SIST EN ISO 18369-1:2017

SLOVENSKI STANDARD
SIST EN ISO 18369-1:2017
01-november-2017
1DGRPHãþD
SIST EN ISO 18369-1:2006
SIST EN ISO 18369-1:2006/A1:2009
2þHVQDRSWLND.RQWDNWQHOHþHGHO6ORYDUVLVWHPUD]YUVWLWYHLQSULSRURþLOD]D
R]QDþHYDQMHVSHFLILNDFLM,62
Ophthalmic optics - Contact lenses - Part 1: Vocabulary, classification system and
recommendations for labelling specifications (ISO 18369-1:2017)
Augenoptik - Kontaktlinsen - Teil 1: Begriffe, Einteilung von Kontaktlinsenmaterialien und
Empfehlungen für die Schreibweise von Kontaktlinsenspezifikationen (ISO 18369-
1:2017)
Optique ophtalmique - Lentilles de contact - Partie 1: Vocabulaire, système de
classification et recommandations pour l'étiquetage des spécifications (ISO 18369-
1:2017)
Ta slovenski standard je istoveten z: EN ISO 18369-1:2017
ICS:
01.040.11 Zdravstveno varstvo Health care technology
(Slovarji) (Vocabularies)
11.040.70 Oftalmološka oprema Ophthalmic equipment
SIST EN ISO 18369-1:2017 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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SIST EN ISO 18369-1:2017

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SIST EN ISO 18369-1:2017


EN ISO 18369-1
EUROPEAN STANDARD

NORME EUROPÉENNE

September 2017
EUROPÄISCHE NORM
ICS 11.040.70 Supersedes EN ISO 18369-1:2006
English Version

Ophthalmic optics - Contact lenses - Part 1: Vocabulary,
classification system and recommendations for labelling
specifications (ISO 18369-1:2017)
Optique ophtalmique - Lentilles de contact - Partie 1: Augenoptik - Kontaktlinsen - Teil 1: Begriffe, Einteilung
Vocabulaire, système de classification et von Kontaktlinsenmaterialien und Empfehlungen für
recommandations pour l'étiquetage des spécifications die Schreibweise von Kontaktlinsenspezifikationen
(ISO 18369-1:2017) (ISO 18369-1:2017)
This European Standard was approved by CEN on 1 July 2017.

CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this
European Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references
concerning such national standards may be obtained on application to the CEN-CENELEC Management Centre or to any CEN
member.

This European Standard exists in three official versions (English, French, German). A version in any other language made by
translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC Management
Centre has the same status as the official versions.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania,
Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,
Turkey and United Kingdom.





EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

CEN-CENELEC Management Centre: Avenue Marnix 17, B-1000 Brussels
© 2017 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 18369-1:2017 E
worldwide for CEN national Members.

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SIST EN ISO 18369-1:2017
EN ISO 18369-1:2017 (E)
Contents Page
European foreword . 3
2

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SIST EN ISO 18369-1:2017
EN ISO 18369-1:2017 (E)
European foreword
This document (EN ISO 18369-1:2017) has been prepared by Technical Committee ISO/TC 172 “Optics
and photonics” in collaboration with Technical Committee CEN/TC 170 “Ophthalmic optics” the
secretariat of which is held by DIN.
This European Standard shall be given the status of a national standard, either by publication of an
identical text or by endorsement, at the latest by March 2018, and conflicting national standards shall
be withdrawn at the latest by March 2018.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN shall not be held responsible for identifying any or all such patent rights.
This document supersedes EN ISO 18369-1:2006.
According to the CEN-CENELEC Internal Regulations, the national standards organizations of the
following countries are bound to implement this European Standard: Austria, Belgium, Bulgaria,
Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, Former Yugoslav Republic of Macedonia,
France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta,
Netherlands, Norway, Poland, Portugal, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,
Turkey and the United Kingdom.
Endorsement notice
The text of ISO 18369-1:2017 has been approved by CEN as EN ISO 18369-1:2017 without any
modification.

