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Study of the tear topography dynamics using a lateral shearing interferometer

Alfredo Dubra, Carl Paterson, and Christopher Dainty

Open Access Open Access

Abstract

The dynamics of the pre-corneal tear film topography are studied on 21 subjects with a purpose-built lateral shearing interferometer. It was found that in most of the recorded data the tear surface is continuous and smooth. Eye movement is identified as a major problem in quantitative tear topography estimation. Based on the reconstructed tear topography maps, the effects of tear dynamics in visual performance, wavefront sensing for refractive surgery and ophthalmic adaptive optics are discussed in terms of wavefront RMS. The potential of lateral shearing interferometry for clinical applications such as dry eye diagnosis and contact lens performance studies is illustrated by the recorded topography features such as post-blink undulation, break-up, eyelid-produced bumps/ridges, bubbles and rough tear surfaces in front of contact lenses.

©2004 Optical Society of America

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Supplementary Material (9)

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Figures (8)
Fig. 1.
Fig. 1. Sketch of the 3 dimensional lateral shearing interferometer. After the light is reflected back from the front surface of the tear, the first glass wedge produces two horizontally sheared and tilted copies of the incident beam by reflection and a third copy by transmission. The second wedge allows the first pair of copies of the beam to go through unchanged, while on reflection produces a second pair of copies of the beam carrying information from the eye, sheared and tilted in the perpendicular direction. The superscript in the lenses indicates focal length in millimeters.

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Fig. 2.
Fig. 2. Movies illustrating tear topography features: (a) smooth surface (2.8MB); (b) post-blink undulation (2.8MB); (c) bubbles and post-blink undulation (2.8MB); (d) eyelid-produced bumps (2.8MB); (e) break-up in an otherwise smooth surface (2.8MB); (f) and (g) dramatic break-up (2.8MB); (h) and (i) rough surfaces in front of hard and soft contact lenses respectively (2.8MB). These movies play at 5 frames per second, that is the rate at which the interferograms were recorded.

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Fig. 3.
Fig. 3. Eye movement: the plot on the left shows a typical eye movement pattern over 30s (the radius of the red circle corresponds to one standard deviation); the center and right plots are the normalized histograms of the horizontal and vertical eye displacement for all the 2450 usable frames from 14 different subjects. Both distributions have zero mean and 0.16 and 0.12mmstandard deviations respectively, which is equivalent to 9nm of wavefront error RMS.

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Fig. 4.
Fig. 4. Correlation between the relative change in the radius of the interferograms and the estimated change in defocus (0.67).

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Fig. 5.
Fig. 5. Movies of estimated wavefront aberration introduced by the tear topography: (a) corresponds to Fig. 2(a) and shows a very smooth tear surface(4MB);(b) corresponds to Fig. 2(d) and shows the formation and flattening of a bump on the top of the pupil (4MB). The change in wavefront height between consecutive contour lines is λ/14.

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Fig. 6.
Fig. 6. Typical wavefront RMS evolution due to the tear topography dynamics. The red horizontal line in the plots indicates the diffraction limit for the wavelength used in the experiment. Plot (a) shows the evolution of the RMS (after subtraction of the mean wavefront of the series). Plot (b) is similar to (a) but with the defocus and astigmatism terms removed, and (c) plots the evolution of the defocus and astigmatism components.

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Fig. 7.
Fig. 7. Mean RMS and residual RMS over the data series corresponding to time intervals of 30s. The red horizontal line shows the diffraction limit (λ=632.8nm). The data is separated in three groups: on the left (light gray background) are the series in which there were no blinks during the data recording; in the center are series in which there was at least one blink and on the right (dark gray) we show the series in which the subject was wearing contact lenses at the time of the experiment. The numbers on top of the error bars are the the maximum correlation coefficient between the corresponding RMS and the eye and head movement.

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Fig. 8.
Fig. 8. Temporal evolution of the estimated wavefront error RMS (a) and residual wavefront error RMS (b) for 30 data series (around 2300 topography maps) corresponding to 19 different subjects. The black lines are the mean RMS at the corresponding time and the gray areas delimited by the blue lines are the areas within one standard deviation from the mean. Again, the red horizontal line indicates the diffraction limit at 632.8nm.

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Equations (1)

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RMS ( θ ) ≈ ( 2.4 μ m ) × θ .
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