prEN ISO 18369-1

SLOVENSKI STANDARD
01-julij-2025
Očesna optika - Kontaktne leče - 1. del: Slovar, sistem razvrstitve in priporočila za
označevanje specifikacij (ISO/DIS 18369-1:2025)
Ophthalmic optics - Contact lenses - Part 1: Vocabulary, classification system and
recommendations for labelling specifications (ISO/DIS 18369-1:2025)
Augenoptik - Kontaktlinsen - Teil 1: Begriffe, Einteilung von Kontaktlinsenmaterialien und
Empfehlungen für die Schreibweise von Kontaktlinsenspezifikationen (ISO/DIS 18369-
1:2025)
Optique ophtalmique - Lentilles de contact - Partie 1: Vocabulaire, système de
classification et recommandations pour l'étiquetage des spécifications (ISO/DIS 18369-
1:2025)
Ta slovenski standard je istoveten z: prEN ISO 18369-1
ICS:
01.040.11 Zdravstveno varstvo Health care technology
(Slovarji) (Vocabularies)
11.040.70 Oftalmološka oprema Ophthalmic equipment
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

DRAFT
International
Standard
ISO/DIS 18369-1
ISO/TC 172/SC 7
Ophthalmic optics — Contact
Secretariat: DIN
lenses —
Voting begins on:
Part 1: 2025-05-15
Vocabulary, classification system
Voting terminates on:
2025-08-07
and recommendations for labelling
specifications
Optique ophtalmique — Lentilles de contact —
Partie 1: Vocabulaire, système de classification et
recommandations pour l'étiquetage des spécifications
ICS: 01.040.11; 11.040.70
THIS DOCUMENT IS A DRAFT CIRCULATED
FOR COMMENTS AND APPROVAL. IT
IS THEREFORE SUBJECT TO CHANGE
AND MAY NOT BE REFERRED TO AS AN
INTERNATIONAL STANDARD UNTIL
PUBLISHED AS SUCH.
This document is circulated as received from the committee secretariat.
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PROVIDE SUPPORTING DOCUMENTATION.
Reference number
ISO/DIS 18369-1:2025(en)
DRAFT
ISO/DIS 18369-1:2025(en)
International
Standard
ISO/DIS 18369-1
ISO/TC 172/SC 7
Ophthalmic optics — Contact
Secretariat: DIN
lenses —
Voting begins on:
Part 1:
Vocabulary, classification system
Voting terminates on:
and recommendations for labelling
specifications
Optique ophtalmique — Lentilles de contact —
Partie 1: Vocabulaire, système de classification et
recommandations pour l'étiquetage des spécifications
ICS: 01.040.11; 11.040.70
THIS DOCUMENT IS A DRAFT CIRCULATED
FOR COMMENTS AND APPROVAL. IT
IS THEREFORE SUBJECT TO CHANGE
AND MAY NOT BE REFERRED TO AS AN
INTERNATIONAL STANDARD UNTIL
PUBLISHED AS SUCH.
This document is circulated as received from the committee secretariat.
IN ADDITION TO THEIR EVALUATION AS
BEING ACCEPTABLE FOR INDUSTRIAL,
© ISO 2025
TECHNOLOGICAL, COMMERCIAL AND
USER PURPOSES, DRAFT INTERNATIONAL
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
STANDARDS MAY ON OCCASION HAVE TO
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Published in Switzerland Reference number
ISO/DIS 18369-1:2025(en)
ii
ISO/DIS 18369-1:2025(en)
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
3.1.1 Basic terms .1
3.1.2 Terms related to contact lens parameters and design .4
3.1.3 Terms related to aspheric contact lenses . 20
3.1.4 Terms related to bifocal and multifocal contact lenses .21
3.1.5 Terms related to scleral contact lenses and shells . 25
3.1.6 Terms related to contact lens material properties .27
3.1.7 Terms related to tinted contact lenses . 30
3.1.8 Terms related to contact lens manufacture .31
3.1.9 Terms related to packaging and labelling of contact lenses and contact lens care
products .31
3.1.10 Terms related to contact lens usage and wear modality .32
3.1.11 Terms related to contact lens hygienic management and contact lens care
products . 34
3.1.12 Miscellaneous terms .37
3.2 Symbols . 39
4 Classification of contact lens material.40
Annex A (informative) Specification of rigid contact lenses .43
Annex B (informative) Specification of soft contact lenses .58
Bibliography .60
Alphabetical index of terms .62

