Alfredo Dubra's PhD thesis - Imperial College London

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5. Preliminary experiments 5.1.1 Normal illumination experiment For this particular experiment, the eye was placed so that the spherically convergent illumination was everywhere normal to the surface of the tear film. Two sequences of 100 images at a 2.5 Hz rate were recorded from each of the two subjects (series coded bb1, bb2, dm1 no wedges and dm2 no wedges). Figures 5.1 and 5.2 show two sequences of 12 images each, where the illuminated area over the tear was around 3.1 mm in diameter. Note the diffraction rings at the pupil edge, which decrease in thickness towards the center of the pupil, the bubbles and other tear features, and finally the small sets of Newton rings with very low contrast (in comparison to the diffraction rings) that are due to interference within optical elements and/or their coatings. These particular sets of images were selected because they illustrate some of the most interesting things that can be seen using this technique. The first sequence (figure 5.1) starts with what is probably the most representative situation, a smooth tear surface with some small bubbles distributed pretty much randomly over the pupil. Before the next frame the top eyelid moved down and then up a fraction of the pupil diameter, leaving a clearly delimited region full of bubbles which smooths out slowly in the following 5 frames (approximately 2 seconds). The subject felt the need to blink and tried to prevent it in the eighth frame of the sequence, where the tear surface becomes very rough. Finally the subject blinked between the eighth and ninth frame, producing again a smooth tear surface that remains so for the rest of the sequence apart from the displacement of a small line of bubbles. The second sequence (figure 5.2) shows a phenomenon that could be seen every time this subject blinked. After the blink shown in the first image, a set of relatively large bubbles are left near the top of the image pupil, and as time passes these set of bubbles move downwards while their size is reduced until they almost disappear before the next blink. We failed to identify clearly an interference pattern on any of the 400 images recorded with this experimental configuration that we could attribute with confidence to interference between the reflections from the front surface and another surface within the eye. 79
5. Preliminary experiments bb2 (t = 0.000s) bb2 (t = 0.390s) bb2 (t = 0.781s) bb2 (t = 1.172s) bb2 (t = 1.562s) bb2 (t = 1.953s) bb2 (t = 2.344s) bb2 (t = 2.734s) bb2 (t = 3.125s) bb2 (t = 3.515s) bb2 (t = 3.906s) bb2 (t = 4.297s) Figure 5.1: Sequence of reflections from the front surface of the tear film (series coded bb2). 80
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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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2. Lateral shearing interferometer
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Page 43 and 44: 3. Data processing If we look at th
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Page 47 and 48: 3. Data processing mk1/33 (a) (b) k
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Page 61 and 62: 3. Data processing Phase discontinu
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Page 67 and 68: Chapter 4 Wavefront reconstruction
Page 69 and 70: 4. Wavefront reconstruction from sh
Page 79: Chapter 5 Preliminary experiments 5
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Page 85 and 86: 5. Preliminary experiments bb_focus
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Page 91 and 92: 6. Tear topography dynamics experim
Page 107 and 108: Chapter 7 Summary and discussion Th
Page 109 and 110: 7. Summary and discussion The value
Page 111 and 112: 7. Summary and discussion ao2_radia
Page 113 and 114: A. Preliminary study of feasibility
Page 125 and 126: B. Laser safety calculations B.1 MP
Page 127 and 128: Bibliography [1] T. Young. The Bake
Page 129 and 130: 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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