Calibration of a trap detector against a cryogenic radiometer

Uncertainty in the transfer of power responsivity from a cryogenic radiomet er to a detector arises from several factors. These include the transmittan ce and scatter of the Brewster window present in the absolute power measure ment and the relative positions in the laser beam of the detector and radio meter. To investigate these, the responsivity of a three-element silicon ph otodiode reflectance trap has been determined against a cryogenic radiomete r at 488 nm. The responsivity measurement was performed in two different po sitions along the path of the irradiating laser beam: firstly in front of t he radiometer's Brewster window, and secondly in the plane of the radiomete r's cavity. The resulting responsivities are compared in terms of the signi ficant differences between each measurement technique and the uncertainty b udget of the two calibrations. In addition, the uncertainty in and feasibil ity of measurements made in vacuum behind the Brewster window and in the ca vity plane are assessed.