Alfredo Dubra's PhD thesis - Imperial College London

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2. Lateral shearing interferometer design He-Ne laser 633nm ND filter microscope objective spatial filter f = 250 1 stop aperture s-polarisation l/4 f 2 = 40 PBS eye p-polarisation circular-polarisation Figure 2.3: Illumination system of the lateral shearing interferometer. which at the wavelength of the He-Ne laser used (632.8 nm) has a refractive index n w = 1.51509. 2.6 Illumination branch The illumination system of the shearing interferometer, shown in figure 2.3, consists of a light source, a neutral density absorption filter, a microscope objective, a pinhole, a collimating lens, a polarising beam-splitter, a quarter waveplate and a lens. The purpose of this part of the optical system is to produce a smooth (ideally perfectly uniform) illumination over a circular region of the front surface of a subject’s precorneal tear and a fixation target. 2.6.1 Source The source to be used needs to be quasi-monochromatic because multiple wavelengths would produce different phase differences and, therefore, multiple overlapping interferograms. 31
2. Lateral shearing interferometer design A polarized source is desirable to keep the number of optical elements to a minimum thus reducing undesired reflections, which as we will show in section 3.4.3 can be a non-negligible source of error in the data processing. Finally, it would also be desirable to use a source with an intensity profile as uniform and symmetrical as possible to produce uniform illumination over the cornea and thus a uniform signal-to-noise ratio across the interferograms. With this in mind we used a single-mode linearly polarized He-Ne laser with an output of 2.5 mW at 632.8 nm (Spectra Physics S/N 47075/1919), with a Gaussian intensity profile. The intensity of the source output is adjusted by using absorption neutral density filters to keep the power reaching the eye within the safety limits described in Appendix B. 2.6.2 Spatial filter and telescope The microscope objective, pinhole, collimating lens and stop aperture shown in figure 2.3 form a telescope to magnify the collimated output of the source and a spatial filter to produce a smooth intensity profile. The microscope objective is ×40 with a numerical aperture (NA) 0.65 and a focal length of approximately 3 mm at 632.8 nm, the pinhole diameter is 10 µm and the collimating lens has a focal length of 250 mm. In a normal telescope, one would like to match the NAs to make use of all the incoming light, but in our case, we have chosen the combination of microscope objective and collimating lens so that the NA of the first one is about 10 times larger than the second, so that the beam at output of the telescope only takes the central part of the intensity profile of the laser, to produce a flatter intensity profile. The stop aperture narrows the beam so that less than the whole diameter of the rest of the optics is used, providing some tolerance for eye movement. 2.6.3 Polarizing beam splitter and quarter waveplate The block formed by the polarising beam-splitter (PBS) and the quarter waveplate serves two purposes, first, to maximize the light reflected back from the eye towards the imaging branch of the optical setup and second, to minimize the reflections from the beam-splitter itself. 32
Page 1 and 2: A Shearing Interferometer for the E
Page 3 and 4: Acknowledgements Pursuing the PhD d
Page 5 and 6: Contents Contents 4 List of Figures
Page 7 and 8: CONTENTS 5.1 Prydal type experiment
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Page 13 and 14: Chapter 1 Introduction Assessing an
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Page 25 and 26: Chapter 2 Lateral shearing interfer
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5. Preliminary experiments dm1_no_w
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5. Preliminary experiments bb_focus
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5. Preliminary experiments the corr
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6. Tear topography dynamics experim
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Chapter 7 Summary and discussion Th
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7. Summary and discussion The value
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7. Summary and discussion ao2_radia
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A. Preliminary study of feasibility
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B. Laser safety calculations B.1 MP
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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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