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Bifocal Lenses 107

Fig. 10.6: The solid bifocal in which the reading addition is the difference between
the RP curve and the DP curve. (A) segment on the concave surface. (B) segment
on the convex surface

The reading addition of a solid bifocal is obtained by raising a second
curve on one surface of the lens to form the segment. The segment can be
worked on either side of the lens. The surface on the segment side of the
lens that forms the part of the distance power is called the DP surface and
the power of this surface is referred to as the DP curve. The raised curve on
the segment side that forms the near power is called the segment surface
and its power is known as RP curve. The reading addition is the difference
between the RP curve and the DP curve. Thus, if the segment is on the
convex surface of the lens and the DP curve is + 6.00D and the RP curve is
+ 8.00D, the reading addition would be + 2.00D. Alternatively, if the
segment is situated on the concave side of the lens and the DP curve is
– 6.00D, then to produce a reading addition of + 2.00D, the RP curve would
have to be – 4.00D.

CEMENTED BIFOCAL
A segment is glued onto the surface of a single vision lens. Theoretically,
almost any prescription can be obtained and it is also possible to get almost
any shape of segment, in any position on the main lens. Traditionally they
are made from glasses but they can be made from plastic materials too.
Until about 1960, the adhesive in common use was Canada balsam – a
resin exuded by the balsam fir tree. Canada balsam was easy to apply and
reapply, but its adhesive is deteriorated after being subjected to mechanical,
thermal or chemical shock. More recently, ultraviolet cured epoxy resins
have replaced Canada balsam and provide excellent long-term adhesion
of glass components.