The determination of the uncertainties of spectral emissivity measurements in air at the PTB

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.