Alfredo Dubra's PhD thesis - Imperial College London

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4. Wavefront reconstruction from shear phase maps pseudoinverse reconstruction method. Our analysis shows that the noise performance of the DFT reconstruction can actually be improved by judicious application of the extension method. 77
Chapter 5 Preliminary experiments 5.1 Prydal type experiments Prydal et al. [83] tried to measure the tear thickness by using the interference between the front and back surfaces of the tear film using a coherent source, reproducing an experiment by Green et al. [82] for the estimation of the corneal thickness. According to the mean refractive index values for the tear (1.337) [81] and epithelium (1.401) [113], if the boundary surfaces were optically smooth, then a large fringe visibility, around 0.44, was to be expected. However, this was not the case, and the fringes present led to a tear thickness value an order of magnitude larger than previous estimations, in clear disagreement with other mechanical and interferometric techniques. Prydal’s model of the tear proved to be an oversimplified one, because the tear has a complex three layer structure, each with a different refractive index. More importantly, the deepest of these, the mucus layer, is very rough on the optical scale. It is believed that what Prydal actually measured was the thickness of the tear and epithelium. We want to study the tear topography and not its absolute thickness, and therefore we want to obtain an interference pattern resulting from back reflections from the front surface of the tear and some other surface within the eye. Provided that the interferogram contrast is high enough, a modified version of Prydal’s experiment could be used to serve this purpose. Two experiments performed to test the viability of this approach are now described and their results discussed. Both experiments use the same experimental setup: that described in sections 2.6 and 2.7 with the glass wedges removed. 78
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A Shearing Interferometer for the E
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Acknowledgements Pursuing the PhD d
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Contents Contents 4 List of Figures
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CONTENTS 5.1 Prydal type experiment
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List of Figures 1.1 Young’s exper
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LIST OF FIGURES 5.2 Sequence of ref
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Chapter 1 Introduction Assessing an
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1. Introduction (a) (b) Figure 1.3:
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1. Introduction was then modified a
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1. Introduction been achieved by us
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1. Introduction A less significant
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1. Introduction It should also be m
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Chapter 2 Lateral shearing interfer
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Page 127 and 128: Bibliography [1] T. Young. The Bake
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BIBLIOGRAPHY [22] P. Artal, S. Marc
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BIBLIOGRAPHY [44] Austin Roorda and
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BIBLIOGRAPHY [67] W.N. Charman and
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BIBLIOGRAPHY [89] J.P. Craig, P.A.
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BIBLIOGRAPHY [115] Frangois C. Delo
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Alfredo Dubra's PhD thesis - Imperial College London

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