Tear film topography and its effect on the optical quality of the eye

A. Dubra*+, C. Paterson*, B. Barry+ and J.C. Dainty+

* Imperial College London
+ National University of Ireland Galway

  More than a century ago, Helmholtz [1] showed that the aberrations of the eye fluctuate in time, with simple experiment that can be repeated by printing 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.

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.

 

These fluctuations lead to variability in wavefront sensing measurements in the eye, and are thought to be due to eye movement, micro-fluctuations of accommodation and tear film evaporation among others. In this project, we designed and built a lateral shear interferometer to quantify the dynamics of the tear film topography and its effect on the optical quality of the eye, in the context of wavefront sensing for refractive surgery and high resolution imaging with adaptive optics. The use of lateral shear interferometry for the study of the tear topography was first proposed Kasprzak et al [2-6].

Figure 2: Schematic diagram of the lateral shear interferometer designed and built for the study of human tear film.

 

We recorded series of interferograms from 20 different subjects, some of them with contact lenses on. The figures below show some of the topography features we encountered.

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.

 

We are currently processing and analysing the recorded data, but we can see in the figure below a typical evolution of the RMS of the wavefront error that the tear topography would induced in the eye, that is the topography multiplied by the difference in refractive index between air and tear.

Figure 4: Typical evolution of wavefront RMS that the tear film topography dynamics would produce. The red horizontal line indicates the limit of RMS equal to wavelength/14 or equivalently a Strehl ratio of 0.8. The wavefront error is measured with respect to the initial topography, and typically the RMS of the change in wavefront is comparable to the diffraction limit.

 

Figure 5: In this graph we plot the RMS evolution of the same wavefront evolution but now without defocus and astigmatism (2nd order Zernike polynomials), because one could argue that the defocus and astigmatism in our measurements could be significantly affected by eye movement. For the data anaylised so far, we have found that this residual RMS is virtually always below the diffraction limit.

 

Figure 6: In this graph we plot the RMS evolution of the individual terms corresponding to defocus and the two astigmatism Zernike polynomials.

 

Selected References

1 - 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.

2 - 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.

3 - 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.

4 - 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.

5 - 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.

6 - 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.

7 - M. Takeda and H. Ina and S. Kobayashi, “Fourier-transform method of fringe-pattern analysis for computer based topography and interferometry”, JOSA 72, pp 156-160 (1982)