More than one decade ago, an InGaAs detector-based transfer standard infrared radiation thermometer working in the temperature range from 150∘C\documentclass[12pt]{minimal}
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\begin{document}$$150\,{^{\circ }}\hbox {C}$$\end{document} to 1100∘C\documentclass[12pt]{minimal}
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\begin{document}$$1100\,{^{\circ }}\hbox {C}$$\end{document} was built at TUBITAK UME in the scope of collaboration with IMGC (INRIM since 2006). During this timescale, the radiation thermometer was used for the dissemination of the radiation temperature scale below the silver fixed-point temperature. Recently, a new radiation thermometer with the same design but with different spectral responsivity was constructed and employed in the laboratory. In this work, we present the comparative study of these thermometers. Furthermore, the paper describes the measurement results of the thermometer’s main characteristics such as the size-of-source effect, spectral responsivity, gain ratio, and linearity. Besides, both thermometers were calibrated at the freezing temperatures of indium, tin, zinc, aluminum, and copper reference fixed-point blackbodies. The main study is focused on the impact of the spectral responsivity of thermometers on the interpolation parameters of the Sakuma–Hattori equation. Furthermore, the calibration results and the uncertainty sources are discussed in this paper.
