The chromaticities of hot bodies (such as the filaments of incandescent lamps) and the various phases of daylight are often expressed on the color–temperature scale. Such chromaticities do not usually coincide precisely with those of the blackbody at any temperature, but are specified by giving the temperature of the blackbody yielding the nearest chromaticity match. Thus each chromaticity point near the Planckian locus is correlated with a chromaticity point on it, and the color temperature corresponding to the Planckian chromaticity is called the correlated color temperature. If the decision as to the nearest Planckian chromaticity is based on a uniform-chromaticity-scale (UCS) diagram, the chromaticity points correlated with each Planckian chromaticity fall on a straight line (isotemperature line) on the chromaticity diagram normal to the Planckian locus at that Planckian chromaticity. The UCS diagram proposed by MacAdam in 1937 and recommended in 1959 by the International Commission on Illumination (CIE) has been used here to define the isotemperature lines. Graphs showing the Planckian locus and the isotemperature lines both in the CIE (x,y)-diagram and in the MacAdam CIE (u,υ)-diagram are shown at every 10 microreciprocal degrees for all color temperatures higher than 1515°K.
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