iii
ISO/DIS 18369-1:2025(en)
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).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent
rights in respect thereof. As of the date of publication of this document, ISO had not received notice of (a)
patent(s) which may be required to implement this document. However, implementers are cautioned that
this may not represent the latest information, which may be obtained from the patent database available at
www.iso.org/patents. ISO shall not be held responsible for identifying any or all such patent rights.
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of 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 www.iso.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 third edition cancels and replaces the second edition (ISO 18369-1:2017), which has been technically
revised.
The main changes are as follows:
— Editorial update of the whole document;
— Add six new definitions related to scleral contact lenses and extend Annex A (informative) to include
representative examples of the specifications for lathe-cut scleral contact lenses of varying complexity
using the standard notation of this document.
A list of all parts in the ISO 18369 series can be found on the ISO website.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.

iv
ISO/DIS 18369-1:2025(en)
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.
v
DRAFT International Standard ISO/DIS 18369-1:2025(en)
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 terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
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) having a total diameter (3.1.2.3.1) 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: In some cases, the back optic zone (3.1.2.2.1) will have minimal corneal touch.

ISO/DIS 18369-1:2025(en)
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.
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 (3.1.2.3.1)
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 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 contact 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
ISO/DIS 18369-1:2025(en)
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 contact
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 contact 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

ISO/DIS 18369-1:2025(en)
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 surface which lies also on the optical axis of the central optic zone
(3.1.2.1.17)
3.1.2.1.3
paraxial front vertex power
F
V
reciprocal of the paraxial front vertex 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 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 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.

ISO/DIS 18369-1:2025(en)
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 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 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 power is zero

ISO/DIS 18369-1:2025(en)
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 bifocal contact lens (3.1.4.15), 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 contact 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 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.

ISO/DIS 18369-1:2025(en)
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 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 fixed points (the two foci) in the plane is
constant
Note 1 to entry: This is a conic section (3.1.2.1.21) having an eccentricity (3.1.2.1.23) greater than zero and less than
one (0 < e < 1), formed by sectioning a cone with a plane in such a way that the angle of the plane with respect to the
cone's base is less than the angle of the cone's side with respect to the base. Each ellipse has a prolate (3.1.2.1.25) curve
and an oblate (3.1.2.1.24) curve. Use of the prolate curve is assumed unless otherwise indicated. Although e is the
same for both prolate and oblate curves, e is sometimes given a minus sign not used in contact lens computations in
order to signify use of the oblate elliptical curve.
3.1.2.1.27
hyperbola
locus of points in a plane whose distance from a fixed point in the plane (the focus) divided by its distance
from a fixed line in the plane (the directrix) is a positive constant greater than one
Note 1 to entry: This is a conic section (3.1.2.1.21) having an eccentricity (3.1.2.1.23) greater than one (e > 1), formed by
sectioning a cone with a plane in such a way that the angle of the plane with respect to the cone's base is greater than
the angle of the cone's side with respect to the base. Hyperbolas are prolate (3.1.2.1.25) curves.
3.1.2.1.28
optical decentration
positioning of the optical centre at a point other than the geometric centre (3.1.2.1.20) of the optic zone
(3.1.2.1.17) or central optic zone (3.1.4.5)
3.1.2.1.29
contact lens axis
line passing through the geometric centre, perpendicular to a plane containing the edge (3.1.2.1.34) of a
contact lens (3.1.1.1)
Note 1 to entry: See Figure 1.
3.1.2.1.30
back vertex
point on the posterior contact lens surface which also lies on the optical axis of the central optic zone
(3.1.2.1.17)
ISO/DIS 18369-1:2025(en)
3.1.2.1.31
vertex sphere
imaginary spherical surface (3.1.2.1.41) touching the back vertex (3.1.2.1.30)
Note 1 to entry: The radius of curvature of the vertex sphere is the same as the steepest back optic zone radius, back
central optic zone radius, or back vertex radius of an aspheric lens (see Figure 1).
Key
1 contact lens axis 4 vertex sphere
2 centre of vertex sphere 5 peripheral junction thickness, t
PJ0
3 back vertex 6 overall posterior sagitta
Figure 1 — Schematic representation of a tri-curve contact lens including symbols of the main
parameters describing its back surface
3.1.2.1.32
sagitta
sagittal depth
sagittal height
maximum distance from a chord which is perpendicular to the axis of rotation of a surface, to the curved surface
3.1.2.1.33
overall posterior sagitta
distance along the contact lens axis (3.1.2.1.29) from the back vertex (3.1.2.1.30) to a plane containing the
contact lens edge (3.1.2.1.34)
3.1.2.1.34
edge
most peripheral part of a contact lens (3.1.1.1) which is contiguous with the front and back surfaces

