Dual wavefront sensing channel monocular adaptive ... - OSA Publishing

Dual wavefront sensing channel monocular adaptive optics system for accommodation studies Karen M. Hampson, Sem Sem Chin and Edward A. H. Mallen Bradford School of Optometry and Vision Science, University of Bradford, Richmond Rd, Bradford, UK k.m.hampson@bradford.ac.uk

Abstract: Manipulation of the eye’s aberrations using adaptive optics (AO) has shown that optical imperfections can affect the dynamic accommodation response. A limitation of current system designs used for such studies is an inability to make direct measurements of the eye’s aberrations during the experiment. We present an AO system which has a dual wavefront sensing channel. The corrective device is a 37-actuator piezoelectric deformable mirror. The measurements used to control the mirror, and direct measurements of the eye’s aberrations, are captured on a single Shack-Hartmann sensor. Other features of the system include stroke amplification of the deformable mirror and a rotating diffuser to reduce speckle. We demonstrate the utility of the system by investigating the impact of aberration dynamics on the control of steady-state accommodation on four subjects. © 2009 Optical Society of America OCIS codes: (220.1080) Adaptive optics; (330.7322) Accommodation.

References and links 1. H. Hofer, P. Artal, B. Singer, J. L. Aragón, and D. R. Williams, “Dynamics of the eye’s wave aberration,” J. Opt. Soc. Am. A 18, 497-506 (2001). 2. J. W. Hardy, J. E. Lefebvre, and C. L. Koliopoulos, “Real-time atmospheric compensation,” J. Opt. Soc. Am. A 67, 360-369 (2001). 3. E. J. Fernández, I. Iglesias, and P. Artal, “Closed-loop adaptive optics in the human eye,” Opt. Lett. 26, 746-748 (2001). 4. H. Hofer, L. Chen, G. Y. Yoon, B. Singer, Y. Yamauchi, and D. R. Williams, “Improvement in retinal image quality with dynamic correction of the eye’s aberrations,” Opt. Express 8, 631-643 (2001). 5. K. M. Hampson, “Topical review: Adaptive optics and vision,” J. Mod. Opt. 55, 3425-3467 (2008). 6. E. F. Fincham, “The accommodation reflex and its stimulus,” Brit. J. Ophthal. 35, 381-393 (1951). 7. G. Walsh and W. N. Charman, “The effect of defocus on the contrast and phase of the retinal image of a sinusoidal grating,” Ophthal. Physiol. Opt. 9, 398-404 (1989). 8. B. J. Wilson, K. E. Decker, and A. Roorda, “Monochromatic aberrations provide an odd-error cue to focus direction,” J. Opt. Soc. Am. A 19, 833-839 (2002). 9. E. J. Fernández and P. Artal, “Study on the effects of monochromatic aberrations in the accommodation response by using adaptive optics,” J. Opt. Soc. Am. A 22, 1732-1738 (2005). 10. L. Chen, P. B. Kruger, H. Hofer, B. Singer, and D. R. Williams, “Accommodation with higher-order monochromatic aberrations corrected with adaptive optics,” J. Opt. Soc. Am. A 23, 1-8 (2006). 11. E. Gambra, L. Sawides, C. Dorronsoro, and S. Marcos, “Accommodative lag and fluctuations when optical aberrations are manipulated,” J. Vis. 9, 1-15 (2009). 12. B. Theagarayan, H. Radhakrishnan, P. M. Allen, R. I. Calver, S. M. Rae, and D. J. O’Leary, “The effect of altering spherical aberration on the static accommodative response,” Ophthal. Physiol. Opt. 29, 65-71 (2009). 13. W. N. Charman and G. Heron, “Fluctuations in accommodation: A review,” Ophthal. Physiol. Opt. 8, 153-164 (1988).

