The significance of changes in pupil size during straylight measurement and with varying environmental illuminance

PURPOSE: In this work, we investigated the pupillary conditions during straylight measurement, and the potential effect this might have on the measured straylight. METHODS: Five young (26–29-years-old) and 15 older (50–68-years-old) individuals participated in this study. First, the pupil diameter of both eyes was measured at three room illuminances. Next, straylight was assessed at two room illuminances. Simultaneously, the change in pupil size of the fellow eye was registered by a camera. RESULTS: Pupil size decreased with room illuminance and with age (both p < 0.05). The dependency of pupil size on age decreased as room illuminance increased (0.018 mm/year at 4 lux, 0.014 mm/year at 40 lux, and 0.008 mm/year at 400 lux illuminances). However, during straylight measurement, pupil sizes hardly differed between 4 and 40 lux illuminances. Respective pupil sizes corresponded with 399 and 451 lux adaptation on average. No statistically significant difference was found between the straylight under the two illuminances with average R(2) = 0.85, p < 0.05. CONCLUSION: We conclude that the illuminance of the examination room during straylight assessment does not affect the outcome in normal eyes. In fact, under mesopic and scotopic conditions, the luminance of the test field is so much higher than that of the room so that it determines the pupil size. Regardless of the lighting level, straylight measured in a laboratory, is valid for photopic pupils at an adaptation level corresponding with about 400 lux room illuminance.