Low_resolution_Thesis_CDD_221009_public - Visual Optics and ...

INTRODUCTION
changes in reflectivity and absorption, among other effects (Manns et al., 2002b,
Fisher and Hahn, 2007).
Pettit and Ediger reported a refractive index of the cornea, at 193 nm, of 1.52. For
visible light, Barbero (Barbero, 2006) proposed a multilayer model of the human
cornea (and tear) with refractive index between 1.40 and 1.33.
1.6.6. Theoretical ablation profiles
Standard algorithms for corneal refractive surgery are based on the Munnerlyn
formula (Munnerlyn et al., 1988). The corneal tissue to be removed is a lenticule with
an anterior radius of curvature equal to the pre-operative corneal radius and the
posterior radius of curvature equal to the post-operative corneal radius (easily related
to the attempted correction).
The Munnerlyn equation can be expressed as:
(1.8)
Where R 1 is the initial radius of curvature, S is the optical power correction and
n= 1.377 is the average index of refraction of the cornea (with visible light) (see (Cano
et al., 2004) for details).
Sometimes the Munnerlyn equation is expressed by its parabolic approximation,
which is obtained by truncating the Taylor expansion (Jiménez et al., 2003):
(1.9)
In this expression, the refractive index of the cornea has already been replaced by its
numerical value, 1.377 (Schwiegerling et al., 2001, Munnerlyn et al., 1988). The
parabolic approximation of the Munnerlyn formula (Eq. 1.9) states that the maximum
depth of the ablation (in microns) per diopter of refractive change is equal to the
square of the optical ablation zone (in millimeters), divided by three.
Figure 1.15 shows ablation profiles based on the Munnerlyn’s equation (Eq. 1.8,
black solid line) and its parabolic approximation (Eq. 1.9, red solid line of Fig. 1.15).
Both profiles are similar in the central part of the profile, but different in the periphery.
The optical effects of the different asphericity associated with both profiles will be
described in Section 1.7.2.
1.7. CORNEAL ABLATION AND OPTICAL ABERRATIONS
1.7.1. Optical aberrations and visual performance after refractive surgery
Although the degree of accuracy in the refractive correction achieved with PRK and
LASIK soon increased with the use of nomograms, some frequent complains include
night vision problems: decreased vision, glare, halos or ghost images in mesopic and
scotopic vision. The low contrast VA and the CS was found to decrease (Verdon et al.,
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