A Further Intercomparison Of Two Cryogenic Radiometers

Two cryogenic radiometers (electrical substitution radiometers cooled to liquid helium temperatures) have been developed at NPL. The first was used to realise thermodynamic temperature and to determine the Stefan-Boltzmann constant. The latter measurement confirmed the calculation that the radiometer is an absolute instrument with an uncertainty of 0.02%. The second radiometer was dedicated to optical radiation measurements with a calculated uncertainty of 0.005%. This paper describes and presents results of the intercomparison of the two radiometers by comparing the spectral responsivity of silicon photodiodes as independently determined by the radiometers.