#113264 - $15.00 USD Received 24 Jun 2009; revised 17 Aug 2009; accepted 17 Sep 2009; published 25 Sep 2009

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14. B. Winn, “Accommodative microfluctuations: a mechanism for steady-state control of accommodation,” in Accommodation and vergence mechanisms of the visual system, O. Franzén, H. Richter, and L. Stark, eds. (Birkhäuser Verlag Basel, Switzerland, 2000), 129-140. 15. L. S. Gray, B. Winn, and B. Gilmartin, “Effect of target luminance on microfluctuations of accommodation,” Ophthal. Physiol. Opt. 13, 258-265 (1993). 16. M. Day, D. Seidel, L. S. Gray, and N. C. Strang, “The effect of modulating ocular depth of focus upon accommodation microfluctuations in myopic and emmetropic subjects,” Vision Res. 49, 211-218 (2009). 17. L. S. Gray, B. Winn, and B. Gilmartin, “Accommodation microfluctuations and pupil diameter,” Vision Res. 33, 2083-2090 (1993). 18. L. R. Stark and D. A. Atchison, “Pupil size, mean accommodation response and the flutuations of accommodation,” Opthal. Physiol. Opt. 17, 316-323 (1997). 19. K. Niwa and T. Tokoro, “Influence of spatial distribution with blur on fluctuations in accommodation,” Optom. Vis. Sci. 75, 227-232 (1998). 20. K. M. Hampson, C. Paterson, C. Dainty, and E. A. H. Mallen, “Adaptive optics system for investigation of the effect of the aberration dynamics of the human eye on steady-state accommodation control,” J. Opt. Soc. Am. A 23, 1082-1088 (2006). 21. K. M. Hampson, S. S. Chin, and E. A. H. Mallen, “Binocular Shack-Hartmann sensor for the human eye,” J. Mod. Opt. 55, 703-716 (2008). 22. British Standards: Safety of Laser Products, 60825-1:1994. 23. R. H. Webb, M. J. Albanese, Y. Zhou, T. Bifano, and S. A. Burns, “Stroke amplifier for deformable mirrors,” Appl. Opt. 43, 5330-5333 (2004). 24. L. N. Thibos, R. A. Applegate, J. T. Schwiegerling, and R. Webb, “Standards for reporting the optical aberrations of eyes,” J. Refract. Surg. 18, 652-660 (2002). 25. L. N. Thibos, W. Wheeler, and D. Horner, “Power vectors: an application of Fourier analysis,” Optom. Vis. Sci. 74, 367-375 (1997). 26. J. M. Bland and D. G. Altman, “Statistical methods for assessing agreement between two methods of clinical measurement,” Lancet 1, 307-310 (1986). 27. K. M. Hampson, E. A. H. Mallen, and C. Dainty “Coherence function analysis of the higher-order aberrations of the human eye,” Opt. Lett. 31, 184-186 (2006). 28. L. Diaz-Santana, V. Guériaux, G. Arden, and S. Gruppetta, “New methodology to measure the dynamics of ocular wave front aberrations during small amplitude changes of accommodation,” Opt. Express 15, 5649-5663 (2007). 29. D. R. Iskander, M. J. Collins, M. R. Morelande, and M. Zhu, “Analyzing the dynamic wavefront aberrations in the human eye,” IEEE Trans. Biomed. Eng. 51, 1969-1980 (2004). 30. M. G. Doane, “Interactions of eyelid and tears in corneal wetting and the dynamics of the normal human eye blink,” A. J. Ophthal. 89, 507-516 (1988). 31. L. N. Thibos, X. Hong, A. Bradley, and R. A. Applegate, “Accuracy and precision of objective refraction from wavefront aberrations,” J. Vis. 4, 329-351 (2004). 32. J. S. Bendat and A. G. Piersol, Random data: analysis and measurement procedures, (Jon Wiley & Sons, Inc., New York, 2000). 33. P. Denieul, “Effects of stimulus vergence on mean accommodation response, micofluctuations of accommodation and the optical quality of the human eye,” Vision Res. 22, 561-569 (1882). 34. S. S. Chin, K. M. Hampson, and E. A. H. Mallen, “Role of ocular aberrations in dynamic accommodation control,” Clin. Exp. Optom. 92, 227-237 (2009). 35. Q. Mu, Z. Cao, D. Li, L. Hu, and L. Xuan, “Open-loop corection of horizontal turbulence: system design and result,” Appl. Opt. 47, 4297-4301 (2008). 36. L. R. Stark, N. C. Strang, and D. A. Atchison, “Dynamic accommodation response in the presence of astigmatism,” J. Opt. Soc. Am. A 20, 2228-2236, (2003). 37. M. J. Collins and B. A. Davis, “Microfluctuations of accommodation and spherical aberration,” Clin. Exp. Optom. 80, 234 (1997).

