Low_resolution_Thesis_CDD_221009_public - Visual Optics and ...

INTRODUCTION
1.4.6.3. Internal Aberrations: interaction between total and corneal aberrations
Internal aberrations can be estimated from the subtraction of anterior corneal
aberrations from ocular (total) aberrations, and include the aberrations of the lens and
the posterior cornea (as the humours are not believed to play a significant role in terms
of aberrations). Knowledge of the relative contribution of the cornea and the lens to
the ocular wave aberration is important for both, basic study of the human eye and
clinical applications, as will be shown in Chapters 6 and 7, respectively.
The existing compensation of the positive corneal spherical aberration by the
negative spherical aberration of the lens has been reported since the 70’s (El Hage and
Berny, 1973, Millodot and Sivak, 1979), and has been confirmed since, using different
techniques (Tomlinson et al., 1993, Smith et al., 2001, Artal and Guirao, 1998, Artal et
al., 2001, Barbero et al., 2002a, Kelly et al., 2004). This compensation had also been
reported for astigmatism (Le Grand and El Hage, 1980, Artal et al., 2001, Kelly et al.,
2004) and is quite well known in clinical practice. The development of techniques to
measure the aberrations has allowed the initial studies on astigmatism and spherical
aberration compensation to be expanded to other aberrations. Artal et al. (Artal et al.,
2001) found compensation in both astigmatism Zernike terms, spherical aberration and
coma. Kelly et al. (Kelly et al., 2004) confirmed Artal’s findings for
Horizontal/vertical astigmatism Z 2 2 , horizontal coma Z 1 3 and spherical aberration Z 0 4 .
The increase in optical aberrations (see section 1.4.6.1) with age has been
attributed to the loss of this compensation with age, reported for spherical aberration.
The changes in the crystalline lens must be responsible for this increase in optical
aberrations with age (although the relative contribution of the surfaces and gradient
index is yet to be determined), as the corneal aberrations have been found to remain
fairly constant (Smith et al., 2001).
1.5. CURRENT METHODS OF CORRECTION
It is interesting to note that myopia and presbyopia are the results of failures in the two
fine active mechanisms of tuning the power of the eye: Emmetropization in the case of
myopia and accommodation in the case of presbyopia. Most people are affected by
one or both conditions (myopia and presbyopia), which compromise their visual
performance. Several ways to correct (or at least reduce) the effects of myopia and
presbyopia have been developed over the years, which re-stablish (completelly or in
part) the functionallity of the eye. This thesis aims at exploring further some of the
pending issues related to methods of correction for myopia and presbyopia.
1.5.1. Myopia correction
Negative lenses, reducing the power of the eye, are needed to correct myopia.
Spectacle lenses and contact lenses are the most common solutions worldwide, as they
are external and removable lenses. There are many different designs and materials for
spectacle and contact lenses.
Spectacle lenses are a non-invasive way of correcting myopia. The optical
coupling between the eye and the spectacle lens is straightforward. However, aside
from an important cosmetic impact, spectacle lenses have some optical disadvantages
over contact lenses (magnification and prismatic effects, distortions), that will be
explained in Section 1.8.2.
Some of the issues present in spectacle lens correction are reduced with the use of
contact lenses. However, the back surface of the contact lens is in contact with the
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