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Chapter 15:
Optical Properties of the Atmosphere
Many properties of the atmosphere affect the quality of images propagating through it. There are atmospheric phenomena that give rise to attenuation of the irradiance of the propagating image, thus reducing the contrast of the final image. There are also atmospheric phenomena that cause blurring of detail. Both types of phenomena prevent small detail from being resolved in the final image, thus degrading image quality. Phenomena that give rise to attenuation are electromagnetic (EM) wave absorption and scattering by the constituent gases and particulates of the atmosphere and airborne particulates. Scattering of photons by airborne particulates is manifested as deflections of the photons to directions other than that of original propagation. If such scattering causes the deflected photons to miss the imaging receiver, then the scattering is manifested as attenuation. The received irradiance of the image propagated through the atmosphere is correspondingly diminished from that in the object plane. However, if the light scattering is at very small angles with respect to the original directions of propagation, and several such small-angle scattering events take place, then forward-scattered radiation can take round-about paths and still be received by the imaging system together with the unscattered radiation. The net effect is image blurring caused by a multitude of angles of arrival at the imaging receiver of radiation, emanating from the same point in the object plane, for one multiply forward-scattered ray and one unscattered ray. Many such multiple-scattering paths give rise to a relatively large blurred-point image rather than a fine sharp-point image. Several adjacent object plane points can then appear as a single blurred image plane point, thus degrading image resolution.
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