Color temperature of a light source is the temperature of an ideal black body radiator that radiates light of comparable hue to that of the light source.
Because most light of the light source is referred as the white light, the surface temperature of light source or correlated color temperature is used to refer to the relative degree of light color, to quantify the light performance of light source. According to Max Planck Theory, to heat a standard black body with absolute absorption and emitting, the luminosity changes when temperature increases gradually; Black body locus of CIE color coordinate shows that black body changes from red, salmon, yellow, yellowish white, white to blueish white.
Black body heated to appear the temperature that is the same or closed to the temperature of photochromic is defined as the correlated color temperature of the light source, called as CCT, the absolute temperature K (Kelvin or Kelvin Temperature ) as a unit ( K= ℃ +273.15 ). Therefore, black body heated to red, the temperature is about 527℃ that is 800K, and other temperature influences the photochromic.