Adaptive Optics for Vision Science

Chapter 10.3.6 - Raster Scanning

10.3.6   Raster Scanning

Two mirrors are generally used to scan the focused spot in a raster pattern
on the retina. One mirror scans the line, defining the line rate, and the other
scans vertically, to set the frame rate. For the line scanner, an SLO can
use a polygon scanner or a galvanometric scanner. The ratio of the line scan
frequency to the vertical scan frequency defines the number of lines per
frame.

Two important considerations in the optical system are the beam size and
the scan angle at the location of the eye. As described in an earlier section,
the pupil size at any conjugate plane is easily computed by multiplying the
size of one pupil by the product of the magnifications of the telescopes that
lie between the two planes. The opposite occurs with field angle, since, as the
beam size is reduced, the scan angle enlarges. This is an invariant in the
optical system and is governed by the following equations:

 

where ma..z is the product of the magnifications of the telescopes between the
conjugate planes, a and z.

These considerations bear on the selection of the scanner. For example, if
the desired field scan angle is 1° and the pupil size is 7 mm, then two examples
of acceptable configurations are:

Option 1: 7-mm beam, 1° scan angle (0.5° mechanical scan angle)
Option 2: 1-mm beam, 7° scan angle (3.5° mechanical scan angle)

Option 1 requires a 1 : 1 magnification between the scanner and the eye,
and option 2 requires a magnification of 7×. Option 2 is less desirable since
the higher scan angle will introduce more aberrations in the final telescope(s)
of the optical system due to off-axis aberrations.

Given the beam and angle requirements, galvanometric scanners are most
suitable for small field/large pupil scanning. Scanners up to 16-kHz frequency
are commercially available and can scan a 2.5-mm beam over a 10° optical
angle. For lower frequencies, many beam sizes and scan angles are available.
However, high-frequency scanners provide the highest line counts at high
frame rates. The 16-kHz frequency is especially convenient because it is
inaudible for most people. An advantage of scanning mirrors over polygons
is that their scan angles can be adjusted, which in turn changes the field angle,
or magnification, of the retinal image. Reducing the scan amplitude can, in
effect, “zoom in” on the image.

High line rates at small scan angles will often require a resonant-type
scanner, which achieves high scan frequencies via a sinusoidal scanning
motion of the mirrors. The sinusoidal scanning motion is not desirable since
it introduces distortions into the image (assuming a fixed-frequency pixel
clock is used), but the distortions are stable and well-characterized so they
can be removed with postprocessing.

 

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