Page 86 - The Indian Digital Edition September-October 2020

Segment Prism at NVP in Δ Jump in Δ
diameter or size
22 0.50 base DOWN 1.1 base DOWN
24 0.60 base DOWN 1.2
25 0.65 base DOWN 1.25
25 x 17 0.15 base UP 0.45
26 x 9 0.15 base UP 0.45
28 0.80 base DOWN 1.4 FIGURE 2
28 x 19 0.10 base UP 0.5 CEMENT
BIFOCAL
30 0.90 base DOWN 1.5
30 x 16 0.50 base UP 0.1
30 x 21 0 0.6
30 x 23 0.20 base DOWN 0.8
34 x 22 0.10 base UP 0.5
35 x 22 0.15 base UP 0.45
38 1.30 base DOWN 1.9
40 x 20 0.60 base UP 0
FIGURE 3
45 1.65 base DOWN 2.25 SOLID PRISM
CONTROLLED
E-style 0.60 base UP 0 BIFOCAL
permits any amount of prism to be is attached to the main lens using Some prism-controlled bifocal
incorporated in the segment and Canada Balsam or a more permanent designs are produced with a given
with its base set in any direction. One epoxy resin adhesive. Note with this amount of prism incorporated in the
purpose of this prism is to neutralise design that the prism base represents segment by the blank manufacturer.
all prism at the near visual point which the thickest point on the dividing line. For example, the only solid prism
arises from the distance prescription Solid visible (or semi-visible) controlled bifocal currently being
and the reading addition. When all bifocals (Figure 3) - where the segment manufactured in the UK has a 30 mm
the prism is neutralised the reading surface is depressed below the level segment diameter and incorporates
portion has its own optical centre of the distance portion surface, and prism in ½ Δ steps from ½ Δ up to 4Δ.
coincident with the near visual point. being depressed may be angled to PARTIALLY PRISM-
Such designs are often referred to as provide any amount of prism and CONTROLLED DESIGNS
centre-controlled bifocals. hence any amount of prism-control. INCLUDE:
BIFOCAL DESIGNS WHICH Note that with this solid prism
PERMIT FULL CENTRE- controlled segment the prism base Solid and fused prism-segment
CONTROL INCLUDE: represents the thinnest point on bifocals whose segment is constructed
the dividing line. It can be imagined with a fixed range of prisms.
Split bifocals (Figure 1) - whose that the segment surface was first Bi-prism bifocals where vertical
distance and reading portions are depressed into the back surface of the prism has been added to or removed
formed from two separate lenses blank and a small prism dropped into from the reading portion by means of
which are cut in half and mounted the depression. the slab-off technique.
together in the same eye of the
frame. (The prism and prescription The method of construction of It has already been seen earlier
possibilities with Split bifocals are these centre-controlled designs that some prism-control may be
almost unlimited, since each eye also permits unusual prescription obtained by judicious selection of an
consists of a pair of single vision lenses requirements to be met such as bifocal appropriate segment diameter. It will
whose powers and centration may be designs where a cylinder is required be seen, shortly, that partial horizontal
chosen at will.) in the distance or reading portion only, prism control may be obtained by
or different cylinder powers or cylinder additional inset of the segment.
Cemented or bonded bifocals axis directions in the distance and
(Figure 2) - where a prism segment reading portions. (To be continued....)

| SEPT-OCT 2020 | 82 LENS TALK