<title>Beryllium oxide: a material for selective supression of thermal emission from domes</title>

Abstract

The possibility of using selectively low emission to reduce thermal signature of domes is pointed out. A material can simultaneously have low radiance in the working range of a detector and cool radiatively to the surrounding atmosphere. If the band of low emission is based on lattice excitation, the signature reduction is compatible with electrically insulating properties and radar transmittance. High density, polycrystalline beryllium oxide is identified as a material with low emittance in the primary atmospheric window 8 - 13 micrometers , owing to a strong reststrahlen band. Bulk reflectance spectra are reported for ceramic BeO of three grades and are used to calculate the average, bulk near normal emittance over various possible detector ranges as well as the radiance for a 50 degree(s)C BeO surface. The results vary: 0.3 - 0.6 and 10 - 37 W/m<SUP>2</SUP>, sr respectively, depending upon the short wavelength threshold of the detector. The calculated values have been compared with radiometer measurements in the 3 - 5 and 8 - 13 micrometers ranges. The possibility to reduce the emittance even further with a second material has been investigated with Fresnel calculations. Very favorable calculated and measured numerical results for a 0.8 micrometers silicon overlayer on BeO are reported.