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Color Temperature and CCT

Any light source whose chromaticity coordinates fall directly on the Planckian locus has a color temperature equal to the blackbody temperature of the Planckian radiator with those coordinates. Color temperature is usually expressed in Kelvin (K). The concept of color temperature is especially useful for incandescent lamps, which very closely approximate a blackbody spectrum throughout the visible region. For these lamps, the color temperature also defines the spectrum in this region.

For white lights that don't have chromaticity coordinates that fall exactly on the Planckian locus but do lie near it, the correlated color temperature (CCT) is used. The CCT of a light source, also expressed in Kelvins, is defined as the temperature of the blackbody source that is closest to the chromaticity of the source in the CIE 1960 UCS (u, v) system. CCT is an essential metric in the general lighting industry to specify the perceived color of fluorescent lights and other nonincandescent white-light sources such as LEDs and high intensity discharge HID lamps.

The difference in perceived color is closely related to the reciprocal of CCT. The reciprocal is expressed in reciprocal megakelvin (MK)^-1, with one (MK)^-1 approximately equal to a just-noticeable color difference:

(MK)^-1=10⁶/CCT.

There are limitless different spectra, all with the same CCT, that may have little or no resemblance to the blackbody curve for that temperature or to each other.

There is no approved method for computing CCT nor is there a simple and accurate closed-form expression. One simple and accurate method is to use a program such as Excel with solver to find the blackbody temperature that minimizes the distance between its (u, v) coordinates and those of the light in question.