Realization of the New Candela (1979) at NRC

Abstract

The paper describes the realization of a new scale of luminous intensity at the National Research Council of Canada, based on the 1979 redefinition of the candela. Two different methods were used. In the first method, the lamps were calibrated directly by means of absolute radiometers equipped with V(λ)-correcting filters. In the second method, secondary radiometers incorporating silicon photodiodes were first calibrated spectrally by means of absolute radiometers, then used in conjunction with V(λ) filters to calibrate the lamps. The sources of error and measurement results for the two methods are analyzed in detail. A comparison of the new scale with the previous platinum-blackbody-based scale indicates that the new NRC candela is 1.2% smaller on average than the old NRC candela.

An optical method for direct determination of the radiometric aperture area at high accuracy

“…The development of cryogenic radiometers [1] and trap detectors [2,3] allows very accurate spectral responsivity measurements of detectors. The uncertainty of the aperture area of the photometer is at present one of the largest components of the uncertainty in detector-based realizations of luminous intensity and illuminance scales [4][5][6][7].…”

High-accuracy optical power measurements by using electrical-substitution cryogenic radiometer

“…The candela, the SI base unit of luminous intensity, was realized by determining the absolute responsivity of a detector from the absolute measurement of optical power using ESCRs working at the temperature of liquid helium. [10][11][12][13][14] At this temperature, the uncertainty in the optical power measurements can be reached 10,15 is of the order of 0.005%. The principal advantages of operating radiometers at low temperatures are the heat capacity of the absorbing material is drastically reduced, which in turn reduces the time constant and measurement time accordingly; heat loss is reduced at low temperatures because the radiant energy is proportional to T 4 ; heat contributions from the resistor leads is eliminated by using superconductive wires; and operating the detector in vacuum eliminates convection losses.…”

Filter-radiometer-based realization of candela and establishment of photometric scale at UME

“…In the past, single photodiode-based detectors were used to realize luminous intensity unit. However due to the high reflectivity of photodiodes ($35%) interreflections between photodiode and V ðlÞ filter reduces the accuracy of the measurements [1,10]. With the design of trap detectors, this problem and, therefore, the uncertainties in the photometric and radiometric realizations were minimized [11].…”