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Below the freezing point of silver, radiation thermometers are generally calibrated by implementing the multi-point interpolation method using blackbody measurements at three or more calibration points, rather than the ITS-90 extrapolation technique. The interpolation method eliminates the need to measure the spectral responsivity and provides greater accuracy at the longer wavelengths required below the silver point. This article identifies all the sources of uncertainty associated with the interpolation method, in particular, those related to the reference blackbody temperatures (either variable-temperature or fixed-point blackbodies) and to the measured thermometer signals at these points. Estimates are given of the 'normal' and 'best' uncertainties currently achievable. A model of the thermometer response is used to propagate all the uncertainties at the reference points and provide a total uncertainty at any temperature within the calibration range. The multi-point method has the effect of constraining the total uncertainty over this range, unlike the ITS-90 technique for which the uncertainties propagate as T 2 . This article is a joint effort of the working group on radiation thermometry of the Consultative Committee for Thermometry (CCT), summarizing the knowledge and experience of all experts in this field.

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The instrumentation of the Physikalisch-Technische Bundesanstalt (PTB) for the measurement of the directional spectral emissivity in air in the temperature range from 80 • C to 400 • C and wavelength range from 4 µm to 40 µm and the subsequent procedures for data evaluation are described. Special emphasis is placed on the calculation of the uncertainty of the directional spectral emissivity and of the uncertainty of the sample surface temperature. For samples with typical emissivities of about 0.6, absolute expanded uncertainties (k = 2) of 0.01 can be reached at a surface temperature of 200 • C in the spectral range from 5 µm to 40 µm. However, it is also shown that-due to the complex dependence of the uncertainty on the sample and the conditions of measurement-general statements about the uncertainty cannot be given. The uncertainty of the emissivity has to be calculated for each individual case of sample and temperature.