The candela and photometric and radiometric measurements

The national measurement system for photometric and radiometric quantities is presently based upon techniques that make these quantities traceable to a high-accuracy cryogenic radiometer. The redefinition of the candela in 19 79 provided the opportunity for national measurement laboratories to base t heir photometric measurements on optical detector technology rather than on the emission from high-temperature blackbody optical sources. The ensuing technical developments of the past 20 years, including the significant impr ovements in cryogenic radiometer performance, have provided the opportunity to place the fundamental maintenance of photometric quantities upon absolu te detector based technology as was allowed by the 1979 redefinition. Addit ionally, the development of improved photodetectors has had a significant i mpact on the methodology in most of the radiometric measurement areas. This paper will review the status of the NIST implementation of the technical c hanges mandated by the 1979 redefinition of the candela and its effect upon the maintenance and dissemination of optical radiation measurements.