The candela and photometric and radiometric measurements

Abstract

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 1979 provided the opportunity for national measurement laboratories to base their 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 improvements in cryogenic radiometer performance, have provided the opportunity to place the fundamental maintenance of photometric quantities upon absolute detector based technology as was allowed by the 1979 redefinition. Additionally, the development of improved photodetectors has had a significant impact on the methodology in most of the radiometric measurement areas. This paper will review the status of the NIST implementation of the technical changes mandated by the 1979 redefinition of the candela and its effect upon the maintenance and dissemination of optical radiation measurements.

A systematic neutron reflectometry study on hydrogen absorption in thin Mg1-xAlx alloy films Special issue on Neutron Scattering in Canada.

“…With detector-based radiometric standards achieving very small uncertainties in the 0.01% range or below (see section IV), several NMIs now measure thermodynamic temperature by radiometric means, traceable to those detector-based standards, rather than using an ITS-90 derived source-based scale [38][39][40][41][42][43][44][45][46]. This practice also anticipates changes in thermometry, where the new mise-en-pratique for the definition of the kelvin (currently being prepared by the Consultative Committee for Thermometry (CCT))…”

Photometric specification of images

“…By the mid-1900s highly stable electronic light detectors became available [29], and vision researchers soon replaced eye-based instruments like the MacBeth photometer with photoelectric ones. Still more recently, Canada, the United States and other countries have specified the candela in terms of a standard radiometer [24,25]. Thereby photometry has returned to its roots as a measure of incident light.…”

“…Indeed, the reliability of Hipparchus' stellar magnitude scale establishes the validity of Francois-Marie's genetically standardized photometer. In doing so, Francois-Marie scooped by 300 years the 'latest' developments in photometry which define the candela in terms of standard radiometers [24,25]. With the ready availability of stable neutral density filters and wedges that attenuate light from 0.1 to 6.0 log, this method can readily be employed today in a variety of stimulus presentation situations.…”

Photometry, radiometry and ‘the candela’: evolution in the classical and quantum world

“…With detector-based radiometric standards achieving very small uncertainties in the 0.01% range or below (see section IV), several NMIs now measure thermodynamic temperature by radiometric means, traceable to those detector-based standards, rather than using an ITS-90 derived source-based scale [38][39][40][41][42][43][44][45][46].…”