28 September 1994 How infrared missile windows degrade the noise-equivalent irradiance of infrared seeker systems
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The purpose of this paper is to provide a firm analytical base for assessing the performance degradation that affects infrared (IR) missile seekers as a result of aerodynamic heating of the protective window or dome. On assuming that the system under consideration operates in a pulsed mode and is limited only by random fluctuations in the arrival rate of background photons, difficulties encountered in evaluating the signal-to-noise ratio can be avoided by writing SNR equals Heff/NEI, where Heff represents an effective target irradiance and NEI is the system's noise-equivalent irradiance at the peak-response wavelength of the detector. The NEI characterizes the capability of the system and is best expressed in terms of a `dark-system NEI,' which refers to the noise-equivalent irradiance in a laboratory environment, and a `photon noise factor' that relates noise originating from the background scene, the infrared window, and the detector surroundings to the photon noise of a dark system. A set of equations is derived for evaluating the performance degradation resulting from the photon exitance of the `hot window,' taking into account the effect of radiation- shielding procedures. The missile window problem is then formulated in a general but compact form compatible with the state of the art of IR technology. An illustration is provided in the context of assessing the feasibility of using pyramidal domes for dual-mode air-to-air missile applications; it is shown how to evaluate the IR performance degradation induced by aerodynamic heating of such domes under conditions that are representative of anticipated thermally most severe trajectories for internal as well as external missile-carry configurations.
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Claude A. Klein, "How infrared missile windows degrade the noise-equivalent irradiance of infrared seeker systems", Proc. SPIE 2286, Window and Dome Technologies and Materials IV, (28 September 1994); doi: 10.1117/12.187366; https://doi.org/10.1117/12.187366

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