Uncertainty estimation for emissivity measurements near room temperature with a Fourier transform spectrometer

A Fourier transform infrared (FTIR) spectrometer was developed to measure spectral radiance of thermal-infrared emission near ambient temperatures with high accuracy at the NMIJ (former NRLM). The spectrometer uses a simple Michelson interferometer that consists of corner cube mirrors and a KBr beam splitter. The spectral range between 5 µm and 12 µm was covered by a liquid nitrogen cooled photovoltaic HgCdTe detector operated in the photoamperic mode. Opaque solid samples with good thermal conductivity were heated directly by the thermostatic fluid up to 100 °C. To reduce the effect of absorption by air, all of the optical components were operated in the vacuum. The spectrometer was calibrated against two high quality reference blackbodies: a fixed-temperature blackbody cooled by liquid nitrogen and a variable-temperature blackbody operated in the temperature range between -20 °C and 100 °C. We measured the normal spectral emissivity of alumina ceramic samples and determined the uncertainties. The combined standard relative uncertainties for the alumina samples at 100 °C was smaller than 1% when the emissivity was close to unity, and less than 3% when the emissivity was approximately 0.2.