Spectral power distributions regulate pupillary adaptive adjustment via Melanopic equivalent daylight illuminance: A basis for optimizing healthy luminous environments

Pupillary adaptive adjustment to the luminous environment directly determines retinal light input, thereby influencing both the visual and non-visual effects of light. Previous studies have predominantly focused on the impact of luminance and correlated color temperature (CCT) on pupillary adaptive adjustment, but the role of spectral power distributions (SPDs) and the underlying mechanism remain unclear. This study investigated how different SPDs regulate pupillary adaptive adjustment and whether the effects of CCT on pupillary adaptive adjustment originate from SPD differences. An 11-channel LED system was employed to generate light stimuli with identical luminance but differing CCTs and SPDs. Pupil diameter, subjective visual brightness ratings, and electroencephalography signals were collected from 20 healthy participants. Results showed that SPDs significantly influenced pupillary adaptive adjustment, an effect primarily determined by the intensity of ipRGCs-weighted illuminance, represented by Melanopic Equivalent Daylight (D65) Illuminance (mel-EDI, lux), with higher mel-EDI leading to smaller relative pupil diameter (RPD). Critically, the effects of CCT on pupillary adaptive adjustment were largely mediated by mel-EDI. When mel-EDI was held constant, CCT had no significant effect on RPD. Furthermore, pupillary adaptive adjustment can serve as a noninvasive physiological marker of brightness perception and brain arousal level. As mel-EDI increases, RPD decreases, while subjective visual brightness and brain arousal levels increase. These findings clarify the SPD-mel-EDI-pupillary adaptive adjustment mechanism, highlight mel-EDI as a key parameter for optimizing the luminous environments, and provide a scientific basis for bridging photophysiological research with practical lighting design in the built environment.