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Adaptive Optics

Adaptive Optics (AO) refers to the set of optical systems which adjust to compensate for optical defects called wavefront aberration introduced by the medium between the object and its image. AO was first used in ground-based astronomical telescopes to remove the wavefront aberration induced by atmospheric turbulence and achieve near-diffraction limited images. The shape of the wavefront in an aberrated system is determined by a wavefront sensor and a wavefront corrector, usually a deformable mirror, is used to restore the plane wavefront. Recently, the same principle has been used for high resolution retinal imaging and improving vision by correcting the aberrations induced by the eye's optics, cornea and crystalline lens (Liang et al, JOSA A 1997, Yoon and Williams, JOSA A 2002 pdf).

  1. AO for vision testing
  2. Large stroke AO in eyes with abnormal cornea
  3. Neural adaptation in abnormal eyes

1. AO for vision testing

Studies of the eye's wavefront aberrations in a normal population have showed significant amounts of higher order aberrations (HOA) apart from conventional sphere and cylinder. This indicates that correcting HOA even in normal eyes can provide additional visual benefit over conventional correction methods such as spectacles and contacts. AO has helped to overcome optical imperfections of the cornea and crystalline lens of the eye to assess their impact on visual performance. Yoon and Williams (pdf) showed the significant visual benefit of correcting HOA in normal eyes using AO under normal viewing (white light) condition. Contrast sensitivity when only monochromatic aberrations were corrected was improved by a factor of 2 on average at 16 and 24 cycles/deg.

2. Large stroke AO for eyes with abnormal cornea

More importantly, eyes with abnormal corneal conditions such as keratoconus and corneal transplant have 5-6 times larger amplitudes of HOA than normal eyes, which implies that the visual benefit of HOA correction will be correspondingly larger. However, correcting large amounts of aberrations in these eyes with AO has been very difficult due to the insufficient stroke of conventional wavefront correctors (deformable mirrors). To overcome this limitation, we have developed the large stroke adaptive optics. Our AO consists of a large-stroke deformable mirror (Imagine Eyes: MIRAO-52D) and a Shack-Hartmann wavefront sensor. The wavefront sensor is optimally designed with high dynamic range to measure highly aberrated eyes. The deformable mirror has a large surface deformation which allows us to compensate for the aberrations in eyes with abnormal corneal conditions. In addition, the highly linear response of the deformable mirror allows us to operate the closed loop at gains up to 100%.

We have been able to demonstrate the feasibility of the large-stroke AO in providing almost perfect optical quality in both normal and keratoconic eyes irrespective of the different magnitudes of aberrations in the two sets of eyes. AO closed-loop correction reduced an average total RMS of 1.73 ± 0.998 µm to 0.10 ± 0.017 µm (higher order RMS of 0.39 ± 0.124 µm to 0.06 ± 0.004 µm) in the 3 normal eyes. For the 2 keratoconic eyes, adaptive optics reduced an average total RMS of 2.73 ± 1.754 µm to 0.10 ± 0.001 µm (higher order RMS of 1.82 ± 1.058 µm to 0.05 ± 0.017 µm) with closed-loop correction. Shown below is the time-course of AO correction in 1 normal subject for a 6-mm pupil.

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Also shown is the PSF based on the measured aberrations. During the time-course, one can notice blinks, which appear as peaks in the time course amidst time intervals of nearly perfect correction.

3. Neural adaptation in abnormal eyes

With the large stroke AO, we wish to study the interaction between optics of the eye and the human visual system, and the mechanism of neural adaptation to optical blur in these eyes. Artal et al suggested that the visual system may be adapted to the eye's particular HOA in normal eyes. This effect could be even more significant in eyes with abnormal cornea because these eyes have experienced poor optical quality for many years. As shown below, with same perfect optical quality after the AO correction in an average of 3 normal and 2 keratoconic eyes, visual acuity is statistically different between the two sets of eyes.

Although the large stroke AO system can provide abnormal eyes with perfect optical quality, whether they can immediately achieve perfect visual performance as predicted by optical theory remains an intriguing question. Moreover, long term adaptation to high quality images may improve vision although optical quality of the eye does not change. Addressing these questions may provide insights into the optimal visual quality obtainable after correcting HOA in these eyes with customized optics.

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