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 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.
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