Low_resolution_Thesis_CDD_221009_public - Visual Optics and ...

INTRODUCTION
1.5.2.3. Alternating vision spectacles
In alternating vision, the lens is divided in different regions each of which have a
different power. For example, in bifocal lenses, there is a bifocal segment added to a
base lens. The base lens provides the distance correction. The bifocal segment (usually
in the lower part of the lens) provides correction for near vision. The bifocal segment
location is carefully selected: the line of sight of the eye for distance vision must be
separated from the bifocal segment, while near vision visual tasks must be performed
through the bifocal segment.
Progressive spectacles are an evolution of the bifocal spectacles. They provide a
“channel” of progressive addition. Different heights on the channel correspond to
different observation distances. Therefore, the position and length of the channel must
match the path followed by the gaze at the lens plane (including binocular
convergence), when looking at near objects at different distances.
All alternating vision designs in spectacle lenses take advantage of the fact that
there is some distance between the eye and the lens, and that it is certainly easy to
orientate the line of sight through different zones of the lens. These possibilities are no
longer available in contact lenses, where the lens is in contact with the eye, and moves
as the eye moves.
1.5.2.4. Alternating vision contact lenses
Theoretically, translating or alternation vision contact lenses (usually bifocal designs)
also provide alternating vision, in a spectacle lens way: the patient looking through a
distance portion for distance vision, the lens moving upwards for close work through
the near portion. A prism ballast is often incorporated to orient the lens correctly and
in addition the lens is often truncated. The lens must move up easily when the eye
looks down for close work, but the lens must not be so loose that the lens rides up
when looking straight ahead causing the near portion to cover the pupil. As the pupil is
usually overlapping the near and distance zones, the retinal image quality is affected
by some out of focus light. Most alternating vision contact lenses are RGP lenses.
Their use is decreasing in favor of soft multifocal contact lenses.
Achieving alternating vision is difficult in contact lenses, and impossible in other
correction alternatives such as intraocular lenses or laser refractive surgery, where
there is no possibility of relative movement between the line of sight and the refractive
correction.
1.5.2.5. Simultaneous vision
Figure 1.12 illustrates the working principle of multifocal corrections by simultaneous
vision. The lens has different focus positions, corresponding to different regions so it
is able to provide focused images of objects at different distances, without an active
focusing mechanism. The concept is explained with a contact lens, but simultaneous
vision is often applied to intraocular lenses and even refractive surgery ablation
pattern designs.
Light from objects at different distances are focused by the different zones of the
lens on the retina, achieving the intended multifocality (Fig. 1.12 (a)). But on the other
hand, when the eye looks at a certain object (Fig. 1.12 (b)), light coming from that
distance passes through the different portions (distance and near) at the same time and
is focused on different planes, in front of and behind the retina. The result (when the
design, the fitting and the coupling with the optics of the eye are correct) is a sharp
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