Page 122 - The Indian Optician Digital Edition March-April 2024

THE PRISMATIC EFFECT

OF DECENTRATION

– PART TWO

Dr Prof Mo Jalie, DSc, SMSA, FBDO (Hons), SLD, Hon FCGI Hon FCOptom,
MCMI, is a Visiting Professor of Optometry at the University of Ulster in
Coleraine, and at the post-graduate facility at Varilux University. He served for
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 and has written several books including Principles of Ophthalmic
Lenses. His most recent book, Ophthalmic Lenses & Dispensing was
translated into Russian. He has authored over 200 papers on ophthalmic,
contact and intra-ocular lenses, and on dispensing; and is a consultant editor
to The Optician (UK) and technical editor to The Indian Optician journal.
He holds patents for aspheric spectacle and intra-ocular lenses. Jalie is a
past-chairman of the Academic Committee of the Association of British
Dispensing Opticians, and was the first Chairman of the Faculty of Dispensing
Opticians. He is the ABDO representative on the BSI committees on
ophthalmic lenses and spectacle frames and a past member of the Education
Committee of the General Optical Council. In 1998 Jalie was thrice honoured:
he was made Honorary Fellow of the British College of Optometrists, a Life
Fellow of the Association of British Dispensing Opticians, and in December of
that year he was granted the Max Wiseman Memorial Research Medal.
Dr Prof Mo Jalie

It is essential that both the PRISMATIC EFFECT OF PLANO-CYLINDERS
optician and the surfacing
laboratory have a good A plano-cylinder has no power along its axis meridian,
grasp of the prismatic hence it can exert no prismatic effect along its axis. The power
effect of a lens. For the of a cylinder lies at right angles to its axis, i.e., along its power
optician this understanding meridian, and so a cylinder can exert prismatic effect only at
is important in preventing right angles to its axis. We may, therefore, consider a cylinder
unwanted prismatic effects to consist of innumerable prisms, joined at the cylinder axis
in lenses, particularly vertical by their bases in the case of a positive cylinder, or by their
differential prism which might apices in the case of a negative cylinder. The prismatic effect
produce vertical diplopia at any point on a plano-cylinder is simply the product of the
between the eyes. For the perpendicular distance of the point in question from the
laboratory, it is necessary to cylinder axis, measured in cm, and the power of the cylinder
know how to calculate how in dioptres.
much prism must be worked
on a lens to shift the optical In Figure 1a, the prismatic effect at the point R is the
centre to the desired position product of the distance PR (measured in centimetres) and the
on the lens, so called “prism- power of the cylinder. Given that the power of the cylinder is
to-cut”. This series is designed +2.00 DC x 90 and the distance PR is 8mm then the prismatic
to explain how prismatic effect at point R, using P = cF, (see Part 1 of this series) is
1
effect is calculated. Part 2 0.8 x 2.00 = 1.6∆ base OUT assuming the cylinder is placed in
deals with plano-cylinders. front of the Right eye.

118 | THE INDIAN OPTICIAN | MAR-APR 2024 LENS TALK