Evaluation of images of visual acuity charts degraded by wavefront aberrations by a human observer and by using cross correlation functions of the images of Landolt rings

Abstract

Purpose: To predict visual acuity from wavefront aberrations of moderately aberrated keratoconic eyes.

Method: Wavefront aberrations of eight moderately keratoconic eyes were measured using a Hartmann-Shack wavefront aberrometer (KR9000PW, Topcon, Japan) and up to 4th order Zernike coefficients were obtained over the central 4 mm circular area of the pupil. Simulated retinal images of Landolt rings (−0.2 to 1.0 in LogMAR) degraded by the measured wavefront aberrations of several subjects were displayed on a CRT screen at the same size regardless the original size of the Landolt rings. A human observer observed the simulated images of the Landolt rings and said where the breaks of rings were located. Estimated visual acuities from this method were compared with visual acuities subjectively obtained in regular visual acuity measurements of the keratoconic subjects.. Cross correlation functions of the retinal images for two differently oriented Landolt rings were calculated and those were compared with the visual acuities obtained with the simulated images to decide the critical values in cross correlation functions for discriminating the directions of Landolt rings.

Results: Statistically significant correlations were found between visual acuities predicted with the method using simulated images and those obtained subjectively. For all eyes, the aberrated image of the smallest Landolt ring which was discriminated by the human observer had roughly same value in the cross correlation functions.

Conclusions: The image quality of the Landolt rings required to discriminate their directions by keratoconic subjects could be predicted from the cross correlations and data from human observer experiments.