Low_resolution_Thesis_CDD_221009_public - Visual Optics and ...

CHAPTER 1
1.10.3. Multifocal contact lenses
There have been very few attempts to explore the on-eye behavior of contact lenses
designed to aid the presbyopic patient. Wavefront aberration measurements should be
useful for such studies in the cases of varifocal lenses with a smooth increase or
decrease in power from the centre to the edge of the optical zone, i.e. lenses which
effectively induce controlled amounts of spherical aberration. As in single vision
lenses, wavefront aberrometry can reveal the way in which lens characteristics interact
with the eye of the individual patient and can lead to a better choice of design for the
particular patient (Charman, 2005).
There are some technical difficulties in the measurement of concentric multifocal
designs in which zonal power alternates rapidly between a distance and a near
correction: The resolution of the aberrometer an its dynamic range is limited, and the
Zernike polynomials are poor descriptors of wavefront changing abruptly.
Martin and Roorda (Martin and Roorda, 2003) studied bifocal soft contact lenses
by simply measuring the aberration of the eye alone and then calculating the
theoretical performance of the eye combined with a particular contact lens design.
They demonstrated that bifocal contact lenses do not always provide bifocal vision
(even if the many fitting effects were not considered in this approach).
In this thesis, we will use a Laser Ray Tracing aberrometer, with high dynamic
range and configurable sampling pattern for the in-vitro and in-vivo measurement of
aberrations, to evaluate both optical coupling and fitting effects. Furthermore, only
progressive designs with soft transitions between distance and near zones will be
considered, to avoid the problems associated with Zernike description of surface
discontinuities.
1.10.4. Correcting aberrations with contact lenses
Intriguingly, Smirnov (Smirnov, 1961) also suggested in 1961 that “in principle it is
possible to manufacture a lens compensating the wave aberration of the eye” … and,
noted that as spectacle lenses do not move with the eye as it rotates, …”the lenses
must obviously be contact ones”.
There is continued interest in extending the concept of a contact lens which
corrects spherocylindrical errors and, perhaps, spherical aberration to one in which all
the monochromatic wave aberration, in both lower and higher orders, is corrected.
Correction of aberrations with adaptive optics have proved to increase optical (Hofer
et al., 2001) and visual performance (Marcos et al., 2008).
Hong et al. (Hong et al., 2001) and Lu et al. (Lu et al., 2003) have demonstrated
that rigid lenses can provide that sort of passive aberration compensation. The fitting
of RGP contact lenses can reduce 66% of the severe HOAs found in postsurgical
corneas (Gemoules and Morris, 2007). Other authors have explored the aberration
correction with customized contact lenses. Full correction of aberrations with contact
lenses is probably impossible, due to dynamic changes associated with tears,
accomodation (Gambra et al., 2009), lens rotations and translations (Guirao et al.,
2001) and other factors as chromatic blur. Nonetheless, the concept is attractive and
can undoubtedly improve the optical performance of eyes with abnormally high
aberrations, as those with keratoconic or post-surgical corneas.
Preliminary results with custom contact lenses for the correction of high order
aberrations show a decrease in HOAs by a factor of 3 in normal and keratoconic eyes
(Lopez-Gil et al., 2002, Sabesan et al., 2007). Although these results are suggestive,
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