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 mum and 12 mum 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 degreesC. 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 degreesC and 100 degreesC. 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 degreesC was smaller than 1% when the emissivity was close to unity, and less than 3% when the emissivity was approximately 0.2.