Correlated Color Temperature in Color Appearance Models

Abstract

Correlated color temperature is the temperature of a Planckian radiator which perceived color closely resembles that of a test stimulus. The International Commission on Illumination (CIE) has defined this measurement as being valid under specified viewing conditions and with the test stimulus being of the same brightness as the point of the Planckian radiator. The current method for calculation of the correlated color temperature dates back to 1968 and is based on an obsolete color space which is nowadays not used for any other calculations. The color space is valid for any viewing condition and the brightness is unknown. Color appearance models use viewing conditions and input stimuli to model perceived appearance. Correlates which include brightness and hue are mathematically defined. These parameters are exactly the requirement of the CIE definition for correlated color temperature but are currently not used in the calculation. In this research the definition of color temperature from the Planckian blackbody equation is used to find their locations in color appearance models. Given the blackbody locus as test stimulus different input situations are tested and the models' outputs are compared. Finally a method for calculating the correlated color temperature in any color appearance model is given which conforms much closer to the CIE definition.