CCT Calculator

To describe the color of white light, typically a correlated temperature and tint value are used, calculated through complex algorithms that rely on x and y chromaticity coordinates or X, Y, and Z tri-stimulus values. This easy-to-use tool implements a fast, high-accuracy bisectional search method based on Robertson’s algorithm.

Instructions

The CIE colorimetric system uses X, Y, and Z tristimulus values, calculated using spectral distributions and color-matching functions, as its basis for various colorimetric models. Lightsource chromaticity —or the color of “white” light— is typically expressed using a correlated color temperature and a —less commonly used— Tint value. Calculating these is complex. There is no simple mathematical formula: the method used here uses a high-accuracy bisectional search method.

Input are (X, Y, Z) tristimulus values, the default setting, or scaled (x,y) chromaticity coordinates, as used in the CIE 1931 chromaticity diagram. The values are entered by selecting the input fields and typing the numbers. The keyboard Enter-key will update the calculated values in the output display area. The correlated color temperature has as unit Kelvin - the Tint value is unitless and is 1000x the distance from the color point to the Planckian curve, measured in the CIE1964 chromaticity diagram, which is sometimes referred to as milli-Δuv.

The initial values shown at start-up are the tri-stimulus values for the CIE D65 illuminant, with a Correlated Color Temperature of 6503.2K and a 3.2-Tint value, corresponding to a distance of Δuv = 0.0032 in the CIE 1964 diagram. Positive Tint values are above and appear more yellow-greenish, while negative values are below the Planckian Locus and typically appear more purple-pinkish.

By clicking the xy selection button, the input field changes to xy chromaticity coordinates, which are 0.31271 and 0.32903 for the D65 illuminant.

The temperature range of the implemented algorithm is 1000 Kelvin to 1,000,000 Kelvin; an error message indicates a temperature is too low or too high. The CIE definition of Correlated Color Temperature limits its calculation to a Tint range of -50 to 50, as it is a quantity mainly used for “whitish” light sources — an error message will be shown if that’s the case.

Invalid input will also result in error messages. Negative values are not allowed, as tristimulus and chromaticity values are always positive. Furthermore, chromaticity values x and y —lowercase— should be between 0 and 1, and their sum should be in this range too. There is no range limit in the tristimulus values, using the uppercase letters X, Y, and Z, except that they need to be positive.