An objective description of the colour quality of a visual stimulus that does not depend on its luminance but which, together with its luminance, completely specifies the colour. The colour quality is defined in terms of chromaticity coordinates, x,y, and z, where

and

X, Y, and Z are the tristimulus values of a light, i.e. they are the amounts of three reference stimuli needed to match exactly the light under consideration in a trichromatic system.