Photometric units in Australia are currently derived from power measurements made with a roomtemperature electrical-substitution radiometer. This basis has been transferred to a cryogenic radiometer. The cryogenic radiometer was used to calibrate the absolute responsivity of a four-element, planar, transmission trap detector (measuring the transmission loss each usage), at laser wavelengths. Modelling of the quantum defect then provided a reference for absolute spectral responsivity throughout the photometric wavelength range. This was used to measure the responsivity of a number of photometers at the mercury green line and to calculate correction factors for CIE Illuminant A. The candela so derived essentially agrees with our existing unit, maintained on a set of lamps. The trap detector was also used to determine the polarization properties of the monochromator.
A single-element, windowless InGaAs photodetector has been calibrated against a cryogenic radiometer using a diode-laser source of wavelength 1296.9 nm (standard uncertainty = 0.4 nm). This source differs from the gas lasers usually used with cryogenic radiometers in that the beam has an elliptical profile, is invisible to the naked eye, is not polarized, and is less spatially coherent. We describe the simple adaptations we have made to use this source with the cryogenic radiometer. In particular, we use a fibre-optic delivery system to prepare the beam, and an anti-reflection-coated window to admit the radiation. The relative uncertainty in this method of calibration is estimated to be 0.08 % (1). The measured response (0.9752 A/W) differs from that obtained from the existing laboratory scale by 0.08 %.
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