Experimental procedures for the comparison of cryogenic radiometers at the highest accuracy

Cryogenic radiometers are recognized as by far the most accurate radiometers for visible and nearinfrared radiation. The now widespread use of such instruments around the world as the basis for optical radiation measurement scales makes it important to carry out a comparison. This will ensure not only that the instruments perform to the uncertainties claimed, a few parts in 10 4 or less, but also that their operators can disseminate such uncertainties. In this paper results of some pilot comparisons, both direct and indirect using transfer standards, are given. The experimental techniques used at the Bureau International des Poids et Mesures (BIPM) to achieve the highest accuracy are described in detail.