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Prisms 47

diplopia. The eyes can cope with maximum 2Δ of vertical differential
prismatic effect for longer hours.
Take for example a prescription in RE – 3.00Dsph and LE – 6.00Dsph,
the vertical prismatic effects that the subject encounters at points 8 mm
below the distance optical centres are RE 2.4Δ base down and LE 4.8Δ base
down. The differential prismatic effect between the eyes is 2.4Δ base down
in the left eye. This prevents comfortable binocular vision and hence some
form of prism compensation may need to be provided for near vision. Such
a compensation for vertical differential prism can be provided in several
ways:
1. Two separate pair of spectacles – one centred for distance vision and
the second pair centred for near vision can be provided.
2. Split design bifocals can be made where separate distance and reading
lenses are cut in half and mounted together in a frame, each component
being separately centred for distance and for near vision.
3. A Plano prism segment can be bonded to the lower half of the lens. For
example – in the above example 2.4Δ base down prism might be bonded
to the right lens to increase the prismatic effect at the right NVP to 4.8Δ
base down which would match that of the left eye.

Different Prism at Distance and Near

There are occasions where different prisms are required at distance and
near. These may be horizontal or vertical and require a different set of
solutions.

Thinning Prism
Because of the decreasing radius of the front curve in the lower portion of
an E-style bifocal or a progressive addition lens, the top of the lens ends up
thicker than the bottom. Removing base up prism (work base down) will
reduce the thickness at the top of the lens. The usual amount of thinning
prism applied is 2/3rd of the near addition power. The benefit of thinning
prism varies depending on the fitting position and the size of the frame. A
progressive addition lens set low in the eye of a shallow frame will not
benefit much from thinning prism but a progressive addition lens set high
in a deeper aviator will definitely benefit because of this.

USES OF PRISM
Diagnostic Use of Prism
Prisms are used to assess the squint and heterophoria. Prism from the trial
lens set and the prism bars are usually used for this purpose. These bars
are composed of adjacent prisms of increasing power.