An improved algorithm for spectral emissivity measurements at low temperatures based on the multi-temperature calibration method

An experimental apparatus for normal spectral emissivity measurements at low temperatures was constructed by using the Fourier transform infrared (FTIR) spectrometer. The FTIR spectrometer was calibrated against a high quality reference blackbody based on the multi-temperature method. The spectral response function R (λ) was computed by the least-square method. To improve the precision of emissivity measurement, an improved algorithm to eliminate disturbances by background radiation was presented. Emissivity uncertainty caused by the spectral response function R (λ) was analyzed. A comparison between the results obtained by the multi-temperature calibration method and the two-temperature method was presented. The linearity of the FTIR spectrometer response in the studied spectral range is better than 1% except spectral bands due to atmospheric absorptions. A high-purity (99 wt%) alumina sample was used to validate emissivity results obtained by the improved algorithm. The excellent agreement with the literature data around Christiansen wavelength demonstrates that our experimental apparatus and the improved algorithm can provide reliable measurements. The calculated relative combined uncertainty of the spectral emissivity for the alumina sample is estimated to be less than 5.1% for the spectral range considered.