Abstract
Under certain viewing conditions, the colour of a light in a surround is determined by the vector from the surround to the colour in a 3-D colour space. This behaviour has been explored with a computer graphics tool that allows sets of such vectors to be drawn from two surrounds in any plane in any colour space, and allows the resulting lights-in-surrounds to be compared haploscopically. In a log cone-excitation space contrast colours with the same surround-to-colour vectors match well, over a surprisingly wide range of chromaticities and luminance contrasts. This implies that the contrast colours are determined by the three cone contrasts; the absolute chromaticities and luminances do not matter. Some limits and implications of this behaviour are discussed.
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