The color temperature is a measure for the quantitative description of the color impression of light sources; the unit of measurement of the color temperature is the temperature unit kelvin (K).
The spectrum of an ideal thermal radiator (“black body”, “black body radiator” or “Planckian radiator”) serves as the reference model for determining the color temperature. This emits electromagnetic radiation in the visible and invisible range, whose wavelength distribution is determined solely by the temperature. For real thermal light sources (flame, light bulb, sun) this is approximately true.
When a black body is slowly heated, it passes through a color scale from dark red, red, orange, yellow, white to light blue. The temperature of the black body at which there is the best possible color match with the light source to be determined is the color temperature of the illuminant. Each natural or artificial light situation can thus be assigned approximately a temperature, which can then be used to describe a light situation mathematically.
Since reddish colors are perceived as “warm” and bluish colors as “cool,” a higher color temperature corresponds to a “cooler” color. Common light sources have color temperatures in the order of magnitude of less than 3,300 K (warm white), 3,300 to 5,300 K (neutral white) to more than 5,300 K (daylight white).
For the practice of photography and digitizing, this means that depending on the existing lighting conditions of the location, a certain color temperature must be set in order to achieve a correct reproduction of colors. In digital photography, this process is called white balance.