High-temperature measurement techniques for the application in photometry, radiometry and thermometry

Abstract

Well characterised sources of thermal radiation are essential for photometry, radiometry, and thermometry. They serve as reference radiators for the calibration of detectors and radiance sources. Thermal radiation sources are advantageous for this purpose compared to other radiance sources such as lamps or LEDs because they possess a continuous spectrum of the emitted spectral radiance, which, for blackbody sources, can be calculated analytically using Planck’s law of radiation. For application in thermometry, blackbody sources starting from temperatures near absolute zero to temperatures up to 3000 ∘C are needed for the calibration of radiation thermometers. For application in photometry and radiometry high intensity sources of radiation in the visible and UV region of the optical spectrum were required. This latter requirement is met by blackbody sources at temperatures well above 2000 ∘C. An ideal reference source should always emit the same amount of radiation at any time of use. This is realised by fixed-point radiators. Such radiators are based on a phase transition of a substance, at high temperatures the melting and freezing points of metals. However, current metal fixed-points are limited to relatively low temperatures. In the present work innovative techniques necessary for research into high-temperature thermal radiation sources are developed and thoroughly described. Starting with variable temperature blackbody sources the techniques required are: Precise apertures determination and detailed characterisation of the applied optical detectors. The described techniques are then used to undertake research into the development of high-temperature fixed-points above the copper fixed-point for application in photometry, radiometry, and thermometry. Applying these sophisticated techniques it was shown that these new high-temperature fixed-points are reproducible and repeatable to better than 100 mK at temperatures up to nearly 3200 K. Finally, a forward look is given that shows the potential of such fixed-points for improving traceability and accuracy in photometry, radiometry, and thermometry. This work forms the foundation for accurate and practical applications of high-temperature fixed-point sources and sets the international benchmark for measurement techniques in photometry, radiometry, and thermometry. This work will open the use of the novel high-temperature fixed-points in an improved International Temperature Scale and will significantly improve and ease the realisation and dissemination of the SI base unit candela, significantly reducing the uncertainty for industrial measurements in these fields.