The relative spectral power distribution and correlated color temperature of the visible radiation from a tungsten filament lamp have been measured by a radiometric comparison with thermopiles of known spectral responsivity making use of a series of glass filters transmitting in different spectral bands. The method is independent of Planck’s formula and the temperature scale.
J. R. Moore, "Verification of the NPL Standards of Relative Spectral Power Distribution by Filter Radiometry," Appl. Opt. 10, 2651-2656 (1971)