ISO/DIS 18369-1:2025(en)
3.1.2.1.35
edge form
edge profile
shape of the edge (3.1.2.1.34) in a plane containing the contact lens axis (3.1.2.1.29)
3.1.2.1.36
bevel
narrow front or back peripheral zone (3.1.2.1.18), of a single spherical or aspherical curvature, adjacent to
the edge (3.1.2.1.34) of a contact lens (3.1.1.1)
3.1.2.1.37
radial lift
l
R
distance between a specified point on the back surface of a contact lens (3.1.1.1) and the vertex sphere
(3.1.2.1.31) measured along a radius of curvature of the vertex sphere
Note 1 to entry: See Figure 2 a).
3.1.2.1.38
radial edge lift
l
ER
distance between a point on the back surface of a contact lens (3.1.1.1) at the edge (3.1.2.1.34) and the vertex
sphere (3.1.2.1.31) measured along the radius of curvature of the latter
Note 1 to entry: See Figure 2 b).
Note 2 to entry: This is often a value computed by the manufacturer and can be altered by the edging process.
3.1.2.1.39
axial lift
l
A
distance between a specified point on the back surface of a contact lens (3.1.1.1) and the vertex sphere
(3.1.2.1.31) measured parallel to the contact lens axis (3.1.2.1.29)
Note 1 to entry: See Figure 2 a).
3.1.2.1.40
axial edge lift
l
EA
distance between a point on the back surface of a contact lens (3.1.1.1) at the edge (3.1.2.1.34) and the vertex
sphere (3.1.2.1.31), measured parallel to the contact lens axis (3.1.2.1.29)
Note 1 to entry: See Figure 2 b).
Note 2 to entry: This is often a value computed by the manufacturer and can be altered by the edging process.
a) Difference between radial and axial lift

ISO/DIS 18369-1:2025(en)
b) Difference between radial and axial edge lift
Key
1 contact lens front surface 6 radial lift, l
R
2 contact lens axis 7 specified point (see NOTE)
3 contact lens back surface 8 axial lift, l
A
4 vertex sphere 9 radial edge lift, l
ER
5 centre of vertex sphere 10 axial edge lift, l
EA
NOTE The specified point's position on the contact lens back surface is measured at right angles to the contact
lens axis from the extreme edge of the contact lens (for radial edge lift) or from the contact lens centre (for radial lift).
Figure 2 — Difference between lift and edge lift
3.1.2.1.41
spherical surface
surface having the same radius of curvature for meridians in all directions
Note 1 to entry: Surfaces which are not spherical would normally be termed aspheric or toroidal.
3.1.2.1.42
sagittal radius of curvature
radius of curvature in the sagittal plane at a specified off-axis point on the surface
Note 1 to entry: The radius at a specified point on the surface is equal to the distance along the normal at that point to
its intersection with the axis of rotation.
Note 2 to entry: The sagittal plane is perpendicular to the tangential plane.
3.1.2.1.43
tangential radius of curvature
radius of curvature in the tangential plane at a specified off-axis point on a surface
Note 1 to entry: The tangential plane contains both the off-axis point and the optical axis.
3.1.2.1.44
bi-curve contact lens
contact lens (3.1.1.1) whose back surface is composed of two intersecting spherical zones
3.1.2.1.45
tri-curve contact lens
contact lens (3.1.1.1) whose back surface is composed of three intersecting coaxial spherical zones
Note 1 to entry: See Figure 1.

