Page 114 - The Indian Optician Digital Edition July-August 2024

THE PRISMATIC

EFFECT OF

DECENTRATION

– PART FOUR

A sphero-cylindrical prescription may be looked upon
in either of two ways. One could look upon a sph-cyl.
as a spherical lens to which has been added a plano-
cylinder, or, we may consider it to be made up from Dr Prof Mo Jalie,
two separate plano-cylinders placed together with DSc, SMSA, FBDO (Hons), SLD, Hon
FCGI Hon FCOptom, MCMI, is a
their axes at right angles to one another. It should be Visiting Professor of Optometry at
apparent that we are only interested in the principal the University of Ulster in Coleraine,
powers of the lens and not in its form, so the remarks and at the post-graduate facility
at Varilux University. He served for
here apply equally to toric lenses. nine years as Head of Department
of Applied Optics at City & Islington
College, where he taught optics,
ophthalmic lenses and dispensing.
He is a recognised international
authority on spectacle lens design
PRISMATIC EFFECT OF SPHERO-CYLINDERS and has written several books
including Principles of Ophthalmic
EXAMPLE (I) Lenses. His most recent book,
Ophthalmic Lenses & Dispensing
was translated into Russian. He
Find the prismatic effects at a point 8mm above has authored over 200 papers on
and 3mm inwards from the optical centre of the ophthalmic, contact and intra-
lens R -6.00 DS/+2.00 DC x 40. ocular lenses, and on dispensing;
and is a consultant editor to The
The graphical construction is shown in Figure Optician (UK) and technical editor
to The Indian Optician journal. He
1. The powers of the lens are -6.00 D along 40 and holds patents for aspheric spectacle
-4.00 D along 130 and these have been marked and intra-ocular lenses. Jalie is a
upon the principal meridians. past-chairman of the Academic
Committee of the Association of
By measurement: British Dispensing Opticians, and
RP = c = 0.75cm and RQ = c = 0.42cm. was the first Chairman of the
40 130 Faculty of Dispensing Opticians.
He is the ABDO representative on
So, P = c x F = 0.75 x 6 = 4.5Δ base up @ 40, the BSI committees on ophthalmic
40
40
40
lenses and spectacle frames and
(Since the power along 40, F , is negative, point P a past member of the Education
40
represents the position of the prism apex). Committee of the General Optical
Council. In 1998 Jalie was thrice
P = c x F = 0.42 x 4 = 1.68 base up @ 130. honoured: he was made Honorary
130
130
130
Fellow of the British College of
These two prisms are now to be constructed to scale Optometrists, a Life Fellow of the
on the graph (P = OX and P = OY) in order to Association of British Dispensing
130
40
compound them into the single resultant prism, PR, Opticians, and in December of
that year he was granted the Max
represented by the length OZ. Wiseman Memorial Research Medal.
110 | THE INDIAN OPTICIAN | JULY-AUGUST 2024 LENS TALK