Page 118 - The Indian Optician Digital Edition July-August 2024
P. 118
NOTE THAT THE SCALE USED TO
COMPOUND THE DECENTRATIONS
NEED NOT BE THE SAME AS THAT
USED TO CONSTRUCT THE PRISMS.
THE MOST CONVENIENT SCALE
SHOULD BE CHOSEN
In Figure 4(b), the single resultant
decentration, OR, has been resolved into vertical
and horizontal components, OV and OH, by
FIGURE 4 - RESOLVING THE DECENTRATIONS INTO dropping perpendiculars, RV and RH onto the
VERTICAL AND HORIZONTAL COMPONENTS
vertical and horizontal meridians respectively.
By measurement, c (OV), = 8mm up and c
H
V
(OH) = 2mm out.
In practice, each step of this solution can be
constructed on a single diagram and this single
construction where all the separate steps have
been combined is shown in Figure 5. The full
solution is repeated here.
The powers of the lens are +4.50 D along
70 and +7.00 D along 160 and these have been
marked on the construction, together with an
indication of the nasal side of the lens.
By measurement:
OX = P = 3.1Δ up @ 70,
70
and OY = P = 3.3Δ up @ 160
160
c = P / F = 3.1/4.50 = 0.69cm = 6.9mm up
70
70
70
@ 70 (since F is positive).
70
c = P / F = 3.3/7.00 = 0.47cm = 4.7mm up
FIGURE 5 - COMPLETE, COMBINED 160 160 160
CONSTRUCTION FOR EXAMPLE (III), R +4.50/+2.50 @ 160 (since F is positive).
160
X 70 WITH 4Δ BASE UP AND 2Δ BASE OUT
The decentrations c (OP) and c (OQ)
160
70
can now be constructed to scale on the
The decentrations c (OP) and c (OQ) must graph. Note that the scale used to compound
160
70
now be resolved into vertical and horizontal the decentrations need not be the same as
components
that used to construct the prisms. The most
In Figure 4(a), the decentration, c , has been convenient scale should be chosen.
70
drawn to scale, up along 70 (OP), and c drawn Completing the rectangle OPRQ, by
160
to scale, up along 160 (OQ). Completing the measurement, OR= 8.3mm up @ 104½ and
rectangle OPRQ, produces the single resultant resolving OR into vertical and horizontal
decentration (OR), which would need to be components, c (OV), and c (OH), we find c =
obtained if the minimum size uncut needed to 8mm up and c = 2mm out. H V
V
be determined. By measurement, OR = 8.3mm H
up @ 104½°. ... to be continued
114 | THE INDIAN OPTICIAN | JULY-AUGUST 2024 LENS TALK
