Accurate calibration of filter radiometers against a cryogenic radiometer using a trap detector

Abstract

At the Physikalisch-Technische Bundesanstalt filter radiometers are used for the comparison of three primary radiometric standards, an electron storage ring, a black body of known temperature and a cryogenic radiometer. The calibration of the filter radiometers is performed in a two-step process. First, a trap detector is calibrated at the cryogenic radiometer at discrete laser lines. In the second step, this trap detector is used to calibrate the filter radiometers at a spectral comparator. Applying a correction for stray light, the responsivities of the filter radiometers can be measured over a dynamic range of more than eight decades in the wavelength range 420 nm to 1200 nm. To overcome drift effects of the interference filters, the filter radiometers were calibrated every night before and after an application. Following this procedure the responsivity was measured with a relative uncertainty of less than 5 × 10-4 at the 1σ level.