The Earth's atmosphere has significant effects on the propagation of electromagnetic (EM) radiation and accordingly degrades the performance of electro-optical systems. These effects are attributed to atmospheric turbulence and to absorption and scattering of EM waves by atmospheric molecules and aerosols. In this paper we develop a detailed model of the effects of absorption and scattering on the optical radiation propagating from the object plane to an imaging system based on the classical theory of EM scattering. Scattering has the effect of de-correlating the light leaving the target from the unscattered light reaching the imaging system, and scattering has the effect of broadening the angle at which the scattered light arrives at the receiver compared to the unscattered light. Absorption has the effect of reducing the amount of power available for the image. Both of these effects depend upon the atmospheric species present, their EM properties, and wavelength. We use this detailed model to compute the average point spread function (PSF) of an imaging system that properly accounts for the effects of the diffraction and scattering, and the appropriate optical power level of both the unscattered and the scattered radiation arriving at the pupil of the imaging system. Since the scattered radiation is temporally and spatially de-correlated from the unscattered radiation, we model the effects of the unscattered radiation and the radiation scattered from the various species as additive in the image plane. The key result of this study is the significant effect of atmospheric scattering on the contrast and spatial resolution of images acquired by imaging systems, due to the increased level of the scattered radiation PSF and the reduced level of the direct radiation PSF, upon increasing the atmospheric optical depth.
Images measured through the atmosphere are degraded by scattering and absorption from aerosols along the path and by atmospheric turbulence. In the presence of heavy scattering at visible and infrared wavelengths, the distances over which reasonable observations are possible are quite short compared to astronomical imaging paradigms, and aerosol scattering effects dominate the degradation of the point spread function (PSF) due to atmospheric effects. In addition to aerosol-induced blurring, measurement noise effects are present in observed images. We examine the problem of reconstructing images degraded by aerosol blur and measurement noise using estimates of the overall PSF, which account for unscattered and scattered radiation detected by the imaging system. Representative images of a spoke target acquired under various conditions of scattering and photon flux levels were simulated, and reconstruction of the degraded images is performed using two linear reconstruction algorithms: a Wiener filter and a constrained least squares filter. Results of the reconstructions show that spatial resolution can be recovered in badly blurred images up to the limit imposed by the noise effective cutoff spatial frequency of the measurement. Downloaded From: http://opticalengineering.spiedigitallibrary.org/ on 05/14/2015 Terms of Use: http://spiedl.org/terms Optical Engineering 033101-7 March 2015 • Vol. 54(3) Hanafy, Roggemann and Guney: Reconstruction of images degraded by aerosol scattering and measurement noise Downloaded From: http://opticalengineering.spiedigitallibrary.org/ on 05/14/2015 Terms of Use: http://spiedl.org/terms Optical Engineering 033101-9 March 2015 • Vol. 54(3) Hanafy, Roggemann and Guney: Reconstruction of images degraded by aerosol scattering and measurement noise Downloaded From: http://opticalengineering.spiedigitallibrary.org/ on 05/14/2015 Terms of Use: http://spiedl.org/terms
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