Regulation of pupil size in natural vision across the human lifespan

Vision is mediated by light passing through the aperture of the eye, the pupil, which changes in diameter from ~2 to ~8 mm from the brightest to the darkest illumination. In addition, with age, mean pupil size declines. In laboratory experiments, factors affecting pupil size can be experimentally controlled or held constant, but how the pupil reflects changes in retinal input from the visual environment under natural viewing conditions is not clear. Here, we address this question in a field experiment (N=83, 43 female, age: 18-87 years) using a custom-made wearable video-based eye tracker with a spectroradiometer measuring spectral irradiance in the approximate corneal plane. Participants moved in and between indoor and outdoor environments varying in spectrum and engaged in a range of everyday tasks. Our real-world data confirm that light-adapted pupil size is determined by light intensity, with clear superiority of melanopic over photopic units, and that it decreases with increasing age, yielding steeper slopes at lower light levels. We find no indication that sex, iris colour or reported caffeine consumption affect pupil size. Taken together, the data provide strong evidence for considering age in personalised lighting solutions and for using melanopsin-weighted light measures to assess real-world lighting conditions.