Temperature-invariant and other narrow-band IR filters containing PbTe, 4-20µm
J S Seeley, R Hunneman, A Whatley
SPIE Proceedings, 246, Pages 83-94 (1980)
PbTe is unique amongst coating materials because of its negative temperature coefficient of refractive index. In combination with a conventional II-VI material it can confer immunity to temperature variation from at least -23°C to +110°C for a narrowband filter. It also has an absolutely large refractive index so that filters are attainable with minimal degradation when tilted, and a minimum number of layers overall particularly towards 20µm. Excellent transmission can be obtained.