Alfredo Dubra's PhD thesis - Imperial College London

7. Summary and discussion
The values found for RMS variability are consistent with the reported variability in
full wavefront sensing in the eye measurements, with paralyzed accommodation.
It was noticed that the front surface of the tear in front of contact lenses is very rough
and although a quantitative analysis of the topography was not possible when the
typical roughness was present, we believe the subject should be studied further given
the high number of contact lens users.
From the data it can also be infered that the characteristic times associated with
change in the tear topography are of the order of seconds, so if one was thinking of
using an AO system in the eye just to correct the tear fluctuations, a bandwidth of
the order of 1 Hz or faster should be considered.
We put to the test the use of the amplitude of the second moment of the intensity
of a lateral shearing interferogram with tilt fringes as a tool to study tear break up
times. We first suggested a normalization of this moment to make it more robust
and independent of certain experimental parameters, and then showed, with a few
experimental examples, that this number is no good on its own for the estimation of
the break up time. It was also found that there is very little correlation of the second
moment with the actual optical quality of the tear, either in terms of wavefront error
RMS or Strehl ratio.
Even though the data processing could not be completely automated and despite
the inherent problems of using back-reflections from a moving eye, we believe that
interferometry (lateral shearing, radial shearing or else) could be useful as a tool
to study the tear film of subjects pre- and post-refractive surgery, and with tear
conditions, but only if the issue of eye movement is addressed in far more depth than
it was here.
After the tear topography experiments, the experimental setup was modified to be
used as a full wavefront sensor in the eye and a similar one was built for a different
wavelength. Each instrument recorded simultaneous lateral shearing interferograms
and SH spot patterns, for a qualitative study on the feasibility of using conventional
interferometry for wavefront sensing in the eye. The experiments performed suggest
that conventional interferometry is not as straightforward to use as other types of
wavefront sensing due to speckle. When recording interferograms with short exposure
times, the interferograms will be severely affected by speckle, while if one tries to
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