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Fang, Lihua PhD; Wang, Yan MD; He, Xingdao PhD

Journal of Cataract & Refractive Surgery: May 2014 - Volume 40 - Issue 5 - p 850-851
doi: 10.1016/j.jcrs.2014.02.024
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In corneal refractive surgery, laser systems are designed to illuminate the corneal surface with a laser beam that propagates parallel through the optical axis of the human eye. Basically, photoablation of corneal tissue is a threshold process and the ablation threshold is the minimum radiant exposure (namely, energy per illuminated area). Above this threshold, the ablation depth per pulse increases in a logarithmic fashion with the radiant exposure. However, the radiant exposure of the laser beam depends on the changes in the effective illumination area and an increase in reflection losses, and this leads to varied effective ablation depth per pulse with the incidence angle of the laser beam on the cornea. Furthermore, previous studies have shown that the effect of oblique incidence across the cornea must be taken into consideration for ablation profile calculations.1 Therefore, an adjustment factor may be used in ablation algorithms to offset the effect of oblique incidence across the cornea. For example, the adjustment factors were used for different conditions in the analysis of experimental data in our paper.

The methodology used to deduce adjustment factors was previously published by Jiménez et al.2 Additionally, adjustment factor K can be expressed as follows:

Here, the non-normal incidence and reflection losses are taken into account. If they are combined with other aspects such as the Gaussian shape of the laser beam3 for correcting ablation algorithms, these factors can be used to decrease the differences between postoperative expected and real corneal shape, resulting in better surgery outcomes. In addition, our results have demonstrated that the methodology of proposing adjustment factors for correcting ablation algorithms is very useful.4

References

1. Cano D, Barbero S, Marcos S. Comparison of real and computer-simulated outcomes of LASIK refractive surgery. J Opt Soc Am A Opt Image Sci Vis. 2004;21:926-936.
2. Jiménez JR, Anera RG, Jiménez del Barco L, Hita E. Effect on laser-ablation algorithms of reflection losses and nonnormal incidence on the anterior cornea. Appl Phys Lett. 81, 2002, p. 1521-1523, Available at: http://hera.ugr.es/doi/15014654.pdf. Accessed February 19, 2014.
3. Jiménez JR, Anera RG, Jiménez del Barco L, Hita E, Pérez-Ocón F. Correction factor for ablation algorithms used in corneal refractive surgery with gaussian-profile beams. Opt Express. 13, 2005, p. 336-343, Available at: http://www.opticsinfobase.org/oe/viewmedia.cfm?uri=oe-13-1-336&seq=0. Accessed February 19, 2014.
4. Fang L, He X, Chen F. Theoretical analysis of wavefront aberration from treatment decentration with oblique incidence after conventional laser refractive surgery. Opt Express. 18, 2010, p. 22418-224311, Available at: http://www.opticsinfobase.org/oe/viewmedia.cfm?uri=oe-18-21-22418&seq=0. Accessed February 19, 2014.
© 2014 by Lippincott Williams & Wilkins, Inc.