Comparison of two cryogenic radiometers by determining the absolute spectral responsivity of silicon photodiodes with an uncertainty of 0.02%.

To substantiate the NPL primary standard cryogenic radiometer as an absolute instrument it has been compared with the cryogenic radiometer which was successfully used to determine the Stefan-Boltzmann constant. The comparison was carried out with an accuracy of better than 0.02% by the independent determination of the spectral responsivity of silicon photodiodes with each radiometer. Only a detector comprising a number of silicon photodiodes (a trap device) had the required stability to achieve the desired accuracy. Four trap devices were found to have near unity internal quantum efficiency, being self-consistent to within 0.01%.