What is color temperature?

warm mid cool
Warm
(2000-3000K)
Mid-range
(3000-4000K)
Cool
(4000K +)

Color Temperature describes certain color characteristics of light sources. A "blackbody" is a theoretical object which is a perfect radiator of visible light. As the actual temperature of this blackbody is raised, it radiates energy in the visible range, first red, changing to orange, white, and finally bluish white.

Color temperature describes the color of a light source by comparing it to the color of a blackbody radiator at a given temperature. For example, the color appearance of a halogen lamp is similar to a blackbody radiator heated to about 3000 degrees Kelvin. Therefore it is said that the halogen lamp has a color temperature of 3000 degrees K- which is considered to be a warm color temperature. The hotter the blackbody, the cooler the color temperature! (Note: The Kelvin temperature scale uses the same size degree as the centigrade scale, but its zero point is at absolute zero, or -273 degrees C). Sunlight can be "warm" or "cool" depending on the time of day and the ambient conditions.

Though color temperature is not a measure of the physical temperature of the light source, it does correspond to the physical temperature of the blackbody radiator when the color appearance is the same as the source being tested.

Here is an example of a standard halogen light bulb, which has a "warm" color temperature vs. a special compact fluorescent lamp, which has a "cool" temperature.