Measurement of the directional emittance of polycrystalline beryllium oxide

The spectral emittance of polycrystalline beryllium oxide has been measured at four different angles from 0° to 75°, at a temperature of 150°C. From the spectral measurements the average band emittance in the 8–13 μm window has been calculated. The results show that the band emittance decreases from 0.44 at the angle of 0°, down to 0.23 at an angle 75° off the normal. The low band emittance is caused by a strong reststrahlen band in the wavelength region 9.2–14 μm. This reststrahlen band has been modelled by a Lorentz one-oscillator model. Experiment and calculations show that this band of low emittance increases its width towards shorter wavelengths at inclined angles and covers almost the entire upper atmospheric window 8–13 μm at the angle of 75°.