Antireflection coating on germanium for dual channel (3-5 and 7.5-10.6 μm) thermal imagers

The dual channel thermal imager, operating in the 3-5 and 7.5-10.6 mu m wavelength bands, is one of the latest achievements in instrumentation for target recognition and acquisition. While the 3-5 mu m band is utilised for detecting hot objects such as engine exhausts of vehicles and fighter planes, the 7.5-10.6 mu m band is employed for human bodies and objects at ambient temperatures. Many substrates are available which transmit in both these wavelength bands and their transmission can be enhanced by providing a suitable antireflection coating. In this paper. a broad band antireflection coating on germanium substrate is reported. The design approach involves achieving a continuously varying refractive index from that of the incident medium to the substrate. The continuously varying refractive index profile may be generated by using a sequence of thin layers of high and low refractive index materials. In this design a continuous refractive index profile is approximated by using a 13-layer stack of thorium fluoride and germanium as low and high index coating materials respectively. This coating conforms to environmental stability standards and shows an average transmission of 91% in 3-5 mu m band and 94.5% in 7.5-10.6 mu m band with a peak of 97% at 9 mu m on 10 mm thick germanium substrate. Polycrystalline germanium has 2.5% absorption for a 10 mm thick substrate. (C) 1999 Elsevier Science B.V. All rights reserved.