Spherical aberration is a key determinant of refraction at night

Abstract

Purpose: For more than 230 years we have known that eyes appear to be more myopic at nighttime. Although spherical aberration has been implicated in this refraction change, its exact role is still unclear. We study if the correction of this refractive change is more closely related to the improvement of the stimulus contrast or its resolution.

Methods: Refractive error was determined for a 7mm pupil, objectively by using techniques based on wavefront refraction and subjectively using small bright point stimuli (<1 arc minute, 1000 cd/m²) of an OLED microdisplay. Experimentally, the amount and sign of ocular spherical aberration were controlled. The objective refraction was computed after a through-focus computational paradigm, which determine the correcting lens that would be expected to optimize retinal image quality according to a visual contrast metric (VSOTF) known to be an unbiased predictor of refractive error for conventional letter charts viewed under daylight conditions. Subjective refractions were determined with a through-focus paradigm on paralyzed or presbyopic eyes, for a contrast detection task (increment detection of a point source on a uniform background) and again for a spatial resolution task (minimum separation of two point sources on a black background). Detection refraction was defined as the power of a correcting lens that maximized contrast sensitivity and resolution refraction was defined as the power of a correcting lens that maximized two-point resolution.

Results: As the amount of the spherical aberration increases, detection refraction becomes myopic relative to the daylight standard refraction or two-point resolution refraction. Positive spherical aberration usually reproduces the classic astronomer’s observation of night myopia while negative spherical aberration reverses this result (i.e. night hyperopia). Nullifying spherical aberration causes detection, resolution and daylight standard refraction to be nearly the same. The refractive change between resolution and detection refraction ranged from 0.25 to 0.5 D for spherical aberration coefficient C₄⁰ = ±0.5 microns (7mm pupil).

Conclusions: Night myopia is a phenomenon more closely related to contrast detection than for spatial resolution. The sign and magnitude of night myopia is determined primarily by the sign and magnitude of ocular spherical aberration. These results have clinical implications for optimum correction of the eye for nighttime vision.