Optical EngineeringImaging through fog and rain
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The main aerosol effect on imaging performance is brightness reduction through scattering losses. This is fairly well understood and modeled. However, one phenomenon that is almost always overlooked is the blurring of images that can result from forward-scattered radiation, i.e., the radiation reaching the image plane after being scattered by airborne particles. The paper describes a new and efficient method of calculation of the forward-scattering effect on the point spread and modulation transfer functions. Solutions are presented that show the dependence of the aerosol blurring effect on particle concentration, particle size, geometry, and size of object features. The paper also reports on a simple experiment for measuring the visible point spread function through fog and rain. In most cases, the measurements are in good agreement with the model predictions. As it turns out, our measurements performed at ranges of 500 and 900 m and optical depths of up to seven show significant aerosol blurring effects only for rain and for some advection fogs with a sufficient number of particles in the size range of about 100 μm.