Examination of a diffraction problem, part of a study to determine the effect of atmospheric turbulence on optical and infrared reconnaissance and guidance equipment. After establishing the validity of extending the Huygens-Fresnel principle to refractive media, a formula is derived for the mean spatial intensity distribution in both the Fresnel and Fraunhofer regions from an arbitrary complex disturbance in a finite aperture. The effect of turbulence is shown to enter through the spherical wave modulation transfer function (MTF). Using a modified Kolmogorov spectrum for the index of refraction fluctuations, the formula is used to calculate the mean intensity distribution from a plane wave diffracting from a circular aperture and to give approximate expressions for the beam spreading at various ranges. It is argued that the spherical wave MTF is the basic quantity to be measured for computing the atmospheric degradation of an intensity pattern from an arbitrary disturbance in a finite aperture. 45 pp. Ref. (Author)
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