Alfredo Dubra's PhD thesis - Imperial College London

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A. Preliminary study of feasibility of interferometric wavefront sensing in the eye a diode laser @ 790nm single mode optical fiber f 1= 60 stop aperture (off axis) NPBS f 2= 40 f 3= 80 eye 120 100 a/2 Figure A.3: Illumination branch of the wavefront sensing experiment using a 780 nm diode laser, which delivers an off-center collimated 1.2 mm beam to the eye. in the previous experiments, but also the light with the orthogonal polarisation state. The price to pay for this gain is the tilting of the beam splitter and illumination branch to eliminate the reflections from the beam splitter, which in this case are even stronger in comparison with the signal in the tear interferometry experiment, because the total power reflected from the retina is lower than that reflected by the tear. It can be demonstrated by using the reflection law that if the optical axis of the part of the illumination branch before the beam splitter is rotated an angle α and the beam splitter is rotated half that angle, then the collimated beam after the beam splitter will be parallel to the optical axis of the Badal optometer and that the undesired back reflections from the beam splitter (leaving the beam splitter through the left surface on the figure) will go off-axis at an angle α, allowing us to spatially filter these reflections. The imaging branch of the experiment (figure A.4) shares the Badal optometer and beam-splitter with the illumination branch. The only purpose of the 4f system to the left of the beam-splitter is to focus the light coming back from the eye so that a stop aperture can be placed in the focal plane of this telescope to filter the off-axis beam splitter reflections. Again, the final optical element before the planes conjugated to the exit pupil of the eye is the wedge at 45 degrees, that produces the two reflections on the camera that will register the interferograms and lets through the light that goes to the SH sensor. 115
A. Preliminary study of feasibility of interferometric wavefront sensing in the eye a/2 CCD camera CCD camera T'' T'' wedge f 5= 40 f 4= 40 NPBS f 2= 40 f 3= 80 T eye lenslet array 40 80 80 stop aperture (BS backreflection filter) 120 100 Figure A.4: Imaging branch of the wavefront sensing experiment using a 780 nm diode laser. The exit pupil of the eye (T) is conjugated to a SH lenslet array (T’) and a CCD camera that will record the laterally sheared interferograms (T”). A.2 Data acquisition The interferograms and SH spot patterns were recorded with two CCD cameras, Retiga 1300 and Retiga EX (manufactured by QImaging), synchronized by an external TTL signal. The exposures of the cameras were equal at all times, and the gain of the electronics of each camera was adjusted to try to optimise the use of the dynamic range and take into account the fact that the SH camera receives approximately 11 times more energy than the one recording the interferograms, and also the light is focused into an array of spots as opposed to the interferogram recording camera in which the energy is more evenly distributed over the pupil. The lenslet array used to produce the SH spot patterns was a Fresnel microlens array on polycarbonate, produced by WelchAllyn, with a focal lens of 7.5 mm and 200 µm pitch. For the data acquisition, the subject was set in front of the experiment and the head kept in position with the aid of a bite bar. In all the wavefront sensing experiments the optical power entering the eye was kept below 1 % of the MPEs for 10 s, as calculated in Appendix B. A.2.1 Data acquired with 632.8 nm source A subject (coded name ad) was tested with three different optical power levels entering the eye and exposure times chosen so that the energy recorded by each camera on each frame was always the same. The optical power entering the eye was adjusted by the use of neutral density absorption filters placed between the laser and the spatial filter to reduce the effect of undesired reflections. The experiment was repeated twice, with 116
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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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P P 2. Lateral shearing interferome
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3. Data processing If we look at th
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3. Data processing term on the retr
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3. Data processing mk1/33 (a) (b) k
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3. Data processing mk1/46 (a) (b) (
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3. Data processing 0.43 mk1/46 (a)
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3. Data processing 3.3 Pupil positi
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3. Data processing (a) (d) (g) (j)
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3. Data processing 3.4 Phase recove
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3. Data processing in I R,R and app
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3. Data processing Phase discontinu
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3. Data processing 0 0.2 0.4 0.6 0.
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Page 67 and 68: Chapter 4 Wavefront reconstruction
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Page 79 and 80: Chapter 5 Preliminary experiments 5
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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
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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
Page 133 and 134: BIBLIOGRAPHY [67] W.N. Charman and
Page 135 and 136: BIBLIOGRAPHY [89] J.P. Craig, P.A.
Page 137: BIBLIOGRAPHY [115] Frangois C. Delo
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Alfredo Dubra's PhD thesis - Imperial College London

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