3

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SIST EN ISO 18369-1:2017

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SIST EN ISO 18369-1:2017
INTERNATIONAL ISO
STANDARD 18369-1
Second edition
2017-08
Ophthalmic optics — Contact lenses —
Part 1:
Vocabulary, classification system
and recommendations for labelling
specifications
Optique ophtalmique — Lentilles de contact —
Partie 1: Vocabulaire, système de classification et recommandations
pour l’étiquetage des spécifications
Reference number
ISO 18369-1:2017(E)
©
ISO 2017

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SIST EN ISO 18369-1:2017
ISO 18369-1:2017(E)

COPYRIGHT PROTECTED DOCUMENT
© ISO 2017, Published in Switzerland
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized otherwise in any form
or by any means, electronic or mechanical, including photocopying, or posting on the internet or an intranet, without prior
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ISO copyright office
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Tel. +41 22 749 01 11
Fax +41 22 749 09 47
copyright@iso.org
www.iso.org
ii © ISO 2017 – All rights reserved

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SIST EN ISO 18369-1:2017
ISO 18369-1:2017(E)

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms, definitions and symbols . 1
3.1 Terms and definitions . 1
3.2 Symbols .38
4 Classification of contact lens materials .39
Annex A (informative) Specification of rigid contact lenses .42
Annex B (informative) Specification of soft contact lenses .52
Bibliography .54
Alphabetical index of terms .56
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SIST EN ISO 18369-1:2017
ISO 18369-1:2017(E)

Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/ directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www .iso .org/ patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation on the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO’s adherence to the
World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see the following
URL: w w w . i s o .org/ iso/ foreword .html.
This document was prepared by Technical Committee ISO/TC 172, Optics and photonics, Subcommittee
SC 7, Ophthalmic optics and instruments.
This second edition cancels and replaces the first edition (ISO 18369-1:2006), which has been
technically revised. It also incorporates the Amendment ISO 18369-1:2006/Amd 1:2009.
A list of all parts in the ISO 18369 series can be found on the ISO website.
iv © ISO 2017 – All rights reserved

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SIST EN ISO 18369-1:2017
ISO 18369-1:2017(E)

Introduction
The ISO 18369 series applies to contact lenses, which are devices worn over the front surface of the eye
in contact with the preocular tear film. This document covers rigid (hard) corneal and scleral contact
lenses, as well as soft contact lenses. Rigid lenses maintain their own shape. Soft contact lenses are
easily deformable and require support for proper shape.
Clause 3 contains the terms and definitions primarily used in the contact lens field. A list of terms
having special symbols is given in Table 1.
The list of terms and definitions does not include all ISO terms, definitions, and symbols used in the
contact lens field. It is intended to be a convenient reference source from which the contents have
been compiled from the text of this and other ISO standards applicable to the manufacture, evaluation,
measurement, labelling and marketing of contact lenses and contact lens care products. An alphabetical
index was added for rapid finding of terms.
Words are grouped under several topics by reference number according to the general category into
which each word logically fits. The preferred form of each term is listed on the first line after its
reference number. Other admitted forms have been placed on subsequent lines after the preferred
form. All admitted terms are given in bold-faced type. A few obsolete and superseded terms are listed
for historical reference and convenience and to aid comprehension but are indicated as deprecated and
are no longer to be used. Obsolete and superseded terms are not in bold-faced type so that they may be
clearly identified as terms used historically.
Contact lenses are primarily used for the correction of refractive errors but they can also be used for
therapeutic purposes and cosmetic reasons. The materials used are divided into two main categories,
rigid and soft. The former is composed mainly of corneal lenses and to a lesser extent, scleral lenses.
Both types can be made from gas-permeable materials or non-gas permeable materials. Soft lenses are
manufactured primarily from hydrogel materials. A small number of lenses incorporate both a rigid
material and a soft material.
In terms of vision correction, contact lenses can be made as single vision, bifocal, multifocal or
progressive lenses. Surface designs can be spherical, aspheric, toric or “complex”.
Wearing modality can be daily wear, flexible wear, or extended wear. Typical replacement schedules for
soft lenses are daily, two weekly, or monthly. Rigid lenses and some soft lenses are replaced less often,
for example, once a year.
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SIST EN ISO 18369-1:2017

