Alfredo Dubra's PhD thesis - Imperial College London

Chapter 7
Summary and discussion
The design of a double shearing interferometer for evaluating the dynamics of the
front surface of the pre-corneal tear film has been described. The instrument was
built and tested on 20 subjects, and the data was analyzed with purpose-developed
software. In order to achieve the topography integration from finite differences, we
generalized an existing reconstruction method based on the DFT, which allows the
processing of data volumes that can not be achieved with current computing powers
and conventional pseudoinverse reconstruction techniques.
The data processing software was tested in a set of recorded interferograms that
illustrate the different features found on the tear topography, to become familiar with
the limits of both the software algorithms and the theory on which these algorithms
are based.
We found that observing the back-reflection of the tear surface with normal coherent
illumination might be a qualitative tool for studying the tear topography evolution,
without the need for data processing and with relatively high resolution. The only
problem that might prevent this technique from being used clinically is the small
tolerance to eye misalignment.
A modified version of Prydal’s experiment was performed to estimate the tear film
thickness [83], with the purpose of estimating the amplitude of back-reflections from
surfaces within the eye other than the front surface of the tear. It was found, based
on fringe contrast estimation that the intensity of these reflections are around 150
times lower than those of the front surface of the tear, thus introducing very little
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