Alfredo Dubra's PhD thesis - Imperial College London

A. Preliminary study of feasibility of interferometric wavefront sensing in the
eye
the wedge in opposite orientations (180 degrees rotation), producing in one a large
shear comparable to the pupil diameter, and in the other an almost negligible shear.
This could be used to get an idea of the spatial coherence of the light emerging from
the eye.
Figure A.5 shows the interferograms and SH patterns that resulted from using 3.5, 1.0
and 0.35 µW with camera exposures 0.2, 0.7 and 4.0 s respectively on each row. The
first and second columns display the lateral shearing interferograms with the wedge in
the orientations that produces large and small shear respectively. The third column
shows the associated SH spot pattern. Apart from the pupil size change in response
to the intensity of the light entering the eye, it can be clearly noticed that the speckle
both in the interferograms and the SH spots is reduced with increasing exposures. It
can also be seen that for the interferograms corresponding to a small lateral shear,
vertical fringes with very poor contrast seem to appear for the 4 second exposure but
these fringes are not of the spatial frequency produced by the wedge. We also looked
at the spectra of the interferograms looking for a peak at the frequency of the fringes
that the wedge would produce, and we found that there is indeed a peak, although
based on the experience gained from processing the tear film data for the previous
chapter, we believe no reliable quantitative data can be extracted.
A.2.2
Data acquired with 780 nm source
The same subject was tested for different exposures and this time with equal optical
power of 4 µW. The rows in figure A.6 show the interferograms and SH patterns
that resulted from the exposures of 0.5, 1.0 and 3.0 s respectively. Notice that the
optical magnification of the system is different from the previous experiment, hence
the increase in number of lenslets over the pupil in the SH spot patterns.
Again, as in the previous experiment, reduction of speckle can be noticed for longer
exposures. Fringes with poor contrast (≈ 0.1) can be observed for the interferograms
with small shear and no fringes at all can be noticed for the larger shearing interferograms.
If one could vary the shear in a continuous way like in the interferometer
proposed by Lohmann [97] and the spatial coherence characteristics of the source were
known, then the fringe contrast could be used to study the spatial coherence of the
retinal reflection. When looking at the amplitude of the spectra at the spatial fre-
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