Calibration of a trap detector against a cryogenic radiometer

Uncertainty in the transfer of power responsivity from a cryogenic radiometer to a detector arises from several factors. These include the transmittance and scatter of the Brewster window present in the absolute power measurement and the relative positions in the laser beam of the detector and radiometer. To investigate these, the responsivity of a three-element silicon photodiode reflectance trap has been determined against a cryogenic radiometer at 488 nm. The responsivity measurement was performed in two different positions along the path of the irradiating laser beam: firstly in front of the radiometer's Brewster window, and secondly in the plane of the radiometer's cavity. The resulting responsivities are compared in terms of the significant differences between each measurement technique and the uncertainty budget of the two calibrations. In addition, the uncertainty in and feasibility of measurements made in vacuum behind the Brewster window and in the cavity plane are assessed.