Relationship between pupillary size, brightness, and photoreceptor responses for unrelated self‐luminous stimuli at low photopic light levels

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Summary

At low photopic light levels (5–80 cd/m²), pupillary response correlates more strongly with perceived brightness than with luminance alone, and ipRGC excitation appears to be the dominant driver of pupil constriction. Highly saturated blue stimuli produce significantly greater pupil constriction than red stimuli of equivalent perceived brightness, highlighting that spectral composition—not just luminance—must be considered in lighting and display design.

Key Findings

Coefficient of determination between pupillary response and perceived brightness exceeded that between pupillary response and luminance, suggesting brightness is a stronger predictor of pupil size than luminance at low photopic levels.
Blue stimuli induced substantially greater pupil constriction than red stimuli of equivalent brightness, implicating ipRGC (melanopsin) excitation as a key differentiating factor.
Among the five photoreceptor classes studied, rod and ipRGC excitations showed the largest effect on pupillary size variation; ipRGC response was identified as the likely dominant mechanism under the tested conditions.
Study used 6 observers (ages 30–39, mean 34.5 years) with stimuli characterized via spectroradiometer and pupil diameter measured via eye tracker using Michelson contrast as the metric.