What limits normal visual performance in the dark?

Abstract

AbstractPurpose Visual performance is affected by changes in the spatial and temporal properties of the retina and/or changes in the quality of the retinal image as a result of increased aberrations and scattered light. The aim of the study was to establish the extent to which retinal and/or optical factors set the limits of visual performance at low light levels.Methods Pupil size, ocular aberrations, scattered light, chromatic sensitivity and contrast acuity were measured as a function of retinal illuminance. Pupil size was measured continuously and display luminance adjusted for constant retinal illuminance.Results Mesopic vision describes the range of light levels over which signals from both rods and cones contribute to the visual response. Visual performance changes with light level: Both the amount of scattered light and the rms wavefront aberration were found to increase rapidly with decreasing light level in the mesopic range. This was paralleled by a massive increase in contrast acuity thresholds and a rapid loss of both red‐green and blue‐yellow chromatic sensitivity.Conclusion The gradual increase in rod signal as the light levels decreases causes changes in the overall spectral sensitivity of the eye with consequences for visual effectiveness. Cone dominated vision is affected most by optical factors; in rod dominated vision the limiting factor becomes the resolving power of the retina. Key aspects of visual performance, such as spatial and temporal contrast sensitivity and acuity, visual delay and colour sensitivity change with light level. Linking such changes allows us to predict visual performance at low light levels where characteristics of the rod and cone system are so different.