p_c = L_d/(L_n+L_d)

where L_d and L_n are the respective luminances of the monochromatic stimulus and of the specified achromatic stimulus that match the colour stimulus considered in an additive mixture

NOTE 1 – In the case of purple stimuli, see Note 1 to 845-03-46.

NOTE 2 – In the CIE 1931 standard colorimetric system, colorimetric purity, p_c, is related to excitation purity, p_e, by the equation p_c = p_e y_d/y where y_d and y are the y-chromaticity coordinates respectively, of the monochromatic stimulus and the colour stimulus considered.

NOTE 3 – In the CIE 1964 supplementary standard colorimetric system, a measure, p_c,10, is defined by the relation given in Note 2, but using p_e, y_d et y instead of p_e,10, y_d,10 and y₁₀.