Low_resolution_Thesis_CDD_221009_public - Visual Optics and ...

CHAPTER 2
surface before and after the ablation. Furthermore, the measurement of the ablation
efficiency effects is obtained after comparing the ablation pattern on flat surfaces from
the ablation pattern on spheres (Chapters 3 and 5). The simulation of the threedimensional
shape of a contact lens fitted on the eye results from the comparison of
the anterior cornea and the anterior surface of the contact lens fitted. These two
surfaces are also compared to obtain the conformity or flexure of the lens to the
underlying cornea. All these applications are described hereinafter.
2.2.2.1. Ablation patterns from flat surfaces
In flat samples ablated with a laser, the measurement of the surface elevation pattern
(or profile) should directly provide a measurement of the ablation pattern. However, in
most cases the raw measurements of flat surfaces are affected by tilt. This happens
when the support of the samples in the profilometry system is not perfectly parallel to
the translation of the probe tip in contact profilometers (as that used in Chapter 3), or
if the support of the samples is not perfectly perpendicular to the optical axis of the
instrument in non-contact profilometers (Chapters 4 and 5). Tilt also appears if the
sample is not perfectly plano-parallel.
To remove this tilt, the non-ablated zone is fitted by a plane surface (a line in the
case of a profile). That plane (or line) is subtracted from the measured surface. The
location of the non-ablated zone is an input parameter, indicated as the ring between
two given radii. The corrected measurement is more symmetrical than the original
measurement (see Fig. 2. 16). Section 2.1.1.2 describes a cross-validation procedure of
the measurement and the processing algorithms based on measuring the sample with
different tilts and orientations (Fig. 2. 7). This cross-validation demonstrates (with the
same sample) that the residual asymmetry obtained is in fact in the original sample,
and is not an artifact introduced by the measurement procedure or the algorithms.
0.08
j j g p
0.06
0.04
0.02
0
-0.02
-0.04
-0.06
-0.08
6
4
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-4 -2 0 2 4 6
j X
j
Fig. 2. 16. Removing tilt from topographies measured on flat ablated surfaces. The
non-ablated zone (green points) of the measured elevation map (blue points) is fitted
to a plane surface. This plane surface is subtracted from the measured surface to give
the corrected measurement (red), which is more symmetrical than the original
measurement (blue). The residual asymmetry is in the original sample, and is not
introduced as an artifact of the measurement procedure, as can be demonstrated by
measuring the sample with different tilts and orientations (Fig. 2. 7 and Section
2.1.1.2)
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