New basis for the Australian realization of the candela

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.