Page 108 - The Indian Optician Digital Edition September-October 2022

meets 2Δ base up. The 2Δ iso-V-prism line must
pass through point J′ lying 5.77mm up along 120
from O and so the line JJ′ must represent the
iso-V-prism line along which the vertical prism is
2Δ base up.

The 0Δ iso-V-prism line must lie parallel with
the line JJ′ passing through the optical centre of
the lens, O.

Points K and K′ lie equidistant from O and the
2Δ base down iso-V-prism line passes through
these points.
The perpendicular distance of point J from
the 0Δ line (JH in Figure 7) can be found as
follows.
tan β = OJ′ / OJ = 5.77 / 20 = 0.2885
FIGURE 5 - LOCATION OF THE COORDINATES (X, Y) FOR
THE ISO-V-PRISM LINES FOR +2.00 DC X 30 from which angle β = 16.1°.
It can be seen that angle JOH = angle β, and that
c = P / F = 2 / 5.00 = 0.4cm or 4mm.
90
90
90
JH = OJ sin β.
When the cylinder axis of a sphero-cylindrical
lens is oblique, it is found that the iso-V-prism JH is the half-width of the iso-V-prism band.
zone is still a band, but it is no longer parallel JH = 20 sin 16.1 = 5.55mm
with the cylinder axis.
The inclination of the band to the horizontal,
Consider the 2Δ iso-V-prism zone for the lens angle α, is found by subtracting angle β from the

-2.00 / -2.00 x 30 (Figure 7). cylinder axis direction, 30,
Along the 30 meridian where the power is so, angle α = 30 - 16.1 = 13.9°
-2.00 D, the value of P30 which gives rise to 2Δ
base up is
given by P = 2 / sin 30,
30
from which, P = 4Δ base 30.
30
Now using c = P / F the eye can roam 4
30
30
30
/ 2 = 2cm or 20mm upwards along 30 before it
meets 2Δ of base up prism. The iso-V-prism line
must pass through point J lying 20mm up along
30 from O.
Along the 120 meridian, where the power is
-4.00, the value of P which gives rise to 2Δ base
120
up is given by
FIGURE 6 -
P = 2 / sin 120, GRAPHICAL
120
CONSTRUCTION
TO FIND ISO-V-
from which, P = 2.31Δ base 120. PRISM ZONE FOR
120
AN OBLIQUE
Since c = P / F , the eye can roam 2.31 / PLANO-CYLINDER
120
120
120
4 = 0.577cm or 5.77mm up along 120 before it
104 | THE INDIAN OPTICIAN | SEPT-OCT 2022 LENS TALK