Displacement analysis for holographic interferometry using fringe tracking
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The pwpose of this thesis was to perform the data analyses of holographic interferograms, of an end-loaded cantilever beam, by using the digital image processing to obtain three-dimensional displacement information. The holographic interferograms analyzed in this work were generated by a previous researcher using double exposure holographic interferometry. The resulting hologram shows macro-interference fringes on the specimen surface. These macro-interference fringes show the optical path length differences which result from the micro-deformation of a diffusely reflecting surface when subjected to a low intensity load. The ability to record surface deformation is of interest in this thesis and of particular interest is the possibility of analyzing three-dimensional surface deformation. The specimen is a transparent cantilever bar, the surface and interior plane of which is diffuse reflecting. The holograms were then reconstructed using digital image processing. It is generally found that the interference fringes observed in the reconstructed holographic images are inherently noisy. This noise may be suppressed substantially by the application of "median filtering." Also, to accurately locate the center-line of the interference fringes, which is critical to determining the displacement data, thresholding and gray level averaging were applied to the digitized images. After locating the fringe center or fringe tracking, each fringe is assigned an order number. A computer program which takes into account the geometry of the experimental set-up permits the calculation of the 3-D displacement components. Comparison to the manual fringe counting method shows that the semi-interactive method of the data analyses presented in this thesis provides consistency and repeatability.