Chromaticity coordinates

The chromaticity coordinates x, y and z are obtained from the tristimulus values X, Y and Z by:

x = X/(X + Y + Z).

y = Y/(X + Y + Z),

z = Z/(X + Y + Z)

From the nature of these equations it can be seen that x, y and z always sum to one, the chromaticity of a colour may be uniquely defined by two of the coordinates only. In the diagram below x is plotted as the abscissa and y as the ordinate. This is the CIE recommendation and the spectrum locus of real colours of maximum saturation is shown plotted in terms of its chromaticity coordinates. Also shown are the positions of the coordinates of two commonly used illuminants, D65 which represents an approximation to daylight and A which represents an incandescent tungsten light.

It should be noted that the term 'illuminant' is used to describe D65 and A and not the term 'source'. The difference is that 'illuminant' can be used to describe a relative energy distribution curve throughout the visible and near visible sections of the spectrum. It may or may not be physically realisable. Any such realisable illuminant may be termed a 'source'. As 'illuminants' A, B and C are realisable they are also 'sources'.