Page 115 - The Indian Optician Digital Edition July-August 2024

in order to compound them into the single
resultant prism, P , represented by the
R
length OZ.

By measurement, P = 2.0Δ base up which
R
represents the total prismatic effect at R.
Note that in this example the lens has been
decentred inwards, resulting in a vertical prism!

DECENTRATION TO PRODUCE PRISM
– THE GENERAL CASE (GRAPHICAL
METHODS)

The graphical construction described above
can be reversed to find the decentration
necessary to produce a prescribed prismatic
effect. The rules for this construction are as
follows.

(1) Construct vertical and horizontal meridians
and the principal meridians and mark the
nasal side of the lens (N).

(2) Draw the prisms to scale with their bases
FIGURE 1 - PRISMATIC EFFECT AT A POINT 8MM
ABOVE AND 3MM INWARDS FROM OC OF LENS, R in the correct directions and construct the
-6.00 / +2.00 X 40 resultant point, Z.

By measurement, P = 4.8Δ base up @ 60½. (3) Resolve the prisms along the principal
R
Finally, resolving P into vertical and meridians of the lens (ZX and ZY) by
dropping perpendiculars from Z to the
R
horizontal components, P (= OV) and P (= OH), principal meridians. Note that the prisms are
H
V
we find:
represented by the lengths, OX and OY and
P = 4.2Δ base up and P = 2.4Δ base in. that the positions of X and Y with respect to
H
V
EXAMPLE (II): O, represent their base directions.
(4) Measure OX and OY and calculate the
Find the prismatic effects introduced by magnitude and the directions of the
decentring the lens L -2.00 / +4.00 x 45, 1cm decentrations necessary to produce them,
inwards. (OP and OQ).
The graphical construction is shown in Figure
2. The powers of the lens are -2.00 D along 45
and +2.00 D along 135 and these have been THE PRINCIPAL POWERS OF THE LENS
marked upon the principal meridians. LIE ALONG THE 70 AND 160 MERIDIANS
By measurement: BUT THE PRISMATIC EFFECT WHICH
RP = c = 0.71cm HAS TO BE PRODUCED IS GIVEN
45
and RQ = c = 0.71cm. AS VERTICAL AND HORIZONTAL
135
COMPONENTS, SO THE FIRST STAGE
So, P = c x F = 0.71 x 2 = 1.42Δ base up @ 45, OF THE SOLUTION IS TO COMPOUND
45
45
45
P = c x F = 0.71 x 2 = 1.42 base up @ 135. THE GIVEN PRISMS INTO A SINGLE
135
135
135
These two prisms can now be constructed PRISMATIC EFFECT...
on the graph to scale (P = OX and P = OY)
45 135
LENS TALK THE INDIAN OPTICIAN | JULY-AUGUST 2024 | 111