Adaptive Optics for Vision Science

Chapter 10.5.3 - Refraction

10.5.3   Refraction

To relieve the demand on the deformable mirror, it is important to correct
for as much of the refractive error of the patient prior to AO correction. The
defocus of the eye can be corrected by placing a trial lens in a suitable pupil
plane or by changing the axial distance of the eye along with the lens that is
closest to the eye, thereby changing the vergence of the beam entering the
eye, without moving the pupil conjugate positions. Astigmatism can be corrected
by placing cylindrical lenses of the appropriate power and orientation
in front of the eye’s pupil or in any other pupil conjugate plane. If the correcting
lens is to be placed in a plane conjugate to the pupil, then the vergence
has to be corrected by the following formula:

 

where m0,N is the magnification between the eye and the plane where the lens
is placed.

The refraction of the eye can be easily determined from the wave aberration.
The formula for converting Zernike coefficients to refraction in diopters
is given below. Start with the three coefficients for the second-order terms,
C3, C4, and C5, respectively. First, compute the axis from the astigmatism
terms,

 

Then compute the two intermediate values A and D,

 

Finally compute the refraction values in diopters.

 

Be sure to convert all distance scales to meters, to get the final values in
diopters. Some care has to be taken when either of the astigmatism terms are
zero, and the axis should be expressed as an angle from 0° to 180°, by convention.
One example of computation code used for determining an appropriate
refraction from the Zernike coefficients follows:

If you are converging on a good correction but are not quite there, then
sometimes an overrefraction is required (and can be obtained by measuring
the refraction through lenses that are already present). Instead of adding
more lenses, which will cause light loss and increase the chance of back reflections,
computing the correct pair of replacement lenses is the better solution.
The computation for an overrefraction follows (summarized from Bennett &
Rabbetts [13]):


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