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 Laborator y (NPL) is described. The radiometers were compared using a laser beam at lambda = 647 nm with power levels of 400 mu W and 650 mu W. They w ere found to be in good agreement, as the measured difference in respo nsivity was found to be 1,5 parts in 10(5), with a combined standard u ncertainty of 8 parts in 10(5).