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

Abstract

Cryogenic radiometers are widely used as national primary standards for radiometric and photometric measurements. The most common comparisons are indirect, using various types of trap detector. The lowest limit of relative uncertainty achievable by indirect comparisons is not much less than 2 × 10−4 (k = 1). To improve this level of uncertainty, it is necessary to compare the cryogenic radiometers directly at the same position using an identical beam. In this case error sources such as beam size, beam power and scatter 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-Technische Bundesanstalt (PTB), was the fourth in a series of direct comparisons worldwide. It took place in the best environment presently available, at the transfer-optimized cryogenic radiometer in the class 1000 clean-room centre at the PTB in Braunschweig (class 100 is available). The two radiometers were sequentially positioned by a translation stage in the power-stabilized 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 × 105, which is currently the lowest obtained in a radiometric comparison of absolute scales.