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SIST EN ISO 18369-1:2017
INTERNATIONAL STANDARD ISO 18369-1:2017(E)
Ophthalmic optics — Contact lenses —
Part 1:
Vocabulary, classification system and recommendations for
labelling specifications
1 Scope
This document identifies and defines the terms applicable to the physical, chemical and optical properties
of contact lenses, their manufacture and uses. It provides a vocabulary of terms and, when appropriate,
the international symbol and abbreviation associated with a specific term. This document also defines
the terms relating to contact lens care products. It also incorporates the classifications of contact lens
materials and gives recommendations for the labelling of the specifications of contact lenses.
2 Normative references
There are no normative references in this document.
3 Terms, definitions and symbols
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— IEC Electropedia: available at http:// www .electropedia .org/
— ISO Online browsing platform: available at http:// www .iso .org/ obp
3.1 Terms and definitions
3.1.1 Basic terms
3.1.1.1
contact lens
ophthalmic lens designed to be worn on the front surface of the eye
Note 1 to entry: This term includes contact lenses of plano power.
3.1.1.2
corneal contact lens
intralimbal contact lens
contact lens (3.1.1.1) having a total diameter less than the visible iris diameter and designed to be worn
in its entirety on the cornea
3.1.1.3
scleral contact lens
contact lens (3.1.1.1) whose scleral zone (3.1.5.12) is supported on the bulbar conjunctiva and whose
optic zone (3.1.2.1.17) vaults over the cornea
Note 1 to entry: Note1 to entry: In some cases, the back optic zone (3.1.2.2.1) will have minimal corneal touch.
Note 2 to entry: Terms such as mini-scleral, semi-scleral and corneo-scleral can be found in the contact lens
literature to describe lenses of different parameters.
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SIST EN ISO 18369-1:2017
ISO 18369-1:2017(E)

Note 3 to entry: See 3.1.5 for specific terms concerning scleral contact lenses.
3.1.1.4
lenticular contact lens
contact lens (3.1.1.1) having a front optic zone (3.1.2.1.17) made smaller than the total diameter
Note 1 to entry: This construction is conventionally used to reduce the centre thickness (3.1.2.4.1) of a positive
power contact lens (3.1.2.1.13) or reduce the edge (3.1.2.1.34) thickness of a negative power contact lens
(3.1.2.1.14).
3.1.1.5
contact shell
contact lens (3.1.1.1) not designed to correct vision
3.1.1.6
scleral shell
rigid contact shell (3.1.1.5) with a scleral zone (3.1.5.12)
Note 1 to entry: See 3.1.5 for specific terms concerning scleral shells.
3.1.1.7
rigid contact lens
contact lens (3.1.1.1) which, in its final state and under normal conditions, retains its form without
support and has a water content (3.1.6.11) less than 10 %
Note 1 to entry: Rigid lenses are made of non-hydrogel rigid materials which can flex slightly but do not
substantially conform to the shape of the cornea when on the eye.
3.1.1.8
rigid gas-permeable contact lens
RGP contact lens
DEPRECATED: hard gas-permeable contact lens
contact lens (3.1.1.1) manufactured from a rigid material containing one or more gas-permeable
polymers in sufficient concentrations to facilitate transport of oxygen through the lens and having a Dk
equal to or greater than 10 Dk units
Note 1 to entry: For an explanation of the meaning of Dk and Dk units, see 3.1.6.8.
3.1.1.9
soft contact lens
contact lens (3.1.1.1) made of a hydrogel material or non-hydrogel material which, in its hydrated final
state and under normal conditions, contains a known water content (3.1.6.11), is easily deformable and
may not retain its form without support
3.1.1.10
hydrogel contact lens
DEPRECATED: hydrophilic contact lens
contact lens (3.1.1.1) made of water-absorbing material having equilibrium water content (3.1.6.11)
greater than or equal to 10 % in standard saline solution as specified in ISO 18369–3 at 20 °C
3.1.1.11
composite contact lens
contact lens (3.1.1.1) composed of two or more different materials
EXAMPLE Laminated lens, fused segment lens, lens with a rigid centre and a flexible periphery.
3.1.1.12
surface-treated contact lens
contact lens (3.1.1.1) whose surfaces have been modified to make the surface characteristics different
to those of the bulk material
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SIST EN ISO 18369-1:2017
ISO 18369-1:2017(E)

