Page 138 - Nov- Dec 2024

THE PRISMATIC

EFFECT OF

DECENTRATION

– PART SIX

It is of interest to establish formulae in order to be
able to calculate and analyse prismatic effects.
Such formulae are, of course, essential in order to
program computers to be able to calculate the Dr Prof Mo Jalie,
prism necessary to shift the optical centre during DSc, SMSA, FBDO (Hons), SLD, Hon
FCGI Hon FCOptom, MCMI, is a
surfacing instruction. These formulae require us Visiting Professor of Optometry at
to formulate a sign convention which informs the University of Ulster in Coleraine,
and at the post-graduate facility
upon the correct base direction and the direction at Varilux University. He served for
of decentration. Also, a symmetrical axis nine years as Head of Department
notation must be chosen to enable the formulae of Applied Optics at City & Islington
College, where he taught optics,
to work equally for the right and left eyes. ophthalmic lenses and dispensing.
He is a recognised international
authority on spectacle lens design
and has written several books
including Principles of Ophthalmic
Lenses. His most recent book,
ANALYSIS OF PRISMATIC EFFECTS Ophthalmic Lenses & Dispensing
was translated into Russian. He
We look first at the sign conventions. The one has authored over 200 papers on
1
chosen for the axis notation is the Binasal convention ophthalmic, contact and intra-
illustrated in Figure 1. The right eye notation is exactly ocular lenses, and on dispensing;
and is a consultant editor to The
the same as Standard notation but the left eye Optician (UK) and technical editor
directions commence at the nasal side of the lens. to The Indian Optician journal. He
To convert left eye axes to Binasal notation the axis holds patents for aspheric spectacle
and intra-ocular lenses. Jalie is a
in Standard notation is simply subtracted from 180 past-chairman of the Academic
(Figure 1). The convention chosen for decentrations is Committee of the Association of
also illustrated in Figure 1. Decentration upwards and British Dispensing Opticians, and
was the first Chairman of the
outwards is positive. This convention is chosen because Faculty of Dispensing Opticians.
the most common problem which occurs involving the He is the ABDO representative on
calculation of prismatic effects, apart from providing the BSI committees on ophthalmic
prism-to-cut, is that of finding the prismatic effect in lenses and spectacle frames and
a past member of the Education
the near visual zones of a pair of lenses. Committee of the General Optical
Council. In 1998 Jalie was thrice
The expressions for the prismatic effect at any point honoured: he was made Honorary
on a lens can be derived from Figure 2. Fellow of the British College of
Optometrists, a Life Fellow of the
Note that the lower, nasal, quadrants of each eye are Association of British Dispensing
the all-positive quadrants, the optical centre of each Opticians, and in December of
that year he was granted the Max
lens is assumed to lie at the origins of the notation for Wiseman Memorial Research Medal.

132 | THE INDIAN OPTICIAN | NOV-DEC 2024 LENS TALK