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Prisms 45

RE 3Δ UP LE ZERO

R COMBINED WITH L
0.5Δ UP 2.5Δ DOWN
1Δ UP 2Δ DOWN
1.5Δ UP 1.5Δ DOWN
2Δ UP 1Δ DOWN
2.5Δ UP 0.5Δ DOWN
3Δ UP ZERO

In practice usually splitting is done evenly.

TYPES OF PRISM
Prisms are used to change the path of the propagation of light. The following
types of prisms may be used in practice:

Prism by Decentration
A larger uncut lens can be used to move the lens so that the optical center
is no longer in front of the pupil and hence create a prismatic effect at the
centre point. In case of anisometropia, the more powerful lens will need
less decentration to create the same prismatic effect. Today a significant
proportion of single vision lenses are aspherics where it is very difficult to
decenter the lens to create prism at the center point. With the spherical
curves, the effect away from the centre is a simple continuous curve, in
case of an aspheric lens, the surface geometry is much more complex and
the design has been calculated to work around the center of rotation of the
eye. If an aspheric lens is decentred then the vision would be compromised,
especially in the periphery opposite to the direction of decent ration. On
an aspheric lens, using prolate ellipsoid surface, the actual surface power

Fig. 5.6: Decentration IN

GC = Geometrical center
OC = Optical center