AbstractThe concept of color temperature has been used for many years to characterize the relative spectral distribution of energy from light sources. Since not all sources of light are blackbody radiators, the term “correlated color temperature” was devised to indicate the temperature of the blackbody with chromaticity nearest that of the source. Since the CIE has recently recommended two additional color spaces superseding the existing universally agreed system, it is important to know what effect the new recommendation will have on the correlated color temperature determination. In view of this, it has seemed appropriate to design and carry out some fundamental studies in acceptable color matching among color‐normal observers. The isotemperature lines derived from the experimental data described here are within one standard deviation of the isotemperature lines derived from the 1960 UCS diagram. Hence, there is no need to revise the isotemperature lines derived for