Low_resolution_Thesis_CDD_221009_public - Visual Optics and ...

CHAPTER 2
aberration estimation from the corresponding excel file stored during the
measurement, and saves the results in a new excel file in the corresponding project
folder.
Fig. 2. 27. Detail of the centroid processing. (a) Raw Image corresponding to a
recorded aerial image. (b) Processed image: background subtraction, filtering and
thresholding. Botton row: zoomed images. The centroids are computed as an intensity
mass center (green cross). (From Sergio Barbero’s Thesis (Barbero, 2004)).
2.3.2.6. Pupil images (passive eye- and lens-tracking)
The system, saving pupil images during the measurement and synchronized with the
retinal images allows for a new functionality in LRT measurements: passive eyetracking.
The edges of the pupil can be detected to estimate the exact pupil position at
each frame. Then, the corrected entrance position of the ray can be considered in the
wave aberration retrieval algorithms to retrieve a more accurate wave aberration
measurement.
Fig. 2. 28 shows two examples of corrected entrance positions of a patient with
important eye movements (a) and a patient with normal eye movements (b). The
hardware needed was set-up and the acquisition software developed during this thesis.
First, a preliminary version was tested in LRT1 and then a fully functional and
synchonized version was incorporated in LRT2. Pupil images served to reject retinal
images (subject to eye motion artefacts or occlusion). Further developments of the
pupil image analysis in the LRT2 system by Lourdes Llorente, have allowed automatic
detection of the pupil edge, and therefore to compensate for potential eye movements
in the measurements (Llorente, 2009, Llorente et al., 2007).
The stored pupillary images also helped in the analysis of LRT measurements
with contact lenses, as they allowed to estimate the lens position, relative to the pupil
position, at each frame, as will be described in Chapter 9.
2.3.2.7. System Calibration
In this section the calibration procedure of the different elements of the system, and
the validation of the measurements provided will be described. Additional descriptions
can be found in Lourdes Llorente’s thesis (Llorente, 2009).
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