The NPL Radiometric Realization of the Candela

Abstract

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.

Advantages of white LED lamps and new detector technology in photometry

“…Before such an instrument can be used in absolute photometric measurements, it has to be calibrated. In a typical photometer calibration, the reading of the detector to be calibrated is compared with the reference value which is produced by a standard light source or by using a reference detector 6,7,8,10 . Tungsten filament incandescent lamps with a correlated color temperature T c 5 2856 K are commonly used as photometric standard light sources to approximate Standard Illuminant A which is defined by the CIE 24 to have a relative spectral power distribution of a Planckian radiator at 2856 K.…”

“…This trend raises some important questions, since tungsten filament incandescent lamps are widely used as source standards in photometry 6,7,8,9,10 . For example, will tungsten filament standard lamps still be available in the future, and if so, what will be their price?…”

The quantum candela: a re-definition of the standard units for optical radiation

“…A photometer generally consists of a detector with a filter whose spectral transmittance modifies the detector spectral responsivity to give an overall response close to that of V(), and is calibrated against the reference detector described in the previous paragraph [9]. While equation (1) may be evaluated using spectral measurements, the speed of using a photometer usually outweighs any gain in information content for tungsten-like sources.…”

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

“…However, there are some problems associated with difficulties of realization like instability in the reproducibility of luminance at the freezing temperature of platinum which brings a variation of brightness more than $2.1% and non-uniform emission of the cavity [3,4]. In order to compare luminous intensity scales of national laboratories an intercomparison was organized in 1969 by the CCPR (ComiteĆ onsultatif de Photome´trie et Radiome´trie) and disagreement was also found on color temperature measurements of light sources [5]. The main factor in disagreement was the color temperature of modern tungsten filament light sources (2856 K), which was far from color temperature of freezing point of platinum.…”