Page 116 - The Indian Optician Digital Edition May-June 2023

again, lies along the “plus-axis” meridian (i.e., The surface powers of the lens are
the meridian which produces against rotation (+3.00 DC x 90 / +7.00 DC x 180) / -11.00 DS
of a crossline limb). The power of the cylinder
may also be written, 5.00 DS/+5.00 DC x 180. The thin edge subs. lies at the extremity of
This rule should be noted for circular astigmatic the vertical meridian.
lenses: the plus axis of a cylindrical or toric lens The thick edge subs. = Sag 11.00 @ 50 minus
represents the meridian of greatest thickness. Sag 3.00 @ 50 plus the centre subs.
In the case of a toric lens, the sags along the
base curve and cross curve meridians must be The centre subs. = thin edge subs. minus (Sag
calculated together with the sag of the spherical 11.00 @ 40 - Sag 7.00 @ 40)
surface as shown in Example (ii) below. So the centre subs. = 3 - (4.42 - 2.73) = 1.31mm.
EXAMPLES: and the thick edge subs. = 7.1 - 1.8 + 1.31 = 6.61mm.
REFERENCES
(i) The lens +3.00 DS / +3.00 DC x 60 is made
up as a 40 round eye with a thin edge substance 1. Jalie M (2021), Principles of Ophthalmic
of 2mm. Lenses (6 Ed.) ABDO, Godmersham.
th
If the lens is made as a plano convex toric
calculate the thick edge substance. (Assume the
use of crown glass, n = 1.523).

The front surface power of the lens is: +3.00
DC x 150 / +6.00 DC x 60. The back surface power
is zero.
The thin edge lies at the extremity of the 150
meridian. The thick edge is the centre substance
minus Sag 3.00 @ 40mm aperture. The centre
subs is the thin edge subs + Sag 6.00 @ 40mm
aperture.

So the centre subs, t = 2 + 2.33mm = 4.33mm.
C
The thick edge subs = 4.33 - 1.15mm = 3.18mm.

(ii) Find the thick-edge substance of the lens:
-8.00 DS / +4.00 DC x 180 made up as a 50 x 40
oval shape with a thin edge substance of 3mm
in toric form with a +3.00 D base curve.

FIGURE 4 - THICKNESS OF PLANO-CYLINDRICAL LENSES

112 | THE INDIAN OPTICIAN | MAY-JUNE 2023 LENS TALK