Page 104 - The Indian Optician Digital Edition September-October 2022

ISO-PRISM ZONES

- 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

he theory of iso-prism zones was From the clinical point of view, vertical
considered in Part One of this paper, an differential prismatic effects are more important
Tiso-prism zone being that zone on a single than horizontal effects owing to the small
vision lens within which the total prismatic tolerance of the eyes to relative rotations in the
effect does not exceed a stipulated amount. In vertical meridian. A zone in which the vertical
practice, it is of more interest to consider the component of the resultant prismatic effect
iso-prism zone dimensions for the differential does not exceed a given amount is called an
1
prescription for a pair of lenses. The iso-prism iso-V-prism zone . An iso-V-prism zone is a
zones then represent the areas on a pair of parallel band, which lies horizontally in the case
lenses within which the eyes will not meet a of spherical lenses, parallel to the cylinder axis in
given amount of differential prismatic effect. For the case of plano-cylindrical lenses but inclined
example, in the case of the prescription, R +3.00, at an angle to the cylinder axis in the general
L +5.00, the differential prescription is +2.00 D, case of sphero-cylindrical lenses. If the iso-V-
i.e., the left lens is +2.00 D stronger than the prism zone is constructed for the differential
right (or the right lens is +2.00 D weaker than prescription it is called an iso-V-differential
the left – it does not matter which way round it prism zone. We will look first at the construction
is considered) and the 1Δ iso-differential-prism of iso-V-prism zones.
zone is a circle of diameter 10mm (its radius, c = Consider a +2.00 D spherical lens. It was seen
бP / бF, where бP is the differential prism and бF in Part One that the 2Δ iso-prism zone is a circle
is the differential prescription). of radius 10mm concentric with the optical

100 | THE INDIAN OPTICIAN | SEPT-OCT 2022 LENS TALK