3.1.1.13
bifocal contact lens
multifocal contact lens (3.1.1.14) having two optic zones (3.1.2.1.17), usually for distance and near-vision
correction
Note 1 to entry: See 3.1.4 for specific terms concerning bifocal contact lenses.
3.1.1.14
multifocal contact lens
contact lens (3.1.1.1) designed to provide two or more zones of different corrective powers
Note 1 to entry: See 3.1.4 for specific terms concerning multifocal contact lenses.
3.1.1.15
progressive power contact lens
varifocal power contact lens
contact lens (3.1.1.1) designed to provide correction for more than one viewing range in which the
power changes continuously, rather than discretely, over a part or the whole of the lens
Note 1 to entry: See 3.1.4 for specific terms concerning progressive power contact lenses.
3.1.1.16
spherical aberration
attribute of an optical system due to variation in the focusing between peripheral and paraxial rays
3.1.1.17
contact lens accessory
article intended specifically by its manufacturer to be used with a contact lens (3.1.1.1) to enable the
lens to be used in accordance with its intended purpose (3.1.9.1)
Note 1 to entry: This term includes all devices to clean, handle, store or manipulate lenses for intended use.
Note 2 to entry: This definition does not include the primary packaging (3.1.9.7), e.g. vials, blister packs (3.1.9.5)
or mailers, intended by the manufacturer to be used only for shipment of the contact lenses.
3.1.1.18
contact lens care product
contact lens accessory (3.1.1.17) intended for use in maintaining the safety and performance (3.1.9.2) of
a contact lens (3.1.1.1) after opening and removal of the lens from its primary container (3.1.9.7)
Note 1 to entry: See 3.1.9 and 3.1.11 for specific terms concerning contact lens care products and the hygienic
management of contact lenses.
3.1.1.19
suction cup
handheld device designed with a small concave flexible tip intended to aid the insertion of a contact lens
(3.1.1.1) onto or removal from the eye by means of suction
Note 1 to entry: A suction cup is designed primarily for use with rigid corneal contact lenses (3.1.1.2) and scleral
contact lenses (3.1.1.3).
3.1.1.20
contact lens container
storage container
contact lens case
storage case
device in which contact lenses (3.1.1.1) are stored either dry (rigid corneal and scleral lenses), or in a
suitable solution (rigid gas permeable lenses, hydrogel and other soft lenses), by the user after removal
from the primary container (3.1.9.7) or the eye
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SIST EN ISO 18369-1:2017
ISO 18369-1:2017(E)

