Color rendering metrics: Status, methods, and future development

Abstract

Color rendition metrics, which assess light sources in terms of the color quality of illuminated objects, are advancing with the development of lighting technology and with the increasing needs of lighting users. This chapter reviews different metrics of assessing the color quality of light sources. We show that the traditional measures of the color fidelity, such as the standard color rendering index (CRI) and its single-figure-of-merit refinements, fail to correctly assess the color rendition properties of illumination, especially for the light sources having spectral power distributions composed of narrow-band components, such as polychromatic light-emitting diode clusters. These metrics (based on the estimation of color shifts for a small number of test color samples) do not account for the ability of the light sources to increase or decrease the chromatic contrast (color saturating or dulling) and clash with the subjective preferences to the color quality of illumination. Supplementing these conventional measures with additional figures of merit accounting for the gamut area of a small number of the test color samples does not completely address this issue. The color rendition vector approach to the color shifts allows for a much more comprehensive assessment of the color rendition properties. In particular, many issues of the color rendition problem can be resolved using the statistical approach based on the color rendition vector sorting for a large number of the test color samples. However despite the availability of advanced measures of color rendition for experts, the need for an improved color rendition metric that could substitute for the outdated CRI still exists. An alternative approach to rating the light sources in terms of color quality is the color rendition engineering. The color rendition engineering allows for the development of light sources having requested or even tunable (and traded off) color rendition properties (color rendition engines). Such color rendition engines can meet individual and group needs in color quality of illumination. When supplemented with the additional functionalities offered by information and communication technology, the color rendition engines could become the preferred tools of the smart lighting revolution.