Title

Incoherent imaging in the presence of atmospheric turbulence and refractivity

Author

Zhijun Yang

Date of Award

2017

Degree Name

Ph.D. in Electro-Optics

Department

Department of Electro-Optics and Photonics

Advisor/Chair

Advisor: M. A. Voront︠s︡ov

Abstract

Atmospheric turbulence, associated with its refractive-index inhomogeneities (refractivity), may severely affect long range incoherent images formation. Example of this impact includes image blurring, motion, warping and anisotropic geometrical distortions. Currently, the effects of turbulence and refractivity on image formation are considered as being mutually independent and analysed separately using the Fresnel diffraction (wave-optics) and geometrical optics (ray tracing) approaches, respectively. Such independent treatment of turbulence and refractivity effects have certain limitations. Atmospheric refractivity may result in significant deviations of optical wave propagation direction. This effect is commonly referred as the ray bending which, in turn, may lead to a change in turbulence characteristics such as the refractive index structure parameter Cn² that is commonly considered as a function of altitude h above the ground. Correspondingly, optical wave refraction, especially in extended-range imaging scenarios, could affect the turbulence-induced optical aberrations. In this work, we analyze the incoherent image formation in atmosphere in the presence of both atmospheric turbulence and refractivity using numerical simulations based on the brightness function (BF) technique. Using the BF technique, the incoherent imaging system modulation transfer function (MTF) estimation is performed via direct numerical analysis of visibility of sine-test patterns of different spatial frequencies. The test patterns are assumed to be imaged through a volume medium with turbulence and refractivity-induced refractive index inhomogeneities. The major effects observed in numerical simulations, include the spatial frequency shift between frequency of a sine-test object and its image, and spatial non-uniformity of the sine-pattern image distortion which is referred as the refractivity-induced image anisoplanatism. Both effects depend on the location and strength of the localized refractive index structure with respect to the imaging (wave propagation) geometry. The MTFs corresponding to distributed (volume) turbulence with and without atmospheric refractivity are also compared. Next, the joint impact of atmospheric turbulence and inverse temperature layer (ITL) on optical mirage formation is analyzed. The dependency of both desert- (superior) and ocean-type (inferior) mirage image formation on ITL characteristics (temperature inversion and location of the ITL) have been studied. The impact of atmospheric turbulence strength on mirage image qualities is also analyzed. Finally, a numerical analysis is conducted to study the impact of localized refractive index inomogeneites on image quality. It is shown that image quality strongly depends on atmospheric turbulence strength and locations along the optical path. To characterize this impact, two metrics are proposed and developed to measure the image quality as a function of turbulence strength and location. The impact of inverse temperature layer on the developed image quality metrics are also studied.

Keywords

Atmospheric turbulence, Refraction, Image processing, Mirages, Atmospheric Sciences, Engineering, Optics, incoherent imaging, atmospheric turbulence, refractivity, modulation transfer function, optical mirage

Rights Statement

Copyright 2017, author

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