3.1.1.21
equilibration
conditioning of a lens or lens material in a test solution at a specified temperature until the parameters
of the lens or material remain stable
Note 1 to entry: Lenses are equilibrated by soaking in an appropriate volume of solution for sufficient time that
the parameters to be measured remain constant on repeated measures within the capability of the method to
measure the parameter.
Note 2 to entry: The key solution parameters, e.g. pH and osmolality (3.1.6.5), are included in the test report.
Note 3 to entry: The equilibration conditions will be determined by the test laboratory.
3.1.2 Terms related to contact lens parameters and design
3.1.2.1 General terms
3.1.2.1.1
dioptre
D
unit of measure of the refractive power of a lens
Note 1 to entry: The power of a lens is equal to the reciprocal of a lens’ focal length measured in metres.
-1
Note 2 to entry: The symbol D is the preferred unit of measure abbreviation over the symbol m , as D is an
established international abbreviation.
3.1.2.1.2
front vertex
point on the anterior contact lens (3.1.1.1) surface which lies also on the optic axis of the central optic
zone (3.1.4.5)
3.1.2.1.3
paraxial front vertex power
F
V
reciprocal of the paraxial front vertex (3.1.2.1.2) focal length
Note 1 to entry: See ISO 13666.
Note 2 to entry: The front vertex power is expressed in dioptres (3.1.2.1.1). This theoretical value is often used
during a design process. Paraxial powers are used in ray-tracing through an optical system and are limited to
very small ray angles and heights.
3.1.2.1.4
paraxial back vertex power
F’
V
reciprocal of the paraxial back vertex (3.1.2.1.30) focal length
Note 1 to entry: See ISO 13666.
Note 2 to entry: The back vertex power is expressed in dioptres (3.1.2.1.1). This theoretical value is often used
during the design process. Paraxial powers are used in ray-tracing through an optical system and are limited to
very small ray angles and heights.
3.1.2.1.5
label front vertex power
F
L
reciprocal of the front vertex (3.1.2.1.2) focal length over the optic zone (3.1.2.1.17) in air, expressed in
dioptres (3.1.2.1.1)
Note 1 to entry: The front vertex focal length is the distance from the front vertex to the optimal focus over the
optic zone, when measured as specified in ISO 18369–3:2017, 4.3.
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SIST EN ISO 18369-1:2017
ISO 18369-1:2017(E)

Note 2 to entry: The measurement of front vertex focal length will be affected by spherical aberration (3.1.1.16).
3.1.2.1.6
label back vertex power
F’
L
reciprocal of the back vertex (3.1.2.1.30) focal length over the optic zone (3.1.2.1.17) in air, expressed in
dioptres (3.1.2.1.1)
Note 1 to entry: The back vertex focal length is the distance from the back vertex to the optimal focus over the
optic zone, when measured as specified in ISO 18369–3:2017, 4.3.
Note 2 to entry: The measurement of back vertex focal length will be affected by spherical aberration (3.1.1.16).
3.1.2.1.7
cylinder power
F’
C
difference in back vertex (3.1.2.1.30) power between the two principal meridians of maximum and
minimum radii of curvature of a lens measured in air, expressed in dioptres (3.1.2.1.1)
3.1.2.1.8
cylinder axis
meridian 90° to the meridian of maximum cylinder power (3.1.2.1.7)
Note 1 to entry: This axis is defined by the angle, in degrees, between the horizontal plane and the cylinder axis.
3.1.2.1.9
prismatic error
unintended optical prism in a contact lens (3.1.1.1), expressed in prism dioptres (3.1.12.10)
3.1.2.1.10
power profile
localized optical power as a function of radial distance from the centre of the lens
3.1.2.1.11
prescribed optical prism
intended optical prism within a contact lens (3.1.1.1), expressed in prism dioptres (3.1.12.10)
3.1.2.1.12
prism axis
defined by the angle, in degrees, between the horizontal plane and the base of the prism
3.1.2.1.13
positive power contact lens
plus-power contact lens
contact lens (3.1.1.1) which causes parallel incident light, incident on a single optic zone (3.1.2.1.17), to
converge to a real focus
Note 1 to entry: Positive power contact lenses are typically used for hyperopic patients.
3.1.2.1.14
negative power contact lens
minus-power contact lens
contact lens (3.1.1.1) which causes parallel light, incident on a single optic zone (3.1.2.1.17), to diverge
from a virtual focus
Note 1 to entry: Negative power contact lenses are typically used for myopic patients.
3.1.2.1.15
plano contact lens
afocal contact lens
contact lens (3.1.1.1) whose back vertex (3.1.2.1.30) power is zero
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SIST EN ISO 18369-1:2017
ISO 18369-1:2017(E)

