Realization of the Candela by Electrically Calibrated Radiometers

The paper describes an experimental realization of the SI base unit for photometry, the candela, according to its new definition, by means of electrically calibrated radiometers. A group of seven stable radiometers has been developed, the value of irradiance measured by each radiometer differing from the mean value for the group by less than 0.1%. Corrections for the reflectance and thermal resistance of the black coating used in the conical cavity of each radiometer, and for shortcomings of the heat sink, have been measured precisely. Two sets of high-quality V (λ) filters have been made by a subtractive combination of coloured optical glasses and their spectral transmittances measured using a double monochromator. BDQ8 and BDQ7 gas-filled tungstenfilament lamps, specially developed as secondary standards for luminous intensity, and stable within ±0.05%, were calibrated against the radiometers. The total uncertainty for the realization of the luminous intensity unit with the electrically calibrated radiometers is ±0.28%.