10.2.4   Wavefront Sensing

Wavefront sensing in current conventional AO ophthalmoscopes is realized
with a Shack–Hartmann wavefront sensor [5–7]. The light source of the
sensor is typically an SLD whose beam is collimated, propagated parallel to
the optical axis of the retinal camera, and then directed into the eye. As
illustrated in Figure 10.1, the SLD is positioned as close as possible to the eye
to minimize the number of reflections from optical elements. The beam at
the pupil is small (~1-mm diameter) and slightly displaced (~1 to 2 mm). The
small beam diameter has two primary advantages: (i) it produces a long depth
of focus that keeps the spot size at the retina relatively constant even when
the eye has refractive error, and (ii) it avoids essentially all of the aberrations
in the eye permitting a nearly round spot at the retina (although not a point
source) that is conducive for finding the centroid locations of the Shack–
Hartmann spots. Slight displacement of the beam removes the corneal reflex
that would otherwise enter the wavefront sensor [8]. Another effective solution
is to insert an aperture conjugate to the retina in the wavefront sensor
path (r4 in Fig. 10.1). The aperture blocks the out-of-focus corneal reflection,
while passing the in-focus retinal reflection.

Typical closed-loop operation of the AO system (>10 Hz) necessitates short
SHWS exposures (<50 ms). Retinal motion over this time scale is minimal
and inadequate for destroying speckle noise in the SHWS spot pattern. An
SLD is preferred over a typical laser (e.g., a HeNe or laser diode) as its short
coherence length (<20 μm) causes the reflected light from different depths in
the thick retina (100 to 400 μm) to incoherently interfere. This effectively
mitigates speckle noise and enables more repeatable centroid determination.
The power level of the SLD at the cornea is typically 4 to 20 μW and depends
on the throughput efficiency of the system and the configuration of the SHWS
(such as the CCD’s quantum efficiency, and the number and NA of the
lenslets).

The ideal wavefront sensor operates simultaneously with the imaging
system and uses the same optical path. The Rochester and Indiana flood-
illuminated systems approach this by sensing and imaging at two different
wavelengths. Custom dielectric beamsplitters were designed to reflect and
transmit the corresponding wavelengths, and this permits simultaneous operation
without the loss or mixing of light. Furthermore, the wavefront sensor
and science camera were positioned as close as possible in the system to
minimize noncommon path errors that would lead to differences in the aberrations
seen at the science camera and the wavefront sensor. The wavefront
sensor typically operates in the near-infrared (near-IR), which confers several
advantages over visible wavelengths (0.4 to 0.7 μm). These include the
following:

• Near-IR is less damaging to retinal tissue, which relaxes the retinal
  safety limits and permits more light for wavefront sensing.
• Near-IR appears less bright, which is more comfortable and less distracting
  for the subject.
• Near-IR reflects more from the retina [9], which provides more light for
  wavefront sensing or equivalently enables the incident light power to be
  reduced for increased eye safety and subject comfort.

A disadvantage of separate wavelengths for AO and retinal imaging is that
the intrinsic chromatic aberrations of the eye cause a shift in focus between
the two wavelengths (Fig. 10.2). The magnitude of this shift can be significant.
Effective compensation is typically realized by axially translating a lens in
the imaging arm of the system (L9 and L10 in Fig. 10.1) or the science camera
itself. Fortunately, the higher order aberrations of the eye are largely insensitive
to wavelength making higher order chromatic compensation largely
unwarranted [10–12]. An additional complication of employing separate
wavelengths is that the retinal reflection is wavelength dependent with near-
IR penetrating deeper into the tissue, the extent of which varies between
subjects. Although chromatic aberration is reasonably stable within a given
eye [13, 14], a unique adjustment for the focus must be made for each individual
after AO correction.

The operational specifics of the wavefront sensing system are described in
Chapter 3.