Cehao Yu¹ (), Sylvia Pont², Ruben Pastilha¹, Anya Hurlbert¹; ¹Centre for Transformative Neuroscience, Biosciences Institute, Newcastle University, United Kingdom, ²Perceptual Intelligence Laboratory, Faculty of Industrial Design Engineering, Delft University of Technology, The Netherlands

Natural diurnal illumination, daylight, consists of sunlight and skylight. Natural crepuscular illumination, twilight, is formed by scattered sunlight, moonlight, starlight, and airglow emissions. Outdoor illumination, encompassing both diurnal and crepuscular light, undergoes intensity and spectral changes throughout the day. These are characterized by spectral irradiance fluctuations across various temporal scales and spatial locations due to atmospheric dynamics. To facilitate future modelling of how these fluctuations influence visual perception and circadian rhythms, we analyzed spectral light-field data (ranging from 360 to 780 nm) collected from dawn to dusk on typical days. Our analysis combined data from two sources: the Delft light field database, covering one cloudy and one sunny day, and additional measurements from Newcastle for an overcast day. We decomposed global illumination into time-varying directional and diffuse components across three distinct weather conditions. This analysis uncovered a clear three-part pattern on the sunny day, marked by major chromaticity shifts from blue to yellow-orange in the early morning and yellow-orange to blue in late afternoon. Weather conditions and solar elevations substantially influenced illuminance, showing rapid transitions at both daybreak and dusk. On overcast days, the dominant illumination direction was upward, with chromaticity remaining stable. Conversely, cloudy days experienced sharp shifts in illuminance and chromaticity when direct light was interrupted by passing clouds, highlighting the variability of natural outdoor illumination. Psychophysical measurements suggest that many daylight variations, apart from those due to rapid cloud movements, will be undetectable. Such changes may be picked up by the slower non-visual light response pathway, potentially affecting circadian rhythms. Initial analyses revealed that directional components alter more swiftly than diffuse components in chromaticity and illuminance, underscoring their distinct roles in natural illumination. These findings highlight the need to differentiate between directional and diffuse effects in understanding natural illumination dynamics and in shaping the anthropogenic light environment.