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150 Ophthalmic Lenses

produce a thinner lens, the lens surfaces generally use an “equithin”
technique which consists of incorporating a vertical prism in order to reduce
the thickness and weight of the lens in the back surface specification of the
lens. Since the same amount of vertical prism is incorporated into both
right and left lenses, there is relatively no prismatic effect for the wearer.
This prism is often referred to as Yoked prism, as no net binocular prismatic
effect is produced.
So prism thinning is the process of grinding prism into a progressive
lens blank to reduce the thickness difference between the upper and the
lower edges. Prism thinning typically involves grinding base down prism
into the progressive lens. In addition to balancing the thickness difference
between the top and the bottom of the lens blank, prism thinning also
reduces the centre thickness of the lens with plus and/or higher add powers.
The overall reduction in thickness also makes the lens lighter in weight.
Prism thinning is accomplished during the generating process by
literally tilting the front surface of the lens on the chuck of conventional
generator, using a base up positioned prism ring. When the back surface is
ground normally, the surfaced lens is left with a prismatic effect at the
centre. Newer three axis generators produce this prismatic effect without
the use of prism rings grinding the back curve with a tilt. The end result in
either case is the reduction of unwanted thickness.

Fig. 11.28: Base down prism is ground into the lens blank using prism-ring (with
its base positioned up) with conventional generators