Optical ray tracing through dynamic finite element solutions of thermo-fluid problems




Wharmby, Andrew William

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The eye or other imaging systems for which there is an absorption of laser energy may exhibit a transient effect called thermal lensing that can give rise to aberrations in a perceived or captured image. The resulting visual effects of the thermal lensing phenomenon occurring in the human eye are investigated in this study by utilizing the finite element method as a means to simulate the heat transfer experienced by physical systems consisting of both solid and fluid mediums when exposed to monochromatic optical radiation. To quantify the optical effect, the solutions to the heat transfer problems produced by the finite element models are then used to compute gradient-refractive index (GRIN) profiles. Constant heating profiles are assumed for the purposes of this study. The solutions to each heat transfer problem are compared to other published models in the literature. Furthermore, the trajectory path is computed for rays of light traveling through the heated medium using geometrical ray tracing algorithms based on various forms of the Runge-Kutta method. Aberrational effects caused by the thermal lensing event are estimated by measuring the amount of deviation from previously calculated trajectory paths, providing preliminary quantitative data on image blurring. Both conductive and convective modes of heat transfer within fluid are considered and used to develop two dimensional and three-dimensional models and then applied to simplified geometries utilizing realistic values for physical constants and boundary conditions are appropriately selected to resemble that of the interior of a human eye for validation purposes. It is found that the inclusion of fluid in the heat transfer problem setting introduces asymmetrical temperature profiles thus producing asymmetrical distributions of refractive index gradients.


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Eye, Finite Element Method, Optics, Ray Tracing, Runge-Kutta, Thermal Lensing



Biomedical Engineering