Contrast-detection thresholds for various combinations of chromaticity and luminance differences were obtained for spatiotemporal square-wave modulation of a yellow field. The results are expressed in terms of excitation of the Vos–Walraven R, G primaries. For every spatiotemporal frequency the thresholds can be approximated by an ellipse in the red–green plane. Large variations were found in the orientation, magnitude, and eccentricity of the discrimination ellipses. It seems that a simple threshold function appears to be sufficient to describe the experimental data. Although the eye does not perceive hue contrast for high spatial frequencies, its sensitivity is not governed mainly by summation of the red and green channels.
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