Comparison of the BIPM cryogenic radiometer with a mechanically cooled cryogenic radiometer from the NPL

The direct comparison at the Bureau International des Poids et Mesures (BIPM) of the BIPM cryogenic radiometer (Radiox) with a mechanically cooled cryogenic radiometer (MCR) from the National Physical Laboratory (NPL) is described. The radiometers were compared using a laser beam at λ = 647 nm with power levels of 400 μW and 650 μW. They were found to be in good agreement, as the measured difference in responsivity was found to be 1,5 parts in 10 5 , with a combined standard uncertainty of 8 parts in 10 5 .