A temperature describing an emitter based on the best match of the shape of its emission spectrum to that of a blackbody at that temperature. The match may be over only a portion of the Planck curve (see Planck's law), or even by noting the wavelength of maximum emission and using Wien's displacement law. Unlike the brightness temperature, color temperature can be used to approximate the physical temperature of objects of unknown distance (especially stars) and also of isothermal emitters that are optically thin.