Page 105 - The Indian Optician Digital Edition September-October 2022
P. 105
point remains within this band, the eye will not
encounter more that 2Δ of vertical prism. Note
that at every point along the upper line the
vertical component of the prismatic effect is 2Δ
base down, whereas along the lower line it is 2Δ
base up.
The iso-V-prism zones for a -4.00 D spherical
lens are illustrated in Figure 2. The iso-V-prism
lines are 2.5mm apart.
In the case of a plano-cylinder, which can only
FIGURE 1 - ISO-PRISM ZONE AND ISO-V-PRISM ZONE
exert prism along its power meridian, at right
angles to the cylinder axis, the iso-V-prism zone
is a band which lies parallel with the cylinder
axis. If the axis lies along 180°, the iso-V-prism
zone will be a horizontal band. With an axis
of 180, the lens only has power along the 90°
meridian and so can only exert prism along the
90° meridian (Figure 3(a)).
If the axis lies along 90°, however, the cylinder
has no power along the 90° meridian and so
cannot exert any prism along 90. The iso-V-prism
band will be of infinite width. (Figure 3(b)).
FIGURE 2 - ISO-V-PRISM ZONES FOR A Figure 3 illustrates the bands for +2.00 DC x
-4.00 D SPHERICAL LENS 180 and +2.00 DC x 90. The width of the band
when the axis lies at 180° is 20mm i.e., each
centre. This is found from Prentice’s rule, boundary lies 10mm from the cylinder axis.
radius of zone (cm) = P / F or 10 P / F mm. When the cylinder axis is oblique, the iso-
V-prism zone will also be oblique, remaining
At any point on this circle the prismatic effect parallel with the cylinder axis. Figure 4 illustrates
is 2Δ, the base direction changing as we move the case for the 2Δ iso-V-prism zone for the
around the circle always lying in the direction cylinder, +2.00 DC x 30.
of the optical centre (Figure 1(a)). Providing the
visual point remains within this zone, the eye will The band lies along the 30° meridian and
not encounter more that 2Δ of prism. its width can be found as follows. We are only
concerned with vertical prismatic effect. The
In the case of an iso-V-prism zone, only power of the +2.00 DC x 30 cylinder lies along
vertical prismatic effect is considered, all the 120° meridian so we must first determine
horizontal prismatic effect being ignored. The how much prism along the 120° meridian gives
iso-V-prism zone takes the form of a horizontal rise to 2Δ of vertical prism.
band the width of which is also found from
Prentice’s rule. Since P = P sin θ, where P = 2 and θ = 120,
R
V
V
Band width of zone = 20 P / F mm P = 2 /sin 120 = 2.309Δ up @120
R
For the 2Δ iso-V-prism zone, the eye can Now the vertical prismatic effect at any point on
roam 10mm from the optical centre before it a plano-cylinder is given by:
meets 2Δ of vertical prism, so the zone is a band P = C cos θ (x sin θ + y cos θ),
20mm wide as illustrated in Figure 1(b) and it V
will be understood that providing the visual a specific sign convention being employed for
LENS TALK THE INDIAN OPTICIAN | SEPT-OCT 2022 | 101
