Study of the dynamic aberrations of the human tear film

The dynamic aberrations introduced by the human tear film are studied by measuring the topography of the tear film surface on 14 subjects using a curvature sensing setup. The RMS wavefront error variation of the data obtained is presented showing the non-negligible contribution of the tear film to overall eye aberrations, and reference to the effect of tear film break up is made. The tear film wavefronts are decomposed in their constituent Zernike terms, showing stronger contributions from 4th order terms and terms with vertical symmetry, and the temporal behaviour of these aberrations is analysed. © 2005 Optical Society of America OCIS codes: (170.4460) Ophthalmic optics, (330.5370) Physiological optics, (010.7350) Wave-front sensing and (010.1080) Adaptive optics. References and links 1. J. Liang, D. R. Williams, and D. T. Miller, “Supernormal vision and high-resolution retinal imaging through adaptive optics,” J. Opt. Soc. Am. A 14(11), 2884–2892 (1997). 2. M. Glanc, E. Gendron, F. Lacombe, D. Lafaille, J.-F. L. Gargasson, and P. L éna, “Towards wide-field retinal imaging with adaptive optics,” Opt. Commun. 230, 225–238 (2004). 3. E. J. Ferńandez, I. Iglesias, and P. Artal, “Closed-loop adaptive optics in the human eye,” Opt. Lett. 26(10), 746–748 (2001). 4. A. Roorda, F. Romero-Borja, W. J. Donnelly III, H. Queener, T. J. Hebert, and M. C. Campbell, “Adaptive optics scanning laser ophthalmoscopy,” Opt. Express 10(9), 405–412 (2002). 5. L. Diaz-Santana, C. Torti, I. Munro, P. Gasson, and C. Dainty, “Benefit of higher closed-loop bandwidths in ocular adaptive optics,” Opt. Express 11(20), 2597–2605 (2003). 6. H. Hofer, P. Artal, B. Singer, J. L. Arag ón, and D. R. Williams, “Dynamics of the eye’s wave aberration,” J. Opt. Soc. Am. A18(3), 497–505 (2001). 7. K. M. Hampson, I. Munro, C. Paterson, and C. Dainty, “Weak correlation between the aberration dynamics of the human eye and the cardiopulmonary system,” J. Opt. Soc. Am. A 22(7), 1241–1250 (2005). 8. J. I. Prydal, P. Artal, H. Woon, and F. Campbell, “Study of human precorneal tear film thickness and structure using laser interferometry,” Invest. Ophth. Vis. Sci. 33(6), 2006–2011 (1992). 9. T. J. Licznerski, H. T. Kasprzak, and W. Kowalik, “Analysis of shearing interferograms of tear film by the use of fast Fourier transforms,” J. Biomed. Optics 3(1), 32–37 (1998). 10. R. Tutt, A. Bradley, C. Begley, and L. N. Thibos, “Optical and visual impact of tear break-up in human eyes,” Invest. Ophth. Vis. Sci. 41(13), 4117–4123 (2000). 11. A. Dubra, C. Paterson, and C. Dainty, “Study of the tear topography dynamics using a lateral shearing interferometer,” Opt. Express 12(25), 6278–6288 (2004). 12. R. Mont́es-Mićo, J. L. Alió, G. Mũnoz, J. J. Ṕerez-Santoja, and W. N. Charman, “Postblink changes in total and corneal ocular aberrations,” Ophthalmology 111, 758–767 (2004). 13. K. Y. Li, G. Yoon, and G. Pan, “Variability in retinal image quality with tear film behavior after blink,” Invest. Ophth. Vis. Sci.46, E–Abstract 848 (2005). 14. S. Gruppetta, L. Koechlin, F. Lacombe, and P. Puget, “A curvature sensor for the measurement of the static corneal topography and the dynamic tear film topography in the human eye,” Opt. Lett. (to be published).


Introduction
The study of the human eye's aberrations has a long history, however it is only recently that interest in the dynamic component of these aberrations has arisen.This interest has been strongly linked with adaptive optics making the jump from astronomy to ophthalmology, where these dynamic aberrations are measured and corrected in a closed loop [1,2,3,4,5].However, whereas in astronomy the atmospheric aberrations are very well understood, their ocular counterparts are less so.Suggestions have been made that the changes in ocular aberrations could be due to, in varying degrees, eye movements, retinal pulsation, microfluctuations of the lens and variations in the tear film layer [6,7].This paper focuses on the latter of these.
The human tear film provides the first and most powerful optical surface in the eye by having a large curvature and the largest refractive index step in the eye's optics.Furthermore, the tear film is a liquid layer, and the effect on this liquid layer of eye movements, pressure exerted by the eye lids, evaporation and other external factors is a non-static air-tear film interface.Consequentially, the aberrations introduced by this layer are also dynamic.
Though considerable work has been done on measuring the average thickness of the tear film layer and the film's break up time [8,9,10], much less is known on the actual aberrations introduced and their temporal behaviour.Dubra et al. [11] have used lateral shearing interferometry to monitor the effect of the tear film on the optical quality showing small but non-negligible variation in the wavefront error with time, while Montés-Micó et al. [12] use a commercial topographer to show a degradation of the optical quality after time intervals of 10s and 20s following a blink.Other work is also currently underway using a Shack-Hartmann sensor [13].

