ACCURATE CALIBRATION OF FILTER RADIOMETERS AGAINST A CRYOGENIC RADIOMETER USING A TRAP DETECTOR

At the Physikalisch-Technische Bundesanstalt filter radiometers are us ed for the comparison of three primary radiometric standards, an elect ron storage ring, a black body of known temperature and a cryogenic ra diometer. The calibration of the filter radiometers is performed in a two-step process. First, a trap detector is calibrated at the cryogeni c radiometer at discrete laser lines. In the second step, this trap de tector is used to calibrate the filter radiometers at a spectral compa rator. Applying a correction for stray light, the responsivities of th e 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 thi s procedure the responsivity was measured with a relative uncertainty of less than 5 x 10(-4) at the 1 sigma level.