where Ld and Ln 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, pc, is related to excitation purity, pe, by the equation pc = pe yd / y where yd 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, pc,10, is defined by the relation given in Note 2 to entry, but using pe,10, yd,10 and y10 instead of pe, yd, and y respectively.
Note 4 to entry: This entry was numbered 845-03-47 in IEC 60050-845:1987.
Note 5 to entry: This entry was numbered 17-195 in CIE S 017:2011.