Effects of Spatial Ambient Illumination on Pupil Diameter and Visual Acuity.

PURPOSE: Although chart luminance is carefully standardised in visual acuity (VA) testing, the luminance and spatial distribution of the surrounding illumination are less clearly defined. This study investigated how central luminance, peripheral field illumination and ambient room illumination influence pupil diameter and VA during small-field visual tasks. METHODS: A programmable light-emitting diode (LED) illumination system was developed to manipulate central chart luminance, peripheral field illumination and ambient room illumination surrounding a central Early Treatment of Diabetic Retinopathy Study (ETDRS) chart independently. VA and pupil diameter were measured in 201 participants under multiple lighting conditions. Relationships among illumination parameters, pupil diameter and VA were evaluated using paired comparisons and regression analysis. RESULTS: Increasing central luminance from 20 to 640 cd/m² reduced mean pupil diameter from 5.93 to 3.61 mm and improved VA from 0.02 to -0.13 logMAR. At constant central luminance (160 cd/m²), increasing peripheral field extent and ambient illumination produced additional pupil constriction and modest improvements in VA. Changes in pupil diameter were significantly associated with changes in VA (r = 0.67, p < 0.001). CONCLUSION: The spatial distribution of illumination significantly influences pupil diameter and VA, even when central chart luminance remains unchanged. Both peripheral field illumination and ambient room lighting were associated with pupil constriction and modest improvements in VA. These findings highlight the importance of spatial illumination in visual performance and suggest that ambient lighting conditions should be considered in visual acuity assessment and visual environment design.