Color temperature may appear to be a somewhat surprising quantity as color and temperature don’t seem to have a direct relationship with each other. However, the relationship is derived from Planck’s black-body radiator. With increasing temperatures, it glows in the red, orange, yellowish white, white, and ultimately bluish white. The color temperature (CT) of a white light source, given in units of kelvin, is the temperature of a planckian black-body radiator that has the same chromaticity location as the white light source considered.

If the color of a white light source does not fall on the planckian locus, the correlated color temperature (CCT), also given in units of kelvin, is used. The correlated color temperature of a white light source is defined as the temperature of a planckian black-body radiator whose color is closest to the color of the white light source.

The correlated color temperature of a light source is determined as follows. On the (u', v') uniform chromaticity diagram, the point on the planckian locus that is closest to the chromaticity location of the light source is determined (i. e. shortest geometrical distance). The correlated color temperature is the temperature of the planckian black-body radiator at that point. The
determination of the correlated color temperature was discussed in CIE publication No. 17.4 (1987) and by Robertson (1968).

On the (x, y) chromaticity diagram, the correlated color temperature cannot be determined by using the shortest distance to the planckian locus due to the non-uniformity of the (x, y) chromaticity diagram. The lines of constant correlated color temperature in the (x, y) chromaticity diagram are shown in Fig. 18.5 (Duggal, 2005).

The chromaticity locations of incandescent light sources are very close to, although not exactly on the planckian locus (Ohno, 2001). For such incandescent light sources, the color temperature can be specified. Standard incandescents have color temperatures ranging from 2 000 to 2 900 K. The common warm incandescent light source has a color temperature of

2 800 K. Quartz halogen incandescent lamps have a color temperature ranging from 2 800 to

3 200 K (Ohno, 1997). Other light sources, such as metal-halide sources, are further removed from the planckian locus. For such light sources, the correlated color temperature should be used. Bluish white lights have a correlated color temperature of about 8 000 K. Color temperatures and correlated color temperatures of common artificial and natural light sources are given in Table 18.1.