Recommended Method for Determining the Correlated Color Temperature and Distance from the Planckian Locus of a Light Source

Abstract

Correlated color temperature (CCT) is one of the primary metrics used to characterize the visual appearance of light and is most informative when coupled with distance from the Planckian locus (Duv). Given a set of chromaticity coordinates—which are calculated from a light source’s spectral power distribution—it is possible to compute CCT and Duv with varying levels of accuracy. Over the last six decades at least a dozen methods have been proposed to compute CCT while balancing accuracy with calculation complexity. CCT values become inconsistent at some level of precision when calculated using different methods, which in turn can lead to discrepancies in dependent or subsequent calculations used by lighting professionals and may be problematic in software. Although methods are now documented that can provide extremely high accuracy, no consensus body has recommended a preferred method. This analysis examines both the calculation speed and the CCT and Duv accuracy of previously proposed and new methods. With consideration of the calculation accuracy, computational burden, calculation complexity, and considerations of practical implementation, we recommend a recent modification of the Robertson method.