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Bifocal Lenses 115

that bifocal lenses are made of two separate components – a main lens
with a distance prescription and a supplementary segment lens with a
power equal to reading addition.
The optical centre of the main lens, i.e., distance optical centre is referred
to as O and the optical centre of the segment lens as O . The total power of
D
s
the near portion is the sum of the distance portion power and the reading
addition of the lens and the prismatic effect at some point in the near portion
is given by the sum of the prismatic effect of the distance portion and the
prismatic effect of the segment lens.
Consider a case of round shape bifocal lens as depicted in Figure 10.18
the prismatic effect at the NVP, 8 mm below the distance optical centre
and 5 mm below the segment top can be determined as follows:
The prismatic effect at 8 mm below the optical centre in + 3.00D lens
can be found out from the decent ration relationship P = cf, where C is the
decent ration in centimeters and F is the power of the lens.
Hence, P = 0.8 x 3.00
= 2.4Δ BU
The segment would exert base down prism at the NVP which can be
deduced as follows:
Suppose the segment diameter is 38 mm. The distance from the dividing
line to the optical centre of the segment is 38/2 or 19 mm. Since, NVP lies 5
mm below the segment top, it must lie 14 mm above the segment centre.
Hence, the prism from segment is:

P = 1.4 × 2.00
= 2.8 Δ BD.

Fig. 10.18: Prismatic effects at the near visual point (NVP) of a bifocal lens. O is
D
the optical centre of the distance portion. O is the optical centre of the segment
s
lens