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.