The spectral power distribution (SPD) of a blackbody radiator can be completely determined from its absolute, or color temperature in Kelvin (K). Correlated color temperature (CCT) is a measure of light source color appearance defined by the proximity of the light source’s chromaticity coordinates to the blackbody locus, as a single number rather than the two required to specify a chromaticity. Practical light sources of different SPD but identical chromaticities will also have identical CCTs. Six isotemperature lines are plotted in the CIE 1976 chromaticity diagram in Figure 1. The CCT of a light source can be determined by extending an isotemperature line from the blackbody locus out to the chromaticity coordinates of the source. For example, Point A in figure 8 represents a light source with chromaticity coordinates of (0.24, 0.59). This point lies on the 3000 K isotemperature line, thus the light source has a CCT of 3000 K.
Since it is a single number, CCT is simpler to communicate than chromaticity or SPD, leading the lighting industry to accept CCT as a shorthand means of reporting the color appearance of “white” light emitted from electric light sources. CCT values of most commercially available light sources usually range from 2700 K to 6500 K. CCT values are intended by the lighting industry to give specifiers a general indication of the apparent “warmth” or “coolness” of the light emitted by the source. According to lighting industry convention, lamps with low CCT values (2700 K to 3000 K) provide light that appears “warm,” while lamps having high CCT values (4000 K to 6500 K) provide light that appears “cool.” This convention may have been established because non-electric light sources with low CCTs, such as fire, connote warmth. However, this industry convention may be confusing to many people because the higher the CCT of the lamp, the “cooler” the light appears.
Another weakness of CCT is illustrated in Figure 8 by points A and B, representing two light sources with the same CCT (3000 K). Although lights A and B have exactly the same CCT they have very different chromaticities and will look very different to the eye. The light emitted by source A will look greenish-white, while the light emitted by source B will look purplish-white. To address the potential problem of lamps with the same CCT having a different color appearance, the lighting industry utilizes a color tolerance system in conjunction with CCT to specify color consistency.