In physics, a black body is an object that absorbs all electromagnetic radiation that falls onto it. A black body, when heated, emits radiation which color depends only on temperature, starting at red, going through orange, yellow and white before ending up at blue.

The color of light emitted by a blackbody radiator at some temperature is the Color Temperature. These colors would lie along the ideal blackbody locus, or Planckian locus (1), the dark curve in the CIE-xy chromaticity diagram (2). The "horseshoe" area represents the colors perceived from the human visual system. The triangle encloses the colors that a typical windows monitor can display (sRGB color space). Strictly speaking, only chromaticities falling on the Planckian locus have true color temperatures. However, for chromaticities falling near the locus, but not on it, colorimetry stated a related property called the Correlated Color Temperature.

Blackbody locus in CIE-xy diagram

This is the color temperature of the point on the blackbody locus that is closest in appearance (chromaticity-wise) to the chromaticity of interest. Technically, this means the point on the blackbody locus nearest the point representing the chromaticity of interest when the blackbody locus is plotted not on the CIE-xy chromaticity diagram but rather on the CIE-uv space chromaticity diagram, on which equal geometric distances represent essentially equal perceived differences in chromaticity.
Sun light and incandescent lights are very close to being a black-body radiator. However, other light sources, such as fluorescent lamps, do not emit radiation in the form of a black-body curve, and are assigned what is known as a Correlated Color Temperature. 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 blackbody locus, the isotemperature lines are lines that indicate equal values of Δuv from the blackbody locus are illustrated in the following figure. For example, a light source which has a color difference of 0.01 in the green direction (Δuv) from a blackbody which has a color temperature of 5500K is indicated as having a correlated color temperature of 5500K+0.01 (uv unit). By convention, chromaticities above the locus have positive values of Δuv, and those below negative values.