When the color temperature is strictly defined, it is classified into "color temperature", "distribution temperature", and "two-color temperature", but these have often been referred to simply as color temperature, without distinguishing them exactly.
In this paper, these three color temperatures will be regarded as distinctive, and the definition and the respective determination methods of these three color temperatures will be proposed. From the viewpoint of measuring the color temperatures mentioned above, some new types of radiators will be defined, based on the characteristics of spectral radiation of the radiator, and then the correlation of each color temperature and the true temperature will be discussed.
When the spectral emissivity of metals (εM(λ, T)) is assumed to be in a linear relationship with wavelength λ, i.e. εM(λ, T)=aλ+b (where a μm-1 and b are constant), each of the color temperatures can be expressed as a function of a/b.
Thus, each of the color temperatures for various a/b values is calculated by the determination methods proposed in this paper, and the correlations with each of the other color temperatures and the true temperature of metals are shown. It is checked up whether the assumption mentioned above is reasonable with tungsten, whose wavelength dependency of spectral emissivity is known. Moreover, the color temperature of liquid iron is treated.