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Progressive Addition Lenses 131

Distance
A designated zones located in the upper portion of the lens, which provides
the necessary distance correction.

Near
A designated zone in the lower portion of the lens, which provides the
necessary near addition or near power.

Intermediate
A corridor in the central portion of the lens connects these two zones, which
increases progressively in plus power from the distance to near. This zone
is also known as “progressive zone”.
These three zones of vision blend together seamlessly, providing the
wearer with a continuous depth of field from near to far.

Fig. 11.4: Typical progressive lens

BASIC DESIGN DIFFERENCE BETWEEN PROGRESSIVE, SINGLE
VISION, BIFOCAL AND TRIFOCAL LENSES

A single vision reading lens consists of a single sphere of appropriate radius
providing correction for near vision only. Distance vision through the lens
is blurred and there is no specific correction for the intermediate vision.
(Fig. 11.5).

Fig. 11.5: Basic design principle of single vision lens