Page 112 - The Indian Optician Digital Edition September-October 2022

It can be seen that angle JOH = angle β,
and that
JH = OJ sin β.
JH is the half-width of the iso-V-prism
band.

JH = 7.7 sin 46.1 = 5.55mm
The inclination of the band to the
horizontal, angle α, is found by
subtracting angle β from the cylinder axis
direction, 60,

so, angle α = 60 - 46.1 = 13.9°.
Thus, the 2Δ iso-V-prism zone is a band
11.1mm wide lying along the 13.9° meridian.
To confirm this result as in the previous
example, the (x, y) coordinates for point J
are given by

x = OJ cos 60 = 7.7 cos 60 = 3.85mm

and y = OJ sin 60 = 6.67mm.
Using the sign convention for prismatic
effects, y = -6.67mm.
The vertical prism at J is given by

P = y S + C cos θ (x sin θ + y cos θ),
V
= -0.667x3 + 2 cos 60(0.385 sin 60 - 0.667
cos 60)
= -2Δ, i.e., 2Δ base down.

As with the previous example, this
result could have been found by means of
a carefully drawn graphical construction.

Points K and K′ have been marked off
equidistant from the origin to represent
the 2Δ iso-V-prism line along which the
prism is 2Δ base up, a line parallel to and
equidistant from these lines and passing
through the origin, O, represents the 0Δ
iso-V-prism line along which there is no
vertical prism.

REFERENCES
1. Jalie M. (2021), Principles of
Ophthalmic Lenses, 6 Ed. ABDO,
th
Godmersham, Kent.

108 | THE INDIAN OPTICIAN | SEPT-OCT 2022 LENS TALK