excitation purity [*p*_{e}]

quantity defined by the ratio NC/ND of 2 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 being that between the point N and the point D on the spectrum locus at the dominant wavelength of the colour stimulus considered, leading to the following expressions:

or

where (*x*, *y*), (*x*_{n}, *y*_{n}), (*x*_{d}, *y*_{d},) are the *x*, *y*-chromaticity coordinates of the points C, N and D respectively

Unit: 1

NOTE 1 In the case of purple stimuli, see NOTE 1 to "purity".

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, *p*_{e}, is related to colorimetric purity, *p*_{c}, by the equation:

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