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Bifocal Lenses 121

The prismatic effect produced by the segment at this point is:
3 x 0.4 = 1.2Δ BD.

Therefore, the resultant prism at this reading point in RE is:
1.2Δ BU.
Similarly, in LE, the resultant prism is 2.0Δ BU, makes a difference of
0.8Δ BU in LE, which may produce discomfort. To overcome this we have
to move the centre of the reading segment in the right eye up, to reduce the
base down effect and in the left eye, we have to move the centre of the
reading segment down to increase the base down effect. The difference to
be eliminated is 0.8Δ, i.e., 0.4Δ in each eye. The amount, therefore, by which
the position of the centres of the segments is to be moved is given by:

A x 3 = 0.4Δ
Therefore, A = 0.4 / 3 = 0.133 cms
Or, A = 1.33 mm
Hence, RE segment will be 17.34 mm diameter and LE segment will be
22.66 mm. A 17 mm segment in right eye and a 23 mm segment in left eye
will practically balance the vertical prism at the NVP. Bigger segment will
be placed in front of more hyperopic eye or least myopic eye.
In case of shaped segment bifocal, prism control by unequal segment
sizes are not useful due to their construction as the optical centre of the
segment is just around the NVP at which we consider the prismatic effect.

BI-PRISM BIFOCALS
A bi-prism is achieved by surfacing the concave rear surface with the base
up prism of equal value to the imbalance between the right and left
prescriptions over the entire lens, i.e., distance and reading. Next base down
prism with the same amount is worked on the distance curve of the front
surface, i.e. on the top part of the lens only. This leaves a lens with zero
prisms at distance and base up prism at near.
The result of this entire process creates a horizontal dividing line
between the two portions of the lens. For this reason, bi-prism technique is
applied to flat top D segments and E-style segments as the bi-prism dividing
line can be made to coincide with exactly the top line of the segment. It is
usually recommended that the minimum prism worked is 2Δ in order to
obtain a clear dividing line between the two zones. Slab off can only provide
a variation in vertical prism and special skill is needed to achieve a neat,
straight slab line coinciding with the segment line. Some higher index resins
are too soft to produce a clean line, the reason why polycarbonate cannot
be slabbed.