Vision is mediated by light passing through the pupil, which
changes in diameter from approximately 2 to 8 mm between bright and dark
illumination. With age, mean pupil size declines. In laboratory
experiments, factors affecting pupil size can be experimentally
controlled. How the pupil reflects the change in retinal input from the
visual environment under natural viewing conditions is unclear. We
address this question in a field experiment (
N = 83, 43 female, 18–87 years) using a
custom-made wearable video-based eye tracker with a spectroradiometer
measuring near-corneal spectral irradiance. Participants moved in and
between indoor and outdoor environments varying in spectrum and engaged
in a range of everyday tasks. Our data confirm that light-adapted pupil
size is determined by light level, with a better model fit of melanopic
over photopic units, and that it decreased with increasing age, yielding
steeper slopes at lower light levels. We found no indication that sex,
iris colour or reported caffeine consumption affects pupil size. Our
exploratory results point to a role of photoreceptor integration in
controlling steady-state pupil size. The data provide evidence for
considering age in personalized lighting solutions and against the use
of photopic illuminance alone to assess the impact of real-world
lighting conditions.