Calculating Correlated Color Temperatures Across the Entire Gamut of Daylight and Skylight Chromaticities

Natural outdoor illumination daily undergoes large changes in its correlated color temperature CCT, yet existing equations for calculating CCT from chromaticity coordinates span only part of this range. To improve both the gamut and accuracy of these CCT calculations, we use chromaticities calculated from our measurements of nearly 7000 daylight and skylight spectra to test an equation that accurately maps CIE 1931 chromaticities x and y into CCT. We extend the work of McCamy Color Res. Appl. 12, 285-287 1992 by using a chromaticity epicenter for CCT and the inverse slope of the line that connects it to x and y. With two epicenters for different CCT ranges, our simple equation is accurate across wide chromaticity and CCT ranges 3000-106 K spanned by daylight and skylight.