highlight the psychological nature of thermal evaluation.