Light stimulation system for measuring pupillary light responses based on Maxwellian view optical system using a retina presentation type viewfinder

Abstract Background Melanopsin-expressing intrinsically photosensitive retinal ganglion cells (ipRGCs) contribute predominantly to non-image-forming effects of light, such as light-induced circadian phase shift, melatonin suppression, and pupillary light reflex. Post-illumination pupil response (PIPR) has been well-used for the evaluation of ipRGC sensitivity for humans since it is able to detect ipRGC function independently from conventional cones. However, pharmacological mydriasis or a specialized light stimulation system, such as the Maxwellian view (MV) optical system, is required to avoid dynamic pupil constriction in the stimulated eye for effective PIPR measurements, which makes these approaches less practical and difficult to implement in standard experimental settings. In this context, we propose the application of a retinal projection viewfinder based on the MV system as a practical approach for measuring PIPR and evaluate the feasibility of this approach by comparing its performance with a typical LED-based optical system. Methods Twenty-two healthy participants underwent pupillometry using both the MV-based viewfinder and a typical LED-based system. Monochromatic red and blue light stimuli were presented for durations of 1 and 10 s. Pupil responses, including maximum constriction, PIPR amplitude after 6 s from the light offset, area under the curve (AUC) values of PIPR, and sustained slopes, were analyzed using a linear mixed-effects model to assess the differences between the two systems. Results The MV-based viewfinder significantly enhanced net PIPR amplitude ( p < 0.05) and sustained slope ( p < 0.01) during 10-s light stimulation compared to the LED system, demonstrating its capability to effectively measure ipRGC-driven responses. In contrast, no significant differences were observed in the net AUC values. These results highlight that the MV-based viewfinder enables effective PIPR measurements by delivering constant and controlled light stimulation directly to the retina, minimizing the effects of dynamic pupil constriction during light stimulation. Conclusions The MV-based viewfinder showed feasibility as an effective method for measuring PIPR without requiring pharmacological dilation or complex optical instrumentation. This approach has strong potential for advancing the assessment of human ipRGC function using pupillary responses.