An Artificial Skylight compared with daylighting and LED: subjective and objective performance measures

In the last decade, innovative systems such as Artificial Skylights (AS) have been developed to reproduce the blue-sky effect and white sunlight, giving the impression that interiors are illuminated by natural light, even when this is not available because of construction- or climate-related reasons. Given the hybrid characteristics between natural and artificial lighting, the aim of this research is to compare an AS with daylight and with artificial lighting to identify similarities and differences in performances, pointing out the most suitable indicators to describe it and to provide useful feedback for the design and integration of these systems. A monitoring campaign was conducted involving nine participants who spent entire work sessions in a full-scale living lab equipped as an office, with different lighting systems (AS, daylight and LED) and different furniture configurations, completing a total of more than 360 questionnaires and analysing the visual and non-visual effects. AS and daylight share many similarities, which may be positive, such as pleasantness, circadian stimuli, colour perception, as well as, in some scenarios, also negative, such as glare perception. As a conventional artificial lighting system, even with AS, Unified Glare Rating correlates more strongly with users' perception of glare than Daylight Glare Probability, although both underestimate it. Overall, the workstation receiving diffuse light was found to be the most comfortable and the higher installed lighting power density of AS with respect to LED is well balanced by a better lighting quality. Further research about AS is needed to define design guidelines and to support a holistic approach, fundamental to high-performing buildings.