Low_resolution_Thesis_CDD_221009_public - Visual Optics and ...

METHODS
The measurement method that was followed for most measurements (confocal
imaging) is slower than standard or interference imaging because a sequence of CCD
frames must be acquired to build each single confocal image. The measurement was
fully automatized, and it took more than 4 hours to measure all the points for each
surface. As dilation of the sample during the measurement is a potential cause of error
(see Fig. 2. 6), the laboratory temperature was monitored and controlled, and the
samples were stored near the measurement instrument.
Fig. 2. 6. Dilation problems in a profilometry measurement. The points marked in red
correspond to a non-ablated flat region and therefore all of them should be on the
same plane. However, some of these points in the left side of the ablation profile are
10 microns bellow their expected position, due temperature changes (and dilation)
occurred during the measurement. The whole ablated pattern is therefore affected by
dilation.
In some of the measurements, we observed important deviations from rotational
symmetry. The asymmetries appeared both on flat and spherical equivalent surfaces
with some of the lasers and the location of the irregularity changed across repeated
ablations. To discard dilation and other effects during the measurement, validation
tests (involving repeated profilometric measurements of the same samples at different
orientations) proved that the irregularities were a consequence of the ablation process
and not of the measurement. Fig. 2. 7 shows an example, in which measurements of
the same sample (an ablation profile) upside down provided similar results. Changing
the orientation also entails changing the measurement axis. This experiment therefore
also represents a cross-validation for the surface analysis algorithms (Section 2.2).
Fig. 2. 7. Two different measurements of the same sample (an ablation profile), at
different orientations, provided similar results demonstrating that the irregularities
were a consequence of the ablation process and not of the measurement.
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