The presence of a solid window at the entrance aperture of an air-borne FLIR can degrade the signal-to-noise ratio as a result of (a) the reduction in target signal-caused by the absorption of radiation by the window and (b) the loss in system detectivity caused by additional quantum noise generated by window-emitted photons. It is also known that, in order to avoid a degradation of the resolution at spatial frequencies of current interest, the window must satisfy specific requirements in terms of its modulation-transfer characteristics. The purpose of this paper is to consider some relevant properties of chemically vapor-deposited ZnS and ZnSe prior to assessing how windows made from these materials may impact the performance of state-of-the-art FLIRs. The degradation in signal-to-noise ratio occurring in the presence of a ZnS window reflects a substantial increase in absorptance/emittance at the long-wavelength end of the FLIR bandpass (8 to 12 iim) and must be attributed to lattice absorption processes, thus imposing intrinsic limitations on the performance, particularly at elevated temperatures. With ZnSe there is essentially no degradation to be anticipated, but since this material is soft, ZnSe windows cannot cope with rain/ice/dust erosions occurring in an operational environment. This emphasizes the need to develop a "new-generation" FLIR window that combines adequate erosion resistance and ZnSe-type optical properties. The present investigation confirms that ZnS/ZnSe laminates represent a highly attractive solution from the point of view of eliminating the loss of sensitivity encountered with ZnS windows in high speed flight.
Claude A. Klein,
"ZnS, ZnSe, and ZnS/ZnSe Windows: Their Impact On FLIR System Performance," Optical Engineering 25(4), 254519 (1 April 1986). https://doi.org/10.1117/12.7973854