Spectral responsivity determination of a transfer-standard pyroelectric radiometer

Abstract

A NIST-developed pyroelectric radiometer was characterized and calibrated to extend the NIST high accuracy spectral responsivity scale from the visible range to the ultraviolet (UV) and infrared (IR). The transmission of the gold-black coated LiNbO₃ pyroelectric material is negligibly small, therefore the absorptance, equal to 1-reflectance, is proportional to the responsivity of the detector. The spectral total reflectance of the coating was measured with integrating spheres and spectrophotometers to determine the relative spectral responsivity from the UV to the IR. The relative spectral responsivity was converted into absolute spectral power and irradiance responsivities by measuring the total power in a 442 nm stabilized laser beam. The reference device for absolute calibration was a Si trap-detector calibrated against the primary standard cryogenic radiometer. The calibrations were repeated with 31 months separation. A 0.3 % long-term shift in the reflectance was measured between 800 nm and 19 &#956;m. The 28 % reflectance at 10.6 &#956;m scaled up the 0.1 % spatial responsivity non-uniformity in the visible (where the reflectance was 0.4 %) to 8 %. The spectral power and irradiance responsivity scales of the pyroelectric radiometer have been realized between 250 nm and 2.5 &#956;m with a relative standard uncertainty of less than 0.34 % (coverage factor <i>k</i>=1).