An aerodynamically heated window can interfere in several ways with FLIR imagery. Uneven window heating causes uneven focal plane irradiation, a contrast gradient across the field, and blur because of point-to-point changes in refractive index. Window heating also increases photon noise, and in some cases reduces signal-to-noise because of reduction in transmission. This paper presents a method for computing aerodynamic heating of a window, starting with point-to-point total pressure patterns and evaluating each parameter in terms of its electrical analog. Window heating time constants are also computed in analogous electrical terms. Temperature patterns are converted into focal plane contrast gradients, image blur and MTF degradation. Examples are given of comparative performance of ZnS, ZSS and GaAs windows in a mach .8 sea level environment.
T. R. Whitney,
"Aerodynamic Window Heating And Its Effect On FLIR Imagery", Proc. SPIE 0095, Modern Utilization of Infrared Technology II, (15 November 1976); doi: 10.1117/12.955166; https://doi.org/10.1117/12.955166