Color temperature (in Kelvin, i.e., K) of a light source is the absolute temperature of an ideal black-body radiator that radiates light of a color comparable to that of the light source. How is the color temperature of a particular light source specified?

In practice, if a standard blackbody is heated from absolute zero (-273°C, i.e., 0 K), when the temperature rises to a certain level, the color starts to change gradually from deep red – light red – orange red – white to blue. In this process, when a light source and a blackbody present the same color at a temperature, we refer to the absolute temperature of the blackbody as the color temperature of the light source. As shown in Fig. 1.3.1, the color temperature of a light source ranges from 2700 K to 6500 K. Whether it comes to nature or home environment, different color temperatures will trigger different sensations.

As can be seen in Fig. 1.3.1, light with a low color temperature is yellowish, and as the color temperature goes higher, the light gives off blueish rays. Since yellow is a warm color, light features a low color temperature is considered a warm light (e.g., Fig. 1.3.2 Lighting effects of a home space illuminated by a low color temperature). By the same token, blue is a cool color tone, so a high-color temperature is called a cool light (e.g., Fig. 1.3.3).