Recent investigations in absolute radiometry

Abstract

Three new absolute radiometers have been constructed for measuring radiation in the visible and infra-red regions of the spectrum. Each works on the electrical substitution principle, and consists in effect of a radiation thermopile with an electrical heating element attached to the receiver. The first instrument is similar to the Guild 'drift' radiometer; the second is a small, quick-acting instrument which has a plane receiver coated with a glossy, or specularlyreflecting, black, for which the absorption factor can be readily determined; the third instrument is again small and quick-acting, but has a cavity or black-body receiver which is almost completely absorbing. A description is given of an intercomparison between these three radiometers and the original Guild radiometer. The readings of the new radiometers agreed to within 0-2 % but differed from the reading of the Guild radiometer by 0.5 %. An account is given of the various sources of error in absolute radiometric measurements, and also of the bearing of this recent work on the present status of the National Physical Laboratory scale of radiation intensity.