Lowest uncertainty direct comparison of a mechanically-cooled and a helium-cooled cryogenic radiometer

Cryogenic radiometers are widely used as national primary standards for rad iometric and photometric measurements. The most common comparisons are indi rect, using various types of trap detector. The lowest limit of relative un certainty achievable by indirect comparisons is not much less than 2 x 10(- 4) (k = 1). To improve this level of uncertainty, it is necessary to compar e the cryogenic radiometers directly at the same position using an identica l beam. In this case error sources such as beam size, beam power and scatte r are significantly reduced. The comparison between the mechanically-cooled cryogenic radiometer (MCR) of the National Physical Laboratory (NPL), and the liquid-helium-cooled radiometer (LaseRad II) of the Physikalisch-Techni sche Bundesanstalt (PTB), was the fourth ill a series of direct comparisons worldwide. It took place in the best environment presently available, at t he transfer-optimized cryogenic radiometer in the class 1000 clean-room cen tre at the PTB in Braunschweig (class 100 is available). The two radiometer s were sequentially positioned by a translation stage in the power-stabiliz ed laser beam. In four different series of measurements, within two weeks, the average stated relative difference of the measurements of radiant power with these two very different cryogenic radiometers was only 3.2 x 10(-5) at the Iie-Ne laser wavelength 632.8 nm. This is well within the overall co mbined uncertainty of 4.0 x 10(-5), which is currently the lowest obtained in a radiometric comparison of absolute scales.