Purity

Excitation purity Pe

On the CIE plot, the ratio of the distance from the sample colour point to the illuminant points, to the distance of the spectral locus to the illuminant point is called the excitation purity.

The excitation purity is often abbreviated to Pe.

The excitation purity of a colour in the purple triangle of the chromaticity diagram bounded by the illuminant point, 400nm and 700nm on the spectrum locus is calculated in the normal manner using the straight line joining 400nm and 700nm as the spectrum locus for this particular purpose.

Excitation purity may be calculated by one of the following two formulae:

where x, y are the chromaticity coordinates of the colour under test. xn, yn are the chromaticity coordinats of the achromatic centre, such as D65. xd, yd are the chromaticity coordinates of the boundary monochromatic wavelength d.

When the line joining the sample colour and the illuminant point is nearer vertical than horizontal, then the second formula using y coordinates is more precise and when it is nearer horizontal, then the first formula gives better results.

Colorimetric purity Pc

Another scale of purity is the colorimetric purity Pc given by:

Unlike the eqations for Pe, with these equations, if the colour is such that it does not have a dominant wavelength d, but instead a complementary wavelength c, the chromaticity coordinates xd, yd in the formula for Pc are the chromaticity coordinates of c.