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48 Ophthalmic Lenses

Therapeutic Use of Prism
The commonest therapeutic use of prisms in the orthoptics is in building
up the fusional reserve of patients with convergence insufficiency. The base
out prism is used during the patients exercise period. They are not worn
constantly. Another therapeutic use of prism is to relieve diplopia in certain
cases of squint. Prisms used in treatment include clip-on spectacle prisms
for trial wear. An improvement on these is fresnel prism. Permanent
incorporation of prism can be done in patients spectacles.

FRESNEL PRISM

In case of high prisms, the thick bases become cosmetically unappealing.
An improvement on these is Fresnel prisms. A Fresnel prism consists of a
plastic sheet of parallel tiny prisms of identical refracting angle. The overall
prismatic effect is the same as that of a single large prism. Fresnel prisms
are moulded from PVC (Polyvinylchloride) to form a flexible sheet of prism.
The sheet can be cut to the shape of the spectacle lens and fixed with the
smooth side attached to the rear surface of the lens.
Functionally, Fresnel prisms reduce visual acuity and contrast
sensitivity, particularly for prisms greater than 10Δ. The reductions in
functions are mainly due to chromatic aberration, and are more pronounced
for Fresnel prisms than for conventional prisms.

Fig. 5.8

COMPOUNDING AND RESOLVING PRISMS
At times prescription calls for a vertical prism in conjunction with a
horizontal prism, for example:

LE 3 Δ BD with 4Δ BO
The combined effect of above example could be produced by a single
prism of appropriate power with its base in some oblique setting. To find
this resultant prism, draw an accurate diagram. Remember standard
notations, which say that the axis starts at zero on the right hand side of
each eye and goes around anti-clockwise from the observer’s point of view.