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Aspheric Lenses 99

Mostly aphakic lenses are made in lenticular design in which central
aperture incorporates the optical correction, and is surrounded by a carrier
or margin of lower power or even afocal. Natural, sharp and foveal vision
is only possible when the wearer views through the aperture. During 1970
aspheric lenses was introduced in which the dividing line between the
aperture and margin was blended to make it invisible, Removal of dividing
line both improved the appearance of the lenses as well as field of vision
by removing the ring scotoma associated with the abrupt change in power
at the edge of the aperture.
With the advent of CNC grinding technology, it became possible to
design and manufacture surfaces of almost any complexities. This led the
introduction of convex surface that is polynomial form.
For example, in the Figure 9.9 the zone ab is having the same optical
properties on aspheric lens that employs an ellipsoidal surface. It could be
free from aberrational astigmatism. Zone bc is seen to be concave in its
tangential section. The surface flexes backwards in this region. Since the
surface is continuous, there is no annular scotoma between the central
ellipsoidal zone and the margin. Zone cd has the same purpose as that of
the margin of the lenticular design, i.e. supporting the central aperture.

There are two important advantages:
Firstly, the aspheric zone has excellent optical properties, and does not
have any aberrational astigmatism of spherical surface.
Secondly, the blending is concave which eliminates the ring scotoma
that exists at the edges of every plus lens, which is so annoying.

Fig. 9.9: Cataract lens design with convex polynomial surface