The adoption in 1979 of a new definition of the candela, which permitted a detector-based approach to the realization of the unit, has emphasized the importance of high-accuracy absolute radiation detectors. This paper describes a new electrical-substitution absolute radiometer operating at 5 K, based on a standard commercial helium cryostat, which has been developed at NPL for optical radiant-power measurements. The principal advantages of operating the radiometer at liquid helium temperatures are that the detector can be a large, highly absorbing cavity (absorptivity 99.998%) and that exact equivalence of electrical and radiant heating can be clearly demonstrated. It will be the primary absolute detector for a new realization of the candela at NPL and will provide the basis for the development of a wider range of radiometric and spectroradiometric standards. The use of the radiometer to measure the radiant power of an intensity-stabilized laser source at the milliwatt level with an uncertainty of 4 parts in 105 is also described, together with its application in the determination of the responsivity of transfer-standard detectors, such as silicon photodiodes.

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.

Deuterium discharge lamps with magnesium fluoride windows sealed to envelopes by means of pure glass-blowing techniques have been developed at NPL as radiance transfer standards between 115 and 370 nm. It is shown how the problems of the complex molecular emission spectrum of deuterium below 165 nm are overcome at the calibration stage provided the bandwidth of the spectroradiometric system to be calibrated is known. Total power dissipation of about 15 W, compact construction, ease of use, high radiance and aging rates less than 0.05% h-1 are some of the advantages discussed.

Measurements have been made of the spectral emission and irradiance from whole body ultraviolet irradiation units used in the treatment of psoriasis by photochemotherapy (PUVA). A total of ten units were studied, located at nine different treatment centres in England. The results showed that within the limits of experimental accuracy there was no significant difference in the profile of all spectra measured, although the measured value of irradiance compared with each centre's own UV-A meter showed wide discrepancies. There is poor documentation provided by manufacturers of UV-A meters on the meaning of the scale reading in mWcm-2, and it would seem expedient that some agreed calibration procedure for these instruments is forthcoming.