Color temperature: When the light source emits light color with the radiation photochromic of black-body under a certain temperature are the same , the temperature of the blackbody referred to as the light source color temperature. Because most of the light source send out the light is known as the white light，so the light source color table temperature or correlated color temperature refer to the degree of light color relative to the white, to quantify the light source light color performance.
According to the theory of Max Planck, use a completely absorbed and a radiation ability standard’s blackbody to heat, the photometric changes with temperature; Blackbody curve on CIE color coordinates shows the changes processes of the red-orange-yellow-yellow-white-blue and white. When the black body heated appear the same as the light sources or close to the light color temperature, so the light source was defined as correlated color temperature–color temperature(Kelvin)(K=℃+273.15).
Therefore, when blackbody heated to red, its temperature up to about 527 ℃ which is 800 k, the other temperature affect light color changes. When the photochromic closer to blue, the color temperature is more higher; The photochromic closer to red, the color temperature is more lower. During the day photochromic changes over time: Photochromic after sunrise 40 minutes is yellow, color temperature is 3000 K; At noon, the temperature rise to 4800-5800 K, at cloudy day noon, about 6500 K; Before the sunset photochromic changes to red, color temperature fell to 2200 K .