The e-ILV provides free access to all the terms and definitions contained in the international standard CIE S 017:2020 ILV: International Lighting Vocabulary, 2nd edition.

To search for another term please use  "Back to the list" to return to the main page or the e-ILV.

For the complete set of terms and definitions CIE S 017:2020 can be purchased from the CIE Webshop. Members of a CIE National Committee or Associate National Committee have access to a 66,7 % discount on the purchase price of the standard.

For advice on CIE Publications contact Note the CIE Helpdesk cannot assist with technical enquiries.


excitation purity


quotient 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 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:


where (x, y), (xn, yn), (xd, yd) are the x, y chromaticity coordinates of the points C, N and D respectively

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: 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 to entry: The excitation purity, pe, is related to the colorimetric purity, pc, by the equation:

Note 4 to entry: The excitation purity has unit one.

Note 5 to entry: This entry was numbered 845-03-48 in IEC 60050-845:1987.

Note 6 to entry: This entry was numbered 17-408 in CIE S 017:2011.

Publication date: 2020-12
Copyright © CIE 2020. All Rights Reserverd.