The refractive errors of a group of young subjects (19-22 years, n=20) were measured under both binocular and monocular conditions of fixation using an infra-red optometer (a CANO R-1). Refraction was determined under normal, photopic consulting room conditions (150 cdm-2), when viewing a well-lit street at night (approximately 0.5 Cdm-2), and under complete darkness. Mean myopic refractive changes in complete darkness were found to be about 1.0 D. However, under the relatively high mesopic levels of street lighting the mean change was less than 0.1 D. It was also found that the myopic shifts in an otherwise completely dark environment were reduced to less than 0.1 D if a small (6 min. Arc, luminance 40 cdm-2) fixation target was present, simulating such features as reflecting road markers. The results are consistent with the view that night myopia is not normally manifest under the environmental lighting conditions applying during night driving and that specific night driving corrections are unlikely to offer significant advantages.
Abstract