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The NPL Radiometric Realization of the Candela

A radiometric realization of the S.I. base unit of photometry, the candela, as performed at the National Physical Laboratory, is described and the results compared with the scale established at NPL in 1937 and disseminated since 1948. The realization has been carried out in accordance with the radiometrically based redefinition given by the Conférence Générale des Poids et Mesures (CGPM) in 1979 which quantifies the spectral luminous efficacy of radiation at a specified frequency (540 × 1012 Hz) as 683 lumens per watt. The excellent performance of the siliconphotodiode photometers developed for this project is demonstrated, together with characteristics of the two types of lamp employed for preserving the results of the realization and methods for improving their alignment and reproducibility.