Because the light emitted by most light sources is collectively referred to as white light, the color surface temperature or correlated color temperature of the light source is used to refer to the relative whiteness of its light color to quantify the light color performance of the light source. According to Max Planck's theory, a standard black body with complete absorption and radiation ability is heated, and the temperature gradually increases, and the luminosity changes accordingly; the black body locus on the CIE color coordinate shows that the black body changes from red to orange-red- - yellow-yellow-white-white-blue-white process. The temperature at which the black body is heated to the same or close to the light color as the light source is defined as the correlated color temperature of the light source, called the color temperature, in absolute temperature K (Kelvin or Kelvin) as the unit (K=℃+273.15). Therefore, when the black body is heated to red, the temperature is about 527°C, or 800K, and other temperatures affect the change of light color.

The bluer the light, the higher the color temperature; the redder the lower the color temperature. The light color of the painted light also changes with time: 40 minutes after sunrise, the light color is yellow, and the color temperature is 3,000K; at noon, the sun is snow-white, rising to 4,800-5,800K; at noon on a cloudy day, it is about 6,500K; the light color before sunset Reddish, the color temperature drops to 2,200K on paper. The correlated color temperature of other light sources, because the correlated color temperature is actually a black body radiation close to the light color of the light source, the evaluation value of the light color performance of the light source is not an accurate color comparison, so two light sources with the same color temperature value, There may still be a slight difference in light color appearance. Color temperature alone cannot tell how a light source can render an object's color, or how well an object's color is reproduced under that light source.