Low_resolution_Thesis_CDD_221009_public - Visual Optics and ...
Low_resolution_Thesis_CDD_221009_public - Visual Optics and ...
Low_resolution_Thesis_CDD_221009_public - Visual Optics and ...
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CHAPTER 3<br />
Ablation efficiency factor<br />
1.1<br />
1<br />
0.9<br />
0.8<br />
Experimental (PMMA)<br />
Experimental (Cornea)<br />
Theoretical (PMMA)<br />
Theoretical (Cornea)<br />
0.7<br />
-3 -2 -1 0 1 2 3<br />
Radial coordinate (mm)<br />
Fig. 3.6. Ablation efficiency factors obtained experimentally (black) for PMMA <strong>and</strong><br />
cornea, compared with theoretical predictions using Jiménez et al’s equations<br />
(Jimenez et al., 2002) for PMMA <strong>and</strong> cornea respectively. Data for PMMA are<br />
represented in thick lines <strong>and</strong> for the cornea in thin lines.<br />
3.4.4. Post - operative asphericities on PMMA<br />
Figure 3.7 shows postoperative asphericities estimated directly from the elevation<br />
maps of PMMA spherical surfaces (red circles), for different spherical corrections (the<br />
values indicate nominal corrections on the cornea, see Fig. 3.1 (b) for the equivalent<br />
corrections on PMMA). We found a systematic increase of asphericity in all the<br />
surfaces tested, the larger the correction the larger the increase. The graph also plots<br />
the predicted asphericities using the experimental ablation efficiency factor found in<br />
this study (orange diamonds) <strong>and</strong> the theoretical efficiency factor (blue triangles)<br />
proposed by Jimenez et al. (Jimenez et al., 2002), calculated with the values<br />
corresponding to PMMA material. The predicted ablation pattern is the result of direct<br />
subtraction of the experimental ablation pattern measured on flat surfaces multiplied<br />
by the efficiency factor considered from a spherical surface of 8 mm-radius. The<br />
asphericities without considering efficiency effects (ablations on flat surfaces<br />
computationally applied to the spherical surfaces) are also shown (green diamonds).<br />
Without efficiency effects, the measured ablations have asphericities close to zero (-<br />
0.03±0.1, mean <strong>and</strong> st<strong>and</strong>ard deviation), <strong>and</strong> no change with correction. We found<br />
experimentally a significant increase of asphericity that can be entirely attributed to<br />
reflection losses <strong>and</strong> non-normal incidence. The mean asphericity of ablated PMMA<br />
spheres was 0.89 ±0.76.<br />
The asphericity predictions using the ablation profile from flat surfaces <strong>and</strong> the<br />
experimental efficiency factor agree well (0.93±0.37) with those obtained directly<br />
from experimental elevation maps on ablated PMMA spheres. For the 12-D sphere<br />
used in the calculation of the experimental efficiency factor, the coincidence is perfect,<br />
as expected. The theoretical efficiency factor predicts a mean increase in asphericity of<br />
0.33±0.18, that is, 37% of the increase found experimentally.<br />
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