Page 116 - The Indian Optician Digital Edition July-August 2024

(5) Compound the decentrations OP and OQ
into a single resultant decentration, OR,
and resolve OR into vertical and horizontal
components, RV and RH. OV represents the
required vertical decentration, c , and OH the
V
required horizontal decentration, c .
H
EXAMPLE (III):

Find the vertical and horizontal decentrations
required to produce 4Δ base up and 2Δ base out
with the lens R +4.50 DS / +2.50 DC x 70.

The principal powers of the lens lie along the
70 and 160 meridians but the prismatic effect
which has to be produced is given as vertical FIGURE 3 - RESOLVING THE PRISMS 4Δ BASE UP & 2Δ
and horizontal components, so the first stage of BASE OUT ALONG THE 70 AND 160 MERIDIANS
the solution is to compound the given prisms In Figure 3(b), this single prism, OZ, has
into a single prismatic effect and then to resolve been resolved along the 70 and 160 meridians
this along the 70 and 160 meridians of the lens. by dropping perpendiculars, ZY and ZX,
This construction is shown in Figure 3. Note respectively, onto the principal meridians of the
that the prisms are compounded following lens.
the rules given above. At this stage, we are not By measurement:
interested in the actual powers of the lens. OX = P = 3.1Δ up @ 70,
70
In Figure 3(a), the prisms 4Δ base up and 2Δ and OY = P160 = 3.3Δ up @ 160
base out have been compounded, graphically Note in which direction the base of the two
into the single resultant prism, OZ, which is prisms, P and P , lie. The origin, O represents
70
160
4.47Δ base up @ 116.6°. the apex of each prism.
When the prism powers are known along
the two principal meridians of the lens, the
decentrations required to produce these
prisms can be found. Now the powers which lie
along the principal meridians, F (+4.50) and
70
F (+7.00), can be considered.
160
From the decentration relationship,

c = P / F = 3.1 / 4.50 = 0.69cm = 6.9mm up
70
70
70
@ 70 (since F70 is positive).
c = P / F = 3.3/7.00 = 0.47cm = 4.7mm up
160
160
160
@ 160 (since F160 is positive).
WHEN THE PRISM POWERS
ARE KNOWN ALONG THE TWO
PRINCIPAL MERIDIANS OF THE
LENS, THE DECENTRATIONS
REQUIRED TO PRODUCE
FIGURE 2 - PRISMATIC EFFECT PRODUCED BY
DECENTERING THE LENS, L -2.00 / +4.00 X 45, THESE PRISMS CAN BE FOUND
10MM INWARDS

112 | THE INDIAN OPTICIAN | JULY-AUGUST 2024 LENS TALK