ISO/DIS 18369-1:2025(en)
3.1.2.1.46
multi-curve contact lens
contact lens (3.1.1.1) whose back surface is composed of more than three intersecting spherical zones
3.1.2.1.47
aspheric contact lens
contact lens (3.1.1.1) with front or back optic zone (3.1.2.2.1) of aspheric form
Note 1 to entry: See 3.1.3 for specific terms concerning aspheric contact lenses.
3.1.2.1.48
toric contact lens
contact lens (3.1.1.1) with front and/or back optic zone (3.1.2.2.1) of toroidal form
3.1.2.1.49
bi-toric contact lens
contact lens (3.1.1.1) having both front and back optic zones (3.1.2.2.1) of toroidal form
3.1.2.1.50
toroidal zone
zone having a surface with its maximum and minimum radii of curvature perpendicular to each other
3.1.2.1.51
toric periphery contact lens
contact lens (3.1.1.1) with one or more back peripheral zones (3.1.2.1.18) of toroidal form that surround a
spherical back optic zone (3.1.2.2.1)
3.1.2.1.52
junction
intersection of two adjacent zones
Note 1 to entry: This applies to both back and front surfaces.
3.1.2.1.53
tangential junction
junction (3.1.2.1.52) where the curvatures of adjacent zones have a common tangent
Note 1 to entry: See Figure 3.

ISO/DIS 18369-1:2025(en)
Key
1 tangent common to both circles
2 contact lens axis
NOTE This is an example of a back surface of a contact lens. It is a bi-curve surface with a tangential junction, T.
The back peripheral zone would be formed by rotating the arc TE around the contact lens axis; the back optic zone is
formed by rotating the arc YT around the contact lens axis. The back optic zone diameter is 2 LT; the total diameter is
2 EA; the overall posterior sagitta is YA.
Figure 3 — Example of a tangential junction
3.1.2.1.54
transition
transition zone
junction (3.1.2.1.52) which has been modified to smooth the change between adjacent curvatures
Note 1 to entry: See Figure 4.
Key
1 zone A
2 zone B
3 original junction of zone A and zone B
4 radius of curvature of the transition
5 transition
Figure 4 — Example of a transition on the back surface of a contact lens

