Measurements using two types of transfer radiometer developed for a monochromator-based cryogenic radiometer facility

A facility for the monochromator-based calibration of transfer standard detectors using a cryogenic radiometer has been set up at the National Research Council of Canada (NRC). A brief status report is given on this facility, which has undergone some modifications since it was first set up in 1994. Measurements so far have concentrated on the spectral range 450 nm to 700 nm. The wavelength calibration of the monochromator used with this facility is discussed in detail. Two types of transfer radiometer have been developed for use with the cryogenic radiometer. The original radiometers use a single photodiode and incorporate a quartz window. The second type of radiometer is a windowless three-detector trap using special 18 mm properties of these two types of transfer radiometer are compared: spectral responsivity, uniformity, polarization sensitivity, and linearity. Results of calibrations of these radiometers over one to two years are presented. These measurements suggest that the long-term reproducibility of the calibrations is approximately 0.025% for a single radiometer, but close to 0.01% for a group of radiometers. The transfer radiometers can be used to calibrate accurately other detectors using auxiliary apparatus. Measurements are presented which suggest that the transfer uncertainty is close to 0.01% on average. The accuracy of the cryogenic-radiometer-based calibrations can be estimated by linking them to the international comparison of spectral-responsivity scales carried out at the Bureau International des Poids et Mesures in 1994 under the auspices of the Consultative Committee for Photometry and Radiometry (CCPR). From this comparison, it is estimated that the overall accuracy of the cryogenic radiometer calibrations is better than 0.05% in the range 450 nm to 700 nm.