A reference radiance-meter system for thermodynamic temperature measurements

Abstract

This paper presents the work carried out to evaluate the radiometric performance of a radiance-meter system which has been built at the LNE-Cnam to determine the thermodynamic temperature of high-temperature fixed points. The work comes as an integral part of the ‘implementing the new Kelvin’ (INK) project in which nine National Metrology Institutes (NMIs) strive to assign the thermodynamic temperatures to the melting curve of high-temperature fixed points with the lowest possible uncertainty. The method used in this research is based on the radiance approach. It exploits a system based on a laser-illuminated integrating sphere source whose radiance is absolutely measured by a trap detector through a well-defined geometry. The trap detector is calibrated traceable to the LNE-Cnam’s cryogenic radiometer. Once the radiance of the sphere is defined, a single grating-based spectroradiometer is used to measure the radiance of the fixed point source at the laser wavelength through direct comparison with the sphere radiance. This allows the thermodynamic temperature of the fixed point to be determined using Planck’s radiation law. The work provides a thorough evaluation of the system along with a detailed study of all related systematic effects and their corresponding uncertainties.