Properties of sphere radiometers suitable for high-accuracy cryogenic-radiometer-based calibrations in the near-infrared

A current project at the National Research Council of Canada (NRC) is to realize a new near-infrared (NIR) spectral-responsivity scale in the range 900 nm to 1600 nm using the NRC monochromator-based cryogenic radiometer facility. A major part of this work consists of developing high-accuracy transfer radiometers for this spectral range. A new type of transfer radiometer incorporating a small integrating sphere and three 5 mm diameter InGaAs detectors is described. The following properties of these so-called sphere radiometers are discussed: spectral responsivity, spatial uniformity, angular variation of responsivity, temperature sensitivity, and linearity. These radiometers have a relatively high responsivity even though they incorporate integrating spheres: the peak responsivity is close to 0.3 A/W, compared with approximately 1.0 A/W for 5 mm or 10 mm diameter InGaAs detectors. In particular, the radiometers are uniform to better than 1 part in 103 and the angular variation is less than 3 parts in 104 for ± 5° incidence, throughout the spectral range 900 nm to 1600 nm. A brief analysis is presented of the uncertainties associated with the realization of a spectral-responsivity scale in the NIR using the above radiometers and the NRC cryogenic radiometer facility.