Page 104 - The Indian Optician Digital Edition May-Junel 2022

SUMMATION OF

PLANO-CYLINDERS

̶PART 2

Dr Prof Mo Jalie, DSc, SMSA, FBDO (Hons), 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

n Part One of this two-part article it was shown how any
two thin plano-cylinders can be added together using
Ieither an algebraic method or a graphical method (for
those who do not like Maths!) attributed to G.G.Stokes
(1849). In this second part some special cases of the
algebraic summation method are considered together
with a different mathematical procedure which is known
as Astigmatic decomposition.

When the cylinder axes are parallel or at right angles to
one another it is not necessary to use the methods outlined
in Part One of this article, the cylinders can be added using
the normal rules for transposition. When the cylinder axes
are parallel, the two cylinder powers can be simply added
together as shown by the following examples.
EXAMPLES

i) The sum of the cylinders -2.00 DC x 90 and +3.00 DC x
90 is +1.00 DC x 90.
ii) The sum of the cylinders -2.00 DC x 90 and +3.00 DC x
180 is -2.00 / +5.00 x 180 (or +3.00 / -5.00 x 90) .

100 THE INDIAN OPTICIAN MAY-JUNE 2022 LENS TALK