Abstract
An Absolute Radiation Detector (ARD) has been designed and constructed. After evaluation it will replace the existing UK primary national standard cryogenic radiometer, with an improved uncertainty. The ARD has been designed to measure black-body and laser radiation with an uncertainty approaching 10-5. From these measurements it will be possible to determine the fundamental constant, the Stefan-Boltzmann constant, confirming the radiometer as an absolute detector, and link this determination to the SI unit of luminous intensity, the candela, and other detector-based radiometric units. Thus source- and detector-based optical units will be tied to an invariant physical quantity ensuring their long-term stability. This paper describes some of the design considerations to enable the ARD to meet its objectives.