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120 Ophthalmic Lenses

portion. Such a design is called a Prism controlled bifocals. This kind of
bifocal design is constructed to incorporate the prism in the segment and is
totally independent of any prism included in the main lens. There are several
methods by which prism may be incorporated in the near segment only.
They are:

SPLIT BIFOCAL
Two half lenses – one for distance and another for reading are made from
two separate lenses for which the power and centration may be chosen at
will. Reading portion carries the corrective vertical prism. Full prism control
is possible in Split bifocals.

CEMENTED BIFOCALS
A separate prism segment is attached to the main lens using permanent
epoxy resin adhesives.

UNEQUAL SEGMENT SIZES
Although, visually a little strange for beholders of those wearing a
combination of unequal segment sizes in two eyes, vertical prismatic effect
due to anisometropia can be controlled by unequal sizes of the round
segment in both the eyes. The heights of top of the segment must be at the
same, as when the eyes are depressed from the line of distance vision they
must meet the upper margin of the reading portion simultaneously. From
Prentice rule, the following expression can be deduced for round segment
down curve bifocal:
Difference in segment radii (in cms) x near addition = Amount of relative
vertical prism overcome.

Example – RE + 3.00 Dsph
LE + 4.00 Dsph
BE Near Add + 3.00 Dsph
Round bifocal 20 mm segment size.
Upper margin 2 mm below optical centre.
Reading point 6 mm below the top edge of segment.
The reading point in the above case is 8 mm below the distance centre
and 4 mm above the centre of the segment.
In the RE, prismatic effect produced by distance correction at this point
will be:
3 x 0.8 = 2.4Δ BU.