Dichromacy characterized by chrominance planes

Dichromacy is described in terms of dichromatic opponent colour spaces. By means of the perceptual criteria 'equally bright', 'neither blue nor yellow' and 'neither red nor green' and embedding in a three-dimensional colour space, it is possible for each type of dichromat to quantify a null-chrominance plane and a null-luminance plane, both of which intersect in the missing colour. These two null planes (or the trace of their intersection with the chromaticity chart) are the chromaticities of the dichromatic opponent primaries. Since a null-luminance plane contains only colour ('chrominance'), it is simply a chrominance plane. Under the assumption that the retinal short-wavelength cones do not contribute to luminance, the chrominance planes of the three types of dichromats intersect in a common straight line, the 'blue' fundamental primary vector. This constellation may serve as a general characterisation of dichromacy.