Definition: | quantity defined by the ratio NC/ND of two collinear distances on the chromaticity diagram of the CIE 1931 or 1964 standard colorimetric systems, the first distance being that between the point C representing the colour stimulus considered and the point N representing the specified achromatic stimulus ; the second distance is that between the point N and the point D on the spectrum locus at the dominant wavelength of the colour stimulus considered. The definition leads to the following expressions :
or
where (x, y), (xn, yn), (xd, yd) are the x, y-chromaticity coordinates of the points C, N, and D, respectively
NOTE 1 – In the case of purple stimuli, see Note 1 to 845-03-46.
NOTE 2 – The formulae in x and y are equivalent, but greater precision is given by the formula which has the greater value in the numerator.
NOTE 3 – Excitation purity pe, is related to colorimetric purity, pc, by the equation : pe = pc y/yd.
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