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96 Ophthalmic Lenses

Fig. 9.5: Aspheric versus spherical surface

Proper base curve selection as recommended by the manufacturer is
critical for an aspheric surface. Even the smaller increments of surface power
affect the base curve selection quite considerably. Consequently, aspheric
lenses have more base curves options. Substituting aspheric base curves
can often have a negative impact on the off-axis performance of the lens.
Flatter base curves produce thinner lenses. It is interesting to note that the
actual geometry of an aspheric surface helps reduce lens thickness also.
This is a consequence of the fact that the sag of an aspheric surface differs
from the sag of a spherical surface. Consider the comparison made in
Figure 9.5. At a given diameter; the aspheric surface has shallower sag
than the spherical surface.

Fig. 9.6: Sagittal depth, conventional versus aspheric lens design

MEASURING AN ASPHERIC SURFACE
Since the curvature of an aspheric surface varies from the centre to the
periphery, normal measuring instruments cannot measure the front curve
value or vertex curvature of an aspheric lens accurately. The manufacturers
occasionally publish the design of their lenses, which enables to determine