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 to entry: In the case of purple stimuli, the monochromatic stimulus is replaced by a stimulus whose chromaticity is represented by a point on the purple boundary.

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

Note 3 to entry: In the CIE 1964 standard colorimetric system, a measure, p_c,10, is defined by the relation given in Note 2 to entry, but using p_e,10, y_d,10 and y₁₀ instead of p_e, y_d, and y respectively.

Note 4 to entry: This entry was numbered 845-03-47 in IEC 60050-845:1987.