Page 142 - PowerPoint 演示文稿

132 Ophthalmic Lenses

They provide larger field of view for the reading and they are readily
available at low cost. But they are inconvenient to use as they correct only
one particular distance at a time.
Bifocal, trifocal and progressive lens designs combine areas of correction
for both distance and near vision in a single lens and link them in different
ways:
In a bifocal lens, a distance vision sphere is placed above a near vision
sphere and is linked by a single “step” creating a visible segment line. They
offer convenience of distance and near in the same spectacle and are
available at lower cost. But they are unappealing cosmetically as there is a
visible demarcation between zones. Intermediate is blurred and the subject
have prismatic image jump (Fig. 11.6).

Fig. 11.6: Design principle of bifocal lens
In a trifocal lens, a third sphere is added between the distance and near
vision sphere to produce an intermediate power. This gives rise to two
segment lines on the lens surface (Fig. 11.7).

Fig. 11.7: Trifocal lens design principle