Investigation of the daylight spectrum in an indoor environment using CIE S 026 melanopic metrics

Since discovering the intrinsically photosensitive retinal ganglion cells (ipRGCs) and their impact on the circadian rhythm, many disciplines have been researching the physiological and psychological effects of light on humans. However, not much research has been conducted on how the spatial configuration of a room (orientation and gaze direction) changes the light quality and quantity. This research examines the variability of the natural lighting spectrum, focusing on light intensity, the spectrum itself, as well as variations across seasons and hours. The metrics examined are the CIE S 026 melanopic equivalent daylight illuminance (melanopic EDI) and melanopic daylight efficacy ratio (melanopic DER). Field measurements were taken in a room with a south orientation located in Cyprus, using a spectrometer. The main parameters under study were eight gaze directions, four seasons (June, September, December and March) and four different hours each day (09:00, 12:00, 15:00 and 18:00) for 20 positions in the test room. The results show the impact of season, time and gaze direction on the melanopic DER and melanopic EDI. Overall, due to the low sun angle and direct sunlight exposure in the winter, the melanopic EDI was much higher compared to the other three seasons, whereas the melanopic DER had the lowest values. For all seasons and times, most of the cases achieved the minimum recommendation for daytime light exposure of melanopic EDI of more than 250 lx.