Calibration of the Irradiance Responsivity of a Filter Radiometer for T Measurement at NIM

At the National Institute of Metrology of China (NIM), silicon photodiode-based narrow-band interference filter radiometers (FRs) have been designed for the radiometric determination of the thermodynamic temperature. The FR calibrations were performed on a new spectral comparator with a trap detector which was calibrated against the cryogenic radiometer at several discrete laser lines. The new spectral comparator is constructed from two grating monochromators assembled to give lower stray light and higher transmitted flux. Applying a transmittance measurement of the filter in the out-of-band region and careful control of the temperature, the irradiance responsivity of a 633 nm centered FR has been obtained over a dynamic range of nearly eight decades in the wavelength range from 450 nm to 1200 nm. The relative standard uncertainty of the responsivity is also analyzed and estimated to be less than 7 × 10−4 at the 1 σ level.