Approximate normal emissivity spectra in the infrared at elevated temperatures of single-crystal and polycrystalline calcium fluoride.

A single-beam ir spectroradiometric system has been used to measure the normal spectral emissivity of specimens of single-crystal and polycrystalline calcium fluoride in the ir at elevated temperatures. The wavelength range was 2-12 micro, and data were taken at sample temperatures of 500 degrees C and 600 degrees C. Room temperature data on the index of refraction of single-crystal calcium fluoride and its rate of change with temperature were extrapolated to 500 degrees C and 600 degrees C; the normal spectral reflectivities were computed from the extrapolated indices at these temperatures. Then the computed reflectivities were used to extract normal spectral absorption coefficients from the emissivity data. It was possible to compute absorption coefficients with reasonable accuracy at wavelengths of 6 micro, 8 micro, and 10 micro; the smallest estimated error was about 2% at 8 micro. The absorption coefficients increased markedly with temperature at these three wavelengths. The absorption coefficients of the polycr stalline material were consistently higher than the corresponding values for the single-crystal calcium fluoride. It was found that the spectral absorption coefficients at 8 micro and 10 micro microns varied approximately as the 2.1 and 1.6 power of the absolute temperature, respectively, between room temperature and 600 degrees C. These exponents lie within the range predicted by theory.