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 luminance (black-white) direction in color space. We report the following results: (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 (1.5 min of visual angle). (ii) The smallest blur thresholds are found for slightly blurred square waves (reference blur of 1 arc min) and not for sharp edges. (iii) Blur thresholds for red-green and black-white follow a Weber law for reference (pedestal) blurs greater than the optimum blur. (iv) Using the model proposed by Watt and Morgan [Vision Res. 24, 1387 (1984)] we estimated the internal blur of the visual system for the black-white and the red-green color directions and arrived at the following estimates: 1.2 arc min for black-white stimuli at 10% contrast and 0.9 arc min for red-green stimuli at 10% cone contrast. Blur tolerance for yellow-blue is independent of external blur and cannot be predicted by the model. (v) The contrast dependence of blur sensitivity is similar for red-green and luminance modulations (slopes of -0.15 and -0.16 in log-log coordinates, respectively) and slightly stronger for yellow-blue (slope = -0.75). Blur discrimination thresholds are not predicted by the contrast sensitivity function of the visual system. Our findings are useful for predicting blur tolerance for complex images and provide a spatial frequency cutoff point when Gaussian low-pass filters are used for noise removal in colored images. They are also useful as a baseline for the study of visual disorders such as amblyopia.