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Frederic Devinck, Peter B. Delahunt, Joseph L. Hardy, Lothar Spillmann, John S. Werner; Color assimilation: Dependence of watercolor spreading on contour luminance contrast and stimulus width. Journal of Vision 2005;5(8):719. doi: https://doi.org/10.1167/5.8.719.
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The Watercolor Effect (WCE) is a long-range color assimilation effect. When a dark chromatic contour (e.g., purple) surrounds a lighter chromatic contour (e.g., orange), the lighter color will assimilate over the entire enclosed area. We performed two sets of experiments to measure the dependence of the WCE on: (1) the luminance contrast between the contours and the background, and (2) the width of the stimulus. The strength of the WCE was determined using a hue-cancellation technique. In the first set of experiments, we measured the color perceived in the enclosed area as a function of the luminance contrast between the orange contour and the background while the luminance contrast for the double contour was held constant. Using a variation of the minimally-distinct border technique, we determined an equiluminance level between the orange inner contour and the white background (CIE xy 0.30 0.33); then four luminance levels of the orange contour below the luminance background were chosen. The data showed that the chromaticity shift moves closer to the orange border when the luminance ratio between the background and the orange contour increased. In the second set of experiments, assimilation of the WCE was measured as a function of stimulus width (the part of the stimulus enclosed by the double contour). The contours had a luminance of 20 cd/m2 (outer contour) and 55 cd/m2 (inner contour) and the background was white with a luminance of 80 cd/m2. Larger shifts in color appearance were observed for smaller stimulus widths ( ≤ 9.3 arcmin), with an exponential decrease in strength of assimilation as stimuli increased in width (up to 7.4 degrees). Correcting for chromatic aberration reduced the magnitude of color spreading for narrow stimulus widths ( ≤ 9.3 arcmin), but not for wide ones. Thus, the strength of the WCE depends on the luminance contrast between the inner contour and the background and also by the width of the stimuli.
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