We describe the development of a radiation thermometer calibrated for spectral radiance responsivity using cryogenic, electrical-substitution radiometry to determine the thermodynamic temperatures of the Ag- and Au-freezing temperatures. The absolute spectral radiance responsivity of the radiation thermometer is measured in the NIST Spectral Irradiance and Radiance responsivity Calibrations using Uniform Sources (SIRCUS) facility with a total uncertainty of 0.15 % (k = 2) and is traceable to the electrical watt, and thus, the thermodynamic temperature of any blackbody can determined using Planck radiation law and the measured optical power. The thermodynamic temperatures of the Ag- and Au-freezing temperatures are determined to be 1234.956 K ( 0.110 K) (k = 2) and 1337.344 K ( 0.129 K) (k = 2) differing from the ITS-90 assignments by 26 mK and 14 mK, respectively. The temperatures were systematically corrected for the size-of-source effect, the linearity of the pre-amplifier and the emissivity of the blackbody. The temperatures are in agreement with the ITS-90 assigned values within the combined uncertainties. The ultimate goal of these thermodynamic temperature measurements is to disseminate temperature scales with lower uncertainties than the ITS-90. These results indicate that direct disseminations of thermodynamic temperature scales are possible.
Citation: Applied Optics
Pub Type: Journals
blackbody radiation, freezing temperature, ITS-90, radiation thermometry, radiometry, thermodynamic temperature