Adaptive Optics for Vision Science

Chapter 10.3.5 - Wavefront Sensing and Compensation

10.3.5   Wavefront Sensing and Compensation

Wavefront sensing and compensation is done in a unique way in an AOSLO.
First, AO is used in both directions; to focus the light to a sharp point on the
retina and to take the scattered light from the eye and focus it to a sharp point
at the confocal pinhole. But a complication arises because the optical path of
the SLO includes scanning mirrors, which means that conventional wavefront
sensing methods that project a stable laser beacon onto the retina will not
work. However, the solution to this complication is elegant and provides
several advantages over conventional AO systems [29, 31]. In the AOSLO,
the wave aberration is measured with the same light that is used to form the
image. This is possible because, although the light is being scanned in a raster
pattern on the retina, the light is descanned on the return path, which renders
the beam stationary. Thus, the Shack–Hartmann wavefront sensor sees the
light from the retina as though it were coming from a single spot, which makes
an aberration measurement possible. This method has several advantages.
First, it automatically implements the method of Hofer et al. in which scanning
on the retina is employed to remove speckle from the short-exposure
retinal images [8, 60]. This is important because the light source in the
AOSLO may be highly coherent. A second advantage is that the average
aberration is measured over the entire field of view of the system. This ensures
more uniformity in the correction over the field that is being imaged. For
small fields, correcting average aberrations likely has minimal benefit since
the eye is isoplanatic over at least one degree [32]. The final advantage is that
the wavefront sensing and the imaging portions of the system use the same
light path and light source, which reduce non-common-path errors and eliminate
noncommon aberrations between the wavefront sensor and imaging
camera due to chromatic effects.

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