A facility is being set up at the National Research Council of Canada for the monochromator-based spectral calibration of transfer standard radiometers using a cryogenic absolute radiometer. General design features and target performance of the new apparatus are discussed. The sources of error inherent to detector calibrations performed using monochromator-based sources are analysed in some detail, with special attention devoted to bandwidth errors, polarization effects and the effect of the angular subtense of the beam. Some detector effects, such as spatial uniformity and temperature dependence, are also discussed. A status report is given on the development of the new cryogenic radiometer facility.
The paper discusses automated spectral linearity measurements using both absolute and relative methods. Results of measurements on typical silicon and germanium photodiodes are presented. These measurements show the agreement between the two methods. They also illustrate some interesting linearity properties: wavelength dependence, detector-to-detector variation, current versus current-density dependence, geometrical effects, and effect of reverse bias. In particular, it is shown that some silicon and germanium detectors must be underfilled with radiation to avoid edge effects. These cause nonlinearities which are generally small (1 % level) but, in certain types of thermoelectrically cooled germanium detectors, can be very large (8 % to 10 %).
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 18 mm silicon photodiodes. Some of the 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.
A method for calibrating incandescent lamps for spectral irradiance by means of absolute radiometers is described in which a secondary radiometer is calibrated spectrally against absolute radiometers and then used in conjunction with a series of filters to calibrate the lamps. Considering both narrowband and wideband filters, an extensive mathematical error analysis is performed. The use of narrowband filters (20-25-nm halfwidth) is found to be advantageous because very little information is required on the spectral distribution of the lamp being measured. The most serious source of error is a wavelength shift in the measured spectral transmittances of the filters, especially at shorter avelengths; for example, at 400 nm, a wavelength shift error of 1 nm can cause an error approaching 3%. It is estimated that the accuracy of spectral irradiance measurements made using the method described here will vary between +/-1 and +/-0.5% from ~350 to 800 nm. Measurements on 500-W quartz-bromine spectral irradiance standards are described. With such lamps, only four or five narrowband filters are required to cover the spectral range from the near UV to the near IR. The measured and calibration values agreed to ~ +/-0.5% on average with a maximum difference of ~1%.
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