Abstract. Over the years, a number of international comparisons have been performed, in the field of radiation thermometry, to compare realisation of temperature scales. These have involved the use of a transfer instrument circulated among participants and were designed to compare the different ITS-90 realisations at different national measurement institutes (NMIs). EUROMET project 658 ('The examination of base parameters for ITS-90 scale realisation') was instigated to investigate the uncertainties of some of the underlying parameters in these ITS-90 realisations, namely: the size-of-source effect, linearity and relative spectral response of a radiation thermometer, and the calculation of the emissivity of different designs of blackbody cavity. The comparison was led by NPL and involved the following NMIs: PTB, VSL, CEM, UME, BNM-INM (Cnam) and IMGC. The comparison was performed by circulating two precision radiation thermometers (an IKE LP3 belonging to PTB and a VEGA TSP2 belonging to IMGC). This report describes the measurement procedures used in each laboratory and compares the results obtained from the measurements by the participants.

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