ISO/DIS 18369-1:2025(en)
3.1.2.1.55
blend
polished, smoothed junction (3.1.2.1.52) or transition zone (3.1.2.1.54) between two different adjacent
surface curvatures
Note 1 to entry: This typically applies to the junction (transition) between posterior zones.
Note 2 to entry: This does not constitute the formation of an aspheric zone (3.1.3.1).
3.1.2.1.56
ballast
rotationally asymmetrical distribution of thickness for the purpose of effecting rotational orientation of a
contact lens (3.1.1.1) on the eye
Note 1 to entry: The most common method of achieving ballast in contact lenses is with the use of base-down
vertical prism.
3.1.2.1.57
prism ballast
vertical prism used to create a wedge design (3.1.2.1.58) that will help stabilize the rotation and orientation
of a contact lens (3.1.1.1) on the eye
Note 1 to entry: A vertical prism can also be used to correct a vertical hyperphoria or hypertropia.
Note 2 to entry: The asymmetrical distribution of thickness, rather than the effect of mass, is responsible for the
rotational orientation of the contact lens that incorporates prism.
3.1.2.1.58
wedge design
rotationally asymmetric distribution of thickness to effect the required rotational orientation of a contact
lens (3.1.1.1) on the eye, or to improve the centration of a high-riding lens
Note 1 to entry: One common way of creating a wedge design is to incorporate base-down vertical prism into a contact lens.
3.1.2.1.59
peripheral thinning
slab-off
thinning, towards the edge (3.1.2.1.34), of the front periphery of the contact lens (3.1.1.1), in one or more
discrete areas
Note 1 to entry: This is normally applied to achieve contact lens rotational stabilization. It is different from both ballast
(3.1.2.1.56) and a lenticular contact lens (3.1.1.4) construction.
3.1.2.1.60
truncation
inferior edge (3.1.2.1.34) of a contact lens (3.1.1.1) that has a straight line or near straight line
Note 1 to entry: See Figure 9.
Note 2 to entry: This construction is typically used in toric and multifocal contact lenses.
3.1.2.1.61
fenestration
specified hole which passes through a contact lens (3.1.1.1)
3.1.2.1.62
carrier
part of a plus or minus lenticular contact lens (3.1.1.4) peripheral to the front optic zone(s) (3.1.2.1.17)
Note 1 to entry: The carrier can be negative, positive or parallel in construction, but it is radially symmetrical.
Note 2 to entry: In Figure 5, the carriers are represented with flat surfaces for clarity, but in reality, they are composed
of curved surfaces.
ISO/DIS 18369-1:2025(en)
3.1.2.1.63
negative carrier
minus carrier
carrier (3.1.2.1.62) having an edge thickness that is greater than the junction thickness
Note 1 to entry: See Figure 5 a).
3.1.2.1.64
parallel carrier
plano carrier
carrier (3.1.2.1.62) having an edge thickness and junction thickness that are equal
Note 1 to entry: See Figure 5 b).
3.1.2.1.65
positive carrier
plus carrier
carrier (3.1.2.1.62) having an edge thickness that is less than the junction thickness
Note 1 to entry: See Figure 5 c).
a) Negative or minus carrier b) Parallel or plano carrier
c) Positive or plus carrier
ISO/DIS 18369-1:2025(en)
Key
1 optic zone 4 negative carrier
2 edge 5 parallel carrier
3 junction 6 positive carrier
NOTE Figures are not to scale.
Figure 5 — Examples of carriers
3.1.2.2 Terms related to the radius of curvature
NOTE 1 Radii relating to zones on the back surface of the contact lens are designated by a numerical subscript
starting with zero (r ). The subscript becomes numerically larger from the centre to the edge of the contact lens. See
Figure 1.
NOTE 2 Radii relating to the front surface of the contact lens have a double subscript, the first part of which is the
letter “a”. The second part is a number from zero upward, for example, r .
a2
NOTE 3 In the case of an aspheric zone (3.1.3.1), a mathematical equation or expression can be used to describe the
curvature of the zone.
3.1.2.2.1
back optic zone
posterior surface area of a single-vision contact lens which contributes to the prescribed optical effect
Note 1 to entry: This area is sometimes called the back central optic zone of a concentric back surface bifocal contact
lens (3.1.1.13) or multifocal contact lens (3.1.1.14), which has more than one optic zone (3.1.2.1.17).
3.1.2.2.2
equivalent posterior radius of curvature
EPC
base curve equivalent
BCE
spherical radius of curvature of a contact lens (3.1.1.1) computed from the overall posterior sagitta (3.1.2.1.33)
of a contact lens
Note 1 to entry: The EPC would normally only apply to soft contact lenses (3.1.1.9).
3.1.2.2.3
back optic zone radius
base curve radius
r
radius of curvature of the back optic zone (3.1.2.2.1) of a surface with a single refractive element
Note 1 to entry: On a toroidal zone (3.1.2.1.50) there will be two radius values.
Note 2 to entry: The term “base curve” used in a contact lens context is no
...