3.1.2.1.16
liquid lens
fluid lens
tear lens
lacrimal lens
refractive element formed by the liquid between the back optic zone (3.1.2.2.1) of the contact lens
(3.1.1.1) and the cornea
3.1.2.1.17
optic zone
area of a contact lens (3.1.1.1) which has a prescribed optical effect
Note 1 to entry: The prescribed optical effect is contributed jointly by the central anterior and posterior surface
curvatures of the contact lens.
Note 2 to entry: The term may be qualified by either the prefix “back” or “front” in the case of a surface with
a single optical component. In the case of an alternating image translating bifocal contact lens (3.1.1.13), the
term may be qualified by either the prefix “distance” or “near”. In the case of a concentric multifocal contact lens
(3.1.4.4), the term may be qualified by the prefix “central” or “peripheral”.
Note 3 to entry: The term may be qualified by the prefix “central” or “peripheral”.
3.1.2.1.18
peripheral zone
region of specified dimensions surrounding the optic zone(s) (3.1.2.1.17) but with no prescribed
refractive effect
Note 1 to entry: There can be more than one peripheral zone.
3.1.2.1.19
displacement of optic
d
decentration of the optic zone (3.1.2.1.17) relative to the lens edge (3.1.2.1.34)
Note 1 to entry: See 3.1.5 for application to scleral contact lenses (3.1.1.3).
3.1.2.1.20
geometric centre
C
centre of the circle containing the contact lens (3.1.1.1) edge (3.1.2.1.34)
Note 1 to entry: For a scleral contact lens (3.1.1.3), the geometric centre is taken as the centre of the optic zone
(3.1.2.1.17). For a truncated contact lens, the geometric centre is taken as the centre of the circle that contains
the circular portion of the edge.
3.1.2.1.21
conic section
one of a group of plane geometric curves at the intersection of a plane with a cone
Note 1 to entry: Conic sections have eccentricities (e) ranging from zero to positive infinity. The group includes
the following two-dimensional curves: circle, ellipse (3.1.2.1.26), parabola, and hyperbola (3.1.2.1.27).
3.1.2.1.22
conoidal surface
surface described by rotating a conic section (3.1.2.1.21) about its axis
Note 1 to entry: These surfaces include spheres, ellipsoids, paraboloids and hyperboloids.
Note 2 to entry: The term “conoidal” in the contact lens field usually refers to surfaces that are not spherical.
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SIST EN ISO 18369-1:2017
ISO 18369-1:2017(E)

3.1.2.1.23
eccentricity
e
value descriptive of a conic section (3.1.2.1.21) and the rate of curvature change away from the apex of
the curve
Note 1 to entry: Circle (e = 0), ellipse (3.1.2.1.26) (0 < e < 1), parabola (e = 1) and hyperbola (3.1.2.1.27) (e > 1).
In order to signify use of an oblate (3.1.2.1.24) curve of the ellipse, e is sometimes given a negative sign that is
not used in contact lens (3.1.1.1) computations. Otherwise, use of the prolate (3.1.2.1.25) curve of the ellipse is
assumed.
3.1.2.1.24
oblate
becoming progressively steeper away from the apex
Note 1 to entry: Oblate is the opposite of prolate (3.1.2.1.25).
3.1.2.1.25
prolate
becoming progressively flatter away from the apex
Note 1 to entry: Prolate is the opposite of oblate (3.1.2.1.24).
3.1.2.1.26
ellipse
locus of points in a plane whose combined distance from two f
...