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.