Shearing Interferometer for the Evaluation of Human Tear Film Topography Dynamics
Alfredo Dubra, Carl Paterson and Chris Dainty
Motivation
More than a century ago, Helmholtz [1] showed that the aberrations of the eye fluctuate in time, with a simple experiment that can be repeated by looking at the picture shown below. The pattern is a set of concentric circles, but because of the aberrations in the eye it does not look perfectly symmetrical, there is some kind of bow-tie pattern, that oscillates due to changes in the optical quality of the eye, mostly due to fluctuations of the accomodation. The asymmetry can be explained as a change in the orientation of the astigmatism with the accommodation state.
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Figure 1: Helmholtz experiment to illustrate the fluctuation of the monochromatic aberrations
of the eye. If one look to the set of concentric circles at a certain distance, then one can see
a bow-tie pattern that oscillates at a frequency of about 2 Hz.
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These fluctuations lead to variability in wavefront sensing measurements in the eye, which is extremely relevant (and unfortunately not always appreciated) in the context of refractive surgery, spectacle prescription and high resolution retinal imaging. Among the known sources of variability of the optical quality of the eye we find: eye movement, fluctuations of accommodation, heart beat and tear film evaporation. Some of these sources have already been studied and quantified, while some remain unexplored. It was in this context that we decided to study effects of the tear film topography dynamics on the optical quality of the eye, and its potential influence on refractive surgery and high-resolution
(diffraction limited) retinal imaging. In order to do so, we designed and built a double-lateral shearing interferometer based on some preliminary work by Kasprzak et al. [2-6].
Shearing interferometer for the evaluation tear film topography
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Figure 2: Schematic diagram of the lateral shearing interferometer.
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The setup consists of an illumination branch and two (partially shared) imaging branches. The illumination arm was designed so that the curvature of the incident wavefronts roughly
matches the curvature of the cornea. Each imaging branch creates a pair of slightly sheared and tilted images of the cornea on a CCD camera, using an uncoated wedge in reflection mode. One branch shears in the x-direction, the other in the y-direction. The resulting pair of interferograms (see below) give the x- and y-slopes of the tear topography. The tilt fringes that modulate the intensity of the interferograms allow the filtering of the effects of uneven intensity across the image. A detailed description of the interferometer can be found in the second chapter of Alfredo Dubra's PhD thesis.
Examples of raw interferograms
Once the interferometer was built and tested, ethical approval from the Local Research Ethichs Committe (St. Mary's Hospital) was obtained and finally, series of interferograms
from 20 different subjects were recorded, some of them with contact lenses on. The figures below show some of the interesting topography features we encountered.
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Figure 3: Examples of tear interferograms: a) normal smooth tear film that is the most representative case,
b) post-blink roughness that usually lasts no longer than two seconds, c) bubbles, d) ridges produced by the
eyelids, e) unusually rough tear surface, f) tear break-up, g) and h) are typical rough tear surface typical
of contact lenses.
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Selected References
H. Helmholtz. Physiological optics, volume I. Optical Society of America, 1924. pages 160-203 and 416-443. Translated version of Handbuch der physiologischen optik, by J.P.C. Southall 1924.
M. Lechna-Marczynska, T. Licznerski and H. Kasprzak. "Interferometry for in vivo testing the artificial tears of the surface of cornea". Proc. SPIE 3820, 386-389.
H. Kasprzak and T. Licznerski. "Influence of characteristics of the tear film break-up on the point spread function of the eye model". Proc. SPIE 3820, 390-395.
T. Licznerski, M. Lechna-Marczynska and H. Kasprzak. "Application of interferometry for in vivo testing the stability of the contact lens". Proc. SPIE 3579, 152-157.
T. Licznerski, H. Kasprzak and W. Kowalik. "Two interference techniques for in vivo assessment of the tear film stability on a cornea and a contact lens". Proc. SPIE 3320, 183-186.
T. Licznerski , M. Lechna-Marczynska and H. Kasprzak. "Application of Twyman-Green interferometer for evaluation of in vivo breakup charactereistic of the human tear film". J. Biomed. Optics 4 (1) 176-182
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