Blur tolerance for luminance and chromatic stimuli

We investigated the blur tolerance of human observers for stimuli modulated along the isoluminant red-green, the isoluminant yellow-blue, and the lumi nance (black-white) direction in color space. We report the following resul ts: (i) Blur difference thresholds for red-green and luminance stimuli (of equal cone contrast) are very similar and as low as 0.5 min of visual angle ; for yellow-blue the lowest blur thresholds are much higher (15 min of vis ual angle). (ii) The smallest blur thresholds are found for slightly blurre d square waves (reference blur of 1 are min) and not for sharp edges. (iii) Blur thresholds for red-green and black-white follow a Weber law for refer ence (pedestal) blurs greater than the optimum blur. (iv) Using the model p roposed by Watt and Morgan [Vision Res. 24, 1387 (1984)] we estimated the i nternal blur of the visual system for the black-white and the red-green col or directions and arrived at the following estimates: 1.2 are min for black -white stimuli at 10% contrast and 0.9 are min for red-green stimuli at 10% cone contrast. Blur tolerance for yellow-blue is independent of external b lur and cannot be predicted by the model. (v) The contrast dependence of bl ur sensitivity is similar for red-green and luminance modulations (slopes o f -0.15 and -0.16 in log-log coordinates, respectively) and slightly strong er for yellow-blue (slope = -0.75). Blur discrimination thresholds are not predicted by the contrast sensitivity function of the visual system. Our fi ndings are useful for predicting blur tolerance for complex images and prov ide a spatial frequency cutoff point when Gaussian low-pass filters are use d for noise removal in colored images. They are also useful as a baseline f or the study of visual disorders such as amblyopia. (C) 2001 Optical Societ y of America.