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.
An increase of 'growth' of optical thickness has been observed in PbTe layers freshly deposited during infrared filter manufacture. The phenomenon is nine parts complete about 15 min after deposition ceases, and is typically 0.03 mu m thick. Fluctuations in receptor surface temperature have been eliminated from being responsible. Negligible correlation is also found with physical thickness, monitoring wavelength (3.4-5 mu m), deposition temperature, receptor surface and underlying layers. An unquantified correlation appears to have been found with deposition rate. Undesirable loss of transparency accompanies any attempt at suppressing the phenomenon; conversely, when allowed to proceed without inhibition the growth sets up errors requiring individual compensation.
Two types of multilayer interference filter are required for temperature sounding the earth's atmosphere. In relation to the 668-cm(-1) nu(2) band of CO(2) these are narrowband (4 cm(-1) wide for the Q branch, 10 cm(-1) for the R branch) or isolation (~60 cm(-1) wide for the complete center of the band). Difficulty in manufacturing the filters for NIMBUS-SCR and -PMR spaceflight has indicated a need for improvement in the monitoring and control of the deposition of layers, and the paper describes a realization of this. The consequent effect on filter performance (utilizing a particular combination of layer materials) is described.
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