1.

Introduction

The human eye suffers from a number of image degrading monochromatic aberrations which fluctuate over time [1]. AO is a powerful technique that can dynamically manipulate these aberrations. The technique was originally developed to increase image quality in ground-based telescopes by compensating for atmospheric turbulence [2]. Less than a decade ago, the first systems capable of real-time correction of the eye’s aberrations were demonstrated [3, 4]. AO has now become an invaluable tool for investigating the effect of the eye’s aberrations on visual acuity and to obtain high resolution in-vivo images of the retina. For a review see [5]. A comparatively new application of AO for the eye is in the study of the effect of the


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monochromatic aberrations on accommodation. The accommodation system is responsible for altering the power of the eye’s lens to bring an object of interest into focus. It is well known that this system responds to a variety of cues such as binocular disparity, chromatic aberration and size, see for example [6]. These cues provide the system with the necessary information to guide the accommodation response in the correct direction. Several investigators, such as Walsh and Charman, have proposed that monochromatic aberrations can also provide such a cue [7]. Even-order aberrations in particular, such as spherical aberration, result in the point spread function (PSF) being different depending upon whether the image is focussed in front of or behind the retina. Wilson and colleagues have demonstrated that subjects can perceive such differences [8]. With the advent of AO it is possible to manipulate these aberrations during the dynamic accommodation response. Correcting them has been shown to adversely affect the dynamic response in some subjects [9, 10]. Aberrations have also been found to affect the steady-state (or static) level of accommodation [11, 12]. Even when the eye is focussed on a stationary target the accommodation level is never truly static however. The eye exhibits small fluctuations in focus about a mean level with an amplitude of around a few tenths of a diopter. For a review of the properties of these so-called microfluctuations in accommodation see [13]. Several investigators such as Winn have proposed that the low frequency component (LFC) of these fluctuations ( 0.05). In the case of the SD, there were no significant differences except for subject KH. For this subject, the SD when correcting defocus (Z20 ) was significantly greater than the baseline condition and when correcting astigmatism (Z22 ) (p < 0.05). On inspection of the time-course records, this proved to be due to noticeable drifts in the records when correcting defocus.

EM

YP

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Fig. 7. (Color online) Typical time-course records of the accommodation fluctuations for the baseline condition for each subject.

1.8

Normalized Area Under PSD

1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0 B

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Fig. 8. (Color online) Average area under the PSD across subjects for each condition in the low frequency region (0.05-0.6 Hz). Error bars indicate ±1 SD.


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6. 6.1.

Discussion Instrument design

We have presented a closed-loop AO system that allows simultaneous measurement of the eye’s aberrations directly. The measurements of both the direct eye’s aberrations and those used to control the deformable mirror are captured by a single Shack-Hartmann sensor. This reduces the cost and complexity of the system. In this investigation we have used the channel that directly measures the eye’s aberrations primarily to determine the fluctuations in accommodation. Hence one could argue that for accommodation studies using AO there is no need to use a Shack-Hartmann sensor as a more simple optometer which only measures accommodation could be used, such as a dynamic retinoscope. However having a system capable of measuring all aberrations in the eye directly, allows more complex investigations to be carried out. The extra channel is essential when carrying out experiments in which the aberrations are dynamically inverted during the accommodation response for example. The common control algorithm used to control an AO system and the one used in this system is vcorrect (ti+1 ) = −g ∗ C ∗ (Zmeas − Zreq ) + vcorrect (ti )

(2)

where v is the vector of voltages sent to the deformable mirror, g is the gain (typically set to 0.3), C is the control matrix, Zmeas is the vector of the measured Zernike coefficients via the deformable mirror and Zreq are the required Zernike coefficients. From Equation 2 it can be seen that it is not possible to dynamically invert aberrations as Zmeas is not a continuous direct measure of the eye’s aberrations. With the AO system presented here dynamic inversion of the aberrations can readily be achieved using a modified control algorithm: vinvert (ti+1 ) = −g ∗ C ∗ (Zmeas + Zeye ) + vinvert (ti )

(3)

where Zeye are the Zernike coefficients as measured in the eye-only channel. As there will inevitably be a minor difference in the baseline aberration measurement in the two channels, the final algorithm would be given by vinvert (ti+1 ) = −g ∗ C ∗ (Zmeas + Zeye − Zbias ) + vinvert (ti )

