Analytical representations of the size-of-source effect

In this paper it is shown that the size-of-source effect (SSE) of a radiation thermometer measured for a range of source diameters D-i, can be fitted by means of empirical analytical approximations F(D), selected on the basis of quite general arguments. This is necessary to analytically process SSE data and to rigorously quantify the uncertainties associated with the correction for the SSE. To test the proposed fitting scheme 35 size-of-source curves, measured by five institutes for different radiation thermometers, have been analysed. In a companion paper the uncertainty in the correction for the SSE will be elaborated in detail for two examples.