As the temperature of an object increases, the emitted thermal radiation also increases. At the same time, the color changes from red through orange to white. A blackbody is an ideal object that absorbs all energy and emits it as radiant energy in such a manner that its temperature is directly related to the color of the emitted radiant energy. The absolute temperature of the blackbody is referred to as the color temperature. These colors would lie along the ideal blackbody locus, as indicated in the xy chromaticity chart shown in Figure 26.

Correlated color temperature is used to apply the general idea of color temperature to those colors that are close to, but not exactly on, the blackbody locus. The correlated color temperature is calculated by determining the isotemperature line on which the color of the light source is positioned. Isotemperature lines are straight lines for which all colors on the line appear visually equal; the correlated color temperature of any color on the isotemperature line is equal to the color temperature at the point where the isotemperature line intersects the blackbody locus.

The blackbody locus, the isotemperature lines and lines that indicate equal values of ∆uv from the blackbody locus are illustrated in Figure 27. For example, a light source which has a color difference of 0.01 in the green direction (∆Euv) from a blackbody which has a color temperature of 7000K is indicated as having a correlated color temperature of 7000K+0.01 (uv unit).

Notes
• See Section IV "Color Terms" for explanation of (∆Euv).
• "K" is an abbreviation for Kelvin. Kelvin is the absolute temperature scale.