Measuring the dynamic tear film aberrations
In the work presented in this paper, a curvature sensor is used to measure the tear film topography.The technique and optical setup used have been described in detail in reference 14.This technique enables fast acquisition and simple and accurate wavefront reconstruction which allows the monitoring of the dynamic tear film surface.These topographies are multiplied by the difference in refractive indices of air (n = 1.000) and the tear film (n = 1.337) to give the wavefront transmitted through the tear film.All further references to tear film wavefronts in this paper refer to the transmitted wavefronts.
Data was collected for 14 subjects with no tear abnormalities; several series of tear film wavefronts were recorded at 22Hz for each subject.The diameter of the measured pupil was 4mm.Most of the subjects were non-contact lens wearers, and for the 2 soft contact lens wearers data was collected with and without the lens worn.The measuring system being very sensitive to eye movements, data was collected only when the cornea was within a tolerance range of ±150µm from the measuring position in the horizontal and vertical directions (< 8% of the pupil diameter), and ±300µm in the axial direction.This sensitivity ensures that for the data collected, the positioning of the eye is very accurate and movements are kept to a minimum; the drawback however is that this makes data collection harder since, even using a chin rest and a restrictive head rest, it is not straightforward for subjects to keep their eyes within the required range.The changes in aberrations measured for a calibrating surface when translated across these tolerance ranges was found to be negligible with respect to the measured tear film aberration changes.In addition, typical power spectra of the aberration changes, which are discussed later, do not show particularly strong contributions in the 2-3Hz region which would correspond to microsaccadic eye movements.
The series of tear film wavefronts obtained vary in length between 2s and 15s depending on how long the subject was able to keep within the required range for data acquisition.Examples of the data collected are shown in the films in figure 1, which show the evolution of the wavefront for 2 subjects following a blink after removal of first and second order Zernike terms.The acquired images also allowed the observation of the effect of blinking on the tear film immediately before and after the blink, as shown in figure 2.

Evolution of the tear film wavefront RMS error
Figure 3 shows the typical evolution of the RMS wavefront error after removal of first and second order Zernike terms.The static component of these plots is largely due to corneal aberrations, whereas the dynamic component is due to the tear film dynamics.In figure 3(b), the dashed line indicates the break up of the tear film and the RMS wavefront error increases steadily thereafter as the dry patches on the cornea grow.In figure 3(c), the evolution of the RMS wavefront error is shown for subject 6 with and without soft contact lenses worn, showing a higher RMS error value and larger variations when the contact lens was worn.The number of contact lens wearers in the group of subjects was however too small to analyse further the effect of contact lenses on the tear film.Average values for the RMS error evolution were calculated over the series of data collected.Figure 4 shows two plots representing the average RMS evolution over all series 2s and 6s in length respectively.These plots show a relatively constant trend due to the large inter-subject variablilty, as shown by the standard deviation on the plots, as well as different tear film break up times between subjects.An increasing trend would possibly be seen for longer time intervals, but long acquisition times were not possible due to the prolonged accuracy in eye positioning required.Different trends might also be observed for pupil sizes larger than the largest pupil possible in this study (4mm) as suggested by Montés-Micó et al. [12].

Zernike polynomial decomposition of the series of wavefronts
To obtain a better insight into the varying tear film aberrations, the series of wavefronts obtained were decomposed into Zernike polynomials.Figure 5 shows the evolution of Zernike terms for orders 3 to 6 for the series for subject 11 represented in figure 3(b).The figure shows that the strongest contributions are due to the lower orders, particularly the 4th order terms.This can also be seen from the histograms in figure 6 representing the Zernike coefficients averaged over all frames of all 2s and 6s series; the average 4th order coefficient is 2.9 times larger than the average 5th order coefficient for the 6s series.

Fig. 3 .Fig. 4 .
Fig. 3. Typical evolution of the RMS wavefront error.(a) Subject 2. (b) Subject 11; the dashed line indicates break up in the tear film.(c) Subject 6 with (blue) and without (red) contact lens.The RMS wavefront error for a static calibration surface is also shown (green.)

Fig. 5 .
Fig. 5. Typical evolution of Zernike terms for the 3rd and 4th orders (left) and the 5th and 6th orders (right) for subject 11.The dashed line indicates tear film break up.