Physical basis of interpolation equations for radiation thermometry

Abstract

A method is described to relate the parameters appearing in interpolation equations for radiation thermometry directly to the radiometric characteristics of the thermometer itself. It is shown that for sufficiently narrow bandwidths these parameters are independent of the shape of the spectral responsivity and can be expressed solely in terms of its mean wavelength and bandwidth as determined by the variance. This allows the parameters to be determined either by direct measurement of the spectral responsivity, by measurements at fixed points, or by a combination of the two, in effect unifying the ITS-90, interpolation, and absolute thermometry methodologies for determining temperature. The development of high-temperature metal–carbon and metal-carbide–carbon eutectic fixed points means that temperatures up to and exceeding 3500 K may be determined by implementing a single simple interpolation equation in a variety of ways.