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AJ Kallmann, J Schramme, C Neumeyer; Colored Shadows - a Quantitative Analysis . Invest. Ophthalmol. Vis. Sci. 2002;43(13):3793.
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Purpose: The colored shadows, first described by Goethe in 1792, are regarded as a special case of simultaneous color contrast. However, this phenomenon is much stronger than the colors induced by a highly saturated colored paper on a gray test field. To understand why the colored shadows are so vivid, we investigated the influence of the parameters brightness, saturation and hue. Methods: The experiments were performed by using a 22 deg surround illuminated by an additive mixture of a colored light and a white light. The induced color appeared on a round central field (10 deg) which was illuminated by the white light only. The perceived colors of center and surround were quantified by matching them on a CRT. All colored areas on screen and CRT were radiometrically analyzed, the CIE x-, y-chromaticity coordinates and the luminance Y were determined. Results: Each of 6 surround colors was tested in three different saturation steps by changing the intensity of the white light. Increasing saturation increased the colorimetric purity of the central field, but its dependency was weaker than expected on the basis of earlier experiments (Valberg, 1974 J. Opt. Soc. Am. 64 1531-1540). Red and purple surrounds showed a weak induction effect only. For the other colors the perceived purity of the central field was about 50-70% of the surround purity. Matching the perceived color of the surrounds indicated that they appeared less saturated; about 70-90% of the actual case. Increasing the luminance of the white central field by adding white light using a third light source, and keeping the saturation of the surround constant, we found that the induction effect decreased proportionally. The effect is particularly strong only if the luminance of the central field is lower than that of the surround. Conclusion: In order to perceive a vivid colored shadow, the brightness of the central field must be lower than that of the surround, a fact which is not given in a «normal» demonstration of simultaneous color contrast. The surround of the colored shadow is the brightest area in the visual field, and seems to be used by the visual system as a reference. If the brightness of the central field increases so that it dominates the visual field, the system is able to perceive it as white and uses this field as a white reference, which determines all colors in the visual field. Thus, the phenomenon of the colored shadows demonstrates a special case of color constancy.
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