Design considerations for the construction of an absolute radiation detector at the NPL

An Absolute Radiation Detector (ARD) has been designed and constructed. Aft er evaluation it will replace the existing UK primary national standard cry ogenic radiometer, with an improved uncertainty. The ARD has been designed to measure black-body and laser radiation with an uncertainty approaching 1 0(-5). From these measurements it will be possible to determine the fundame ntal constant, the Stefan-Boltzmann constant, confirming the radiometer as an absolute detector, and link this determination to the SI unit of luminou s intensity, the candela, and other detector-based radiometric units. Thus source- and detector-based optical units will be tied to an invariant physi cal quantity ensuring their long-term stability. This paper describes some of the design considerations to enable the ARD to meet its objectives.