Contrast sensitivity and efficiency as a function of grating area and spectral density of spatial noise

Abstract

We measured contrast sensitivity for cosine gratings by using various grating areas and spectral densities of white two-dimensional spatial noise. Contrast sensitivity increased with grating area similarly at all noise spectral density levels. Also, contrast sensitivity decreased with increasing noise spectral density similarly at all grating areas. The experimental data was described by Eq. (1): S = Smax(1 + A c /A) − 0.5(1+N e /N o ) − 0.5, where RMS contrast sensitivity S is the dependent variable, grating area A and spectral density of noise N e are the independent variables, and the maximum contrast sensitivity Smax, critical area A c of spatial integration, and critical spectral density of noise N o are the parameters of Eq. (1). The ideal detector for a signal known exactly is a matched filter. Its energy threshold is d' 2 (N o + N e ), whereas the human energy threshold is S −2 A. Efficiency is their ratio, which is equal to d' 2 Smax 2 N o (A c + A) − 1, where d' is the detectability index. The maximum efficiency was 30%. Our results can be explained by a suboptimal matched filter with a sampling aperture equal to A c .