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Extrapolation of PbTe/II-VI multilayer interference-filter technique from 20 to beyond 40 microm is described and PbTe transparency reviewed; improvements below 20 microm are reported. A composite filter cutting on steeply at 40 microm is described that uses absorptive films of ZnS and As(2)S(3), thin quartz, and supplementary multilayer interference. Absorptive filters are described containing the II-VI compounds since these are found transparent at wavelengths shorter and longer than their reststrahl.

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 20 pm. Excellent transmission can be obtained.

We describe the spectral design and manufacture of the narrow-bandpass filters and 6-18-microm broadband antireflection coatings for the 21-channel High Resolution Dynamics Limb Sounder. A method of combining the measured spectral characteristics of each filter and antireflection coating, together with the spectral response of the other optical elements in the instrument, to obtain a predicted system throughput response is presented. The design methods that are used to define the filter and coating spectral requirements, choice of filter materials, multilayer designs, and deposition techniques are discussed.