Direct comparison of cryogenic radiometers from the BIPM and the PTB

For the first time, the cryogenic radiometer of the Bureau International des Poids et Mesures (BIPM) has been taken to a national metrology institute for direct comparison with another cryogenic radiometer. This paper describes a comparison at the Physikalisch-Technische Bundesanstalt (PTB, Braunschweig, Germany) with the PTB cryogenic radiometer. The two systems have been shown to differ, in relative terms, by 2,3 × 10-4 in the measurement of optical power at λ = 632,8 nm with a power level of about 700 μW. The standard uncertainty in the determination of this relative difference is estimated to be 5,7 × 10-5. As the source of this discrepancy, which is larger than the uncertainties, has not yet been found, a closer investigation of the two systems is necessary.