Alfredo Dubra's PhD thesis - Imperial College London

Chapter 3
Data processing
The methods used to process the recorded data, from the raw interferograms to the
topography slope maps are now presented and discussed. The algorithms described
here might not be optimum either mathematically or computationally, but they keep
the complexity to a minimum and, within reasonable levels, minimize the user input
(automation). The quantitative use of an interferometric technique to study the tear
film topography is already a novel field, and we think that by keeping the data processing
simple, not only will this help in its understanding, but it will also keep artifacts
to a minimum. We have sometimes, therefore, traded performance and accuracy for
simplicity.
Before processing, a visual inspection of all the raw interferograms was made, and
those with vignetting, a blink, or where the eye was too far (or too close) to the
experimental setup were tagged as non-usable. From now on we will refer to all the
other interferograms as usable, and these are the only ones to be processed as described
below.
3.1 Carrier frequency estimation
3.1.1 Theory
The first step of the data processing is the estimation of the carrier frequency ⃗ f c of
the interferograms described by equation 2.3 that we recall here once more
I(⃗r) = I o (⃗r) + I o (⃗r + ⃗s) + 2 √ [
I o (⃗r) I o (⃗r + ⃗s) cos φ(⃗r + ⃗s) − φ(⃗r) + 2πf ⃗ ]
c · ⃗r .
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