Influence of melanopic illuminance on spatial brightness in interior spaces: A controlled-variable study with comparison spectra

Spatial brightness, a visual perception related to the magnitude of overall ambient lighting, has become a key aspect of indoor lighting quality. Traditional lighting measures, however, cannot accurately assess spatial brightness across different lighting spectra as the spectral sensitivity of spatial brightness differs from the V(λ) function, particularly in the short-wavelength range. Previous studies have reported that intrinsically photosensitive retinal ganglion cells (ipRGCs) may contribute to spatial brightness, but conclusive evidence is lacking, as potential interference from other photoreceptors and spectral attributes has not been excluded.

This study aims to investigate the direct impact of ipRGCs on spatial brightness of interior spaces, with other influencing spectral attributes strictly controlled. A color-mixing lighting system with nine spectral channels was used to generate five comparison spectra, covering a wide tuning range of ipRGC-related melanopic illuminance at fixed levels of photopic illuminance, cyanopic illuminance, chromaticity coordinates, gamut index (R_g), while maintaining color fidelity (R_f) values greater than 70. Human-factor studies were conducted to compare spatial brightness across these comparison spectra with controlled variables. The results indicated that increasing melanopic illuminance alone leads to a small but clear increase in spatial brightness when other spectral factors are held constant, proving the positive contribution of ipRGCs to spatial brightness. Additionally, this contribution is influenced by the corneal illuminance levels: when corneal illuminance is between 50 and 150 lx, ipRGCs have a clear positive impact on spatial brightness; however, when the corneal illuminance is outside of this range, changing ipRGC stimulation alone does not affect spatial brightness.