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Aspheric Lenses

The term “aspheric” has been widely used in recent years to describe the
surfaces of the various lens designs. The literal meaning of “aspheric” is
simply “not spherical”, which means cylinder surfaces or toroidal surfaces
used for astigmatic correction. Quite often progressive addition lens surface
is also described as “aspheric”. In terms of “lens designing” the term
“aspherical” usually refers to a surface that is rotationally symmetrical,
but at the same time not spherical. A change in curvature is noticed over
the lens surface, rather than constant curvature like a spherical surface.
The change is the same in all direction or meridians of the lens. The
“asphericity” is achieved by flattening the periphery of the front surface in
plus lens and by steepening the periphery of the front surface in minus
lens and/or by flattening the periphery of the back surface to reduce the
edge thickness. Using the “asphericity” the lens designers are able to
produce thinner and flatter lens that also provide peripheral vision
comparable to the best form lens. The off-axis performance of the lens
improves considerably. The lens looks flatter which makes the eyes look
more natural due to reduced magnification or minification. Also flatter
lens fits better ensuring the attractive look of the spectacles, thereby, offering
the subjects a wider variety of frames to choose from. If the aspheric lens
design is coupled with high index material, it will definitely provide thinner,
lighter and flatter lens that looks cosmetically the best.

Fig. 9.1: Comparison between aspheric and spheric surface

WHY ASPHERIC?
Unfortunately several lens aberrations affect the quality of peripheral vision
through a spectacle lens. Oblique astigmatism is the principal lens
aberration that is considered to be the most serious defect in designing the