(4)

where Zbias is a vector containing the differences in the baseline measurements of the Zernike coefficients between the two channels. This algorithm has been successfully implemented in another study [34]. Another advantage of having a two-channel system is that it is more convenient to determine the bandwidth of the system. This is determined from the ratio of the eye’s aberrations with and without the correction device operating and so can be readily found from the ratio of the output from both channels. Additional advantageous features that are employed in this system include a rotating diffuser to reduce laser speckle. This can be placed outside the wavefront sensing path and so is more convenient than other methods such as scanning. The light strikes the deformable mirror twice to produce a cost effective way of doubling the stroke of the device, which only requires two lenses and a plane mirror. We found that upon doing this the dynamic range of the sensor was slightly less than the capabilities of the mirror. Although this has not been an issue in this study, in future work increased dynamic range of the wavefront sensor may be necessary in order to cope with larger levels of aberration inversion for example. Future experiments may also benefit from the inclusion of a more flexible target that can be varied in terms of luminance, spectral composition, contrast, and spatial frequency. As previously mentioned, all AO systems for the eye are in a closed-loop configuration. One reason is that open-loop systems require very accurate calibration, which is difficult owing to


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hysteresis and non-linearity of the correction device. With the development of liquid crystal on silicon devices (LCOS), open-loop systems have been realized in the field of Astronomy. See for example [35]. The application of open-loop systems to the eye in the future would simplify the system presented here as two wavefront sensing paths would not be required. 6.2.

Aberration dynamics and steady-state accommodation

For three out of our four subjects used in this study, we did not find a significant effect of aberration correction on the magnitude of the accommodative microfluctuations both in terms of the power of the LFC and the SD of the fluctuations in accommodation. For one subject we found that only the SD for the correction of defocus (Z20 ) was significantly greater than the baseline condition and when correcting astigmatism (Z22 ). Upon inspection of the time traces this was found to be due to considerable drifts when correcting defocus. Several investigators have proposed that the eye may use accommodative microfluctuations, in particular the LFC, to maintain the required steady-state accommodation level. This is based on observed increases in the magnitude of the LFC with increasing depth of focus, see for example [14]. As we found an effect in only one subject for a limited amount of conditions, our results may at first sight appear counter to the current literature. However in our investigation, prior to each experimental run, including when the baseline fluctuations were measured, the static aberration levels were corrected. Hence the depth of focus was minimized prior to each run and so we would not expect the subsequent correction of the aberration microfluctuations to significantly impact the depth of focus. A limited number of studies have directly assessed the effect of aberrations on the LFC of accommodation microfluctuations. In these studies static levels of aberrations have been induced. Stark and colleagues found that inducing astigmatism could cause the fluctuations in accommodation to increase in magnitude in two out of their seven subjects [36]. Collins et al. found no increase in the LFC when inducing spherical aberration in their two subjects [37]. To the authors’ knowledge, this is the first study to investigate the effect of aberration dynamics on steady-state accommodation control. Using AO, Gambra et al. found that the fluctuations were generally smallest when the subject had their natural aberrations present and when all aberrations were corrected as compared to when inducing spherical aberration [11]. However in the corrected state, they operated the mirror in closed-loop then stopped it before the measurements. Hence no dynamic correction was performed during the measurements. Currently, we do not know why only one subject was affected by the correction of aberrations and others were not. The effect of aberration dynamics on the dynamic accommodation response has also been proven to be subject dependent [10]. The closed-loop bandwidth of our system is limited to around 1 Hz. As aberration fluctuations extend well beyond this, it is not possible to completely remove these fluctuations. It may be that for three of the subjects, their remaining aberration fluctuations were sufficient to keep the eye in good focus. 7.

Conclusion

This paper has described an AO system for the study of the role of ocular aberration dynamics in steady-state accommodation control. A key feature of this system is its ability to apply closed-loop aberration modifications while simultaneously acquiring an independent measure of aberration dynamics and accommodation accuracy. The utility of this system for the study of accommodation function has been demonstrated in a small cohort of human subjects. 8.

Acknowledgements

The authors are grateful to the Engineering and Physical Sciences Research Council for funding under grant EP/D036550/1.


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