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Ophthalmic Lens Material and Design 19

the laboratory calculates the variables needed to grind the back surface
and thus creates the finished lens with the given prescription. The
manufactured front surface is defined as base curve.
In the earliest days, when the lenses were very simple, they were only
ground as sphere. But with the advancement in the lens design, toric
cylinder grinding was introduced. The type of lens produced was called
cylinder lens, the two principal meridians of curvature were ground on
the front side of the lens. The flatter of these two principle meridians of
curvature was referred to as the base curvature of the toric surface
(Figs 2.5 and 2.6).
Sometimes in early 1970s, the industry saw a dramatic change in the
frame style. Consequently, bulgy front surface of the plus cylinder was
found unsuitable. The hand edgers were pushing the vee bevel back and
forth to achieve a good fit. The introduction of more automated system of

Fig. 2.5: Plus cylinder

Fig. 2.6: Minus cylinder