Improved Method for Evaluating and Specifying the Chromaticity of Light Sources

This article describes a method for calculating and specifying light source chromaticity using the International Commission on Illumination (CIE) 2015 10° color matching functions (CMFs), which, according to analysis of existing psychophysical experiment data, can reduce visual mismatch compared to specifications based on the traditional CIE 1931 2° CMFs in architectural lighting applications. Specifically, this work evaluates, documents, and recommends for adoption by lighting standards organizations a supporting system of measures to be used with the CIE 2015 10° CMFs: a new uniform chromaticity scale (UCS) diagram with coordinates (<i>s, t</i>), a measure of correlated color temperature (CCTst), and a measure of distance from the Planckian locus (Dst). It also presents options for updating nominal classification quadrangles. A complete method of this nature has not yet been standardized, which may be contributing to the slow uptake of the CIE 2015 CMFs. The proposed tools are analogous to <i>u, v</i>, CCT, Duv, and the American National Standards Institute (ANSI) C78.377 chromaticity specifications that are all currently defined in the CIE 1960 UCS diagram using the CIE 1931 2° CMFs. While conceptually equivalent, the differences between the current standard method and the proposed <i>st</i> system are important for reducing unintended visual mismatch in the chromaticity of light. The implications of changing chromaticity specification methods are identified by a comparison over a diverse set of real light source spectral power distributions.