Alfredo Dubra's PhD thesis - Imperial College London

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5. Preliminary experiments The recorded patterns look very similar, if not identical, to what the subject sees when the experiment is being performed. We believe the image projected on the retina not only shows information about the tear topography, but also about floaters within the eye, and although this was not explored further, it should also show opacity in the eye due to for example cataracts. Typically when the subject participates for the first time in the experiment, asked to concentrate on one particular spot of these images and to remain looking always in the same direction, they would choose a floater or a bubble on the tear. Then because this feature is rigidly attached to the eye, the subject would start moving the eye trying (unsuccessfully) to bring that object to the fovea, resulting in a very frantic eye movement (one could push the analogy with a dog chasing its own tail to its very limit). The subject would then be asked to try to fixate on one of the set of small Newton rings that are due to the optics (and therefore static with respect to the optical setup). In any case, it is an interesting experiment to participate in as a subject, not only because it allows one to see what happens on ones own tear film but also to see other structures (e.g. floaters) within ones own eye. 5.1.2 Focused illumination and estimation of undesired eye reflections Despite the failure of the previous experiment to provide a quantitative tool for studying the tear topography, a similar experiment with the same optical setup was performed, but with the eye further away from the experiment, so that the focus of the beam incident on the eye coincided with the front surface of the tear. In this way, if there was a reflection from a surface within the eye other than that from the tear surface, it would show up by producing an interferogram with concentric circular fringes. These fringes would carry information on the optical path between the front surface of the tear and the second reflecting surface. This is a similar experiment to that performed by Prydal, the only difference being that in this case the axis of the incident beam is perpendicular to the surface of the tear rather than at 45 o . For this experiment, a series of 100 images on two subjects (series coded bb focused and lm focused) were recorded. The two images shown at the top in figure 5.3 are representative of the recorded series of images, and show three types of ringing: those at the edges, caused by diffraction, small sets due to interference from reflections within optical elements in the setup and the very dim set almost centered covering the 81
5. Preliminary experiments dm1_no_wedges (t = 0.000s) dm1_no_wedges (t = 0.188s) dm1_no_wedges (t = 0.391s) dm1_no_wedges (t = 0.594s) dm1_no_wedges (t = 0.781s) dm1_no_wedges (t = 0.969s) dm1_no_wedges (t = 1.172s) dm1_no_wedges (t = 1.360s) dm1_no_wedges (t = 1.563s) dm1_no_wedges (t = 1.750s) dm1_no_wedges (t = 1.9380s) dm1_no_wedges (t = 2.156s) Figure 5.2: Sequence of reflections from the front surface of the tear illustrating bubble movements on the tear. (series coded dm1). 82
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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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2. Lateral shearing interferometer
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Page 43 and 44: 3. Data processing If we look at th
Page 45 and 46: 3. Data processing term on the retr
Page 47 and 48: 3. Data processing mk1/33 (a) (b) k
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Page 53 and 54: 3. Data processing 3.3 Pupil positi
Page 55 and 56: 3. Data processing (a) (d) (g) (j)
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Page 59 and 60: 3. Data processing in I R,R and app
Page 61 and 62: 3. Data processing Phase discontinu
Page 63 and 64: 3. Data processing 0 0.2 0.4 0.6 0.
Page 65 and 66: 3. Data processing 8 6 4 2 0 -2 -4
Page 67 and 68: Chapter 4 Wavefront reconstruction
Page 69 and 70: 4. Wavefront reconstruction from sh
Page 79 and 80: Chapter 5 Preliminary experiments 5
Page 81: 5. Preliminary experiments bb2 (t =
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Page 87 and 88: 5. Preliminary experiments the corr
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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
Page 131 and 132: 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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