SIST EN ISO 19980:2021
(Main)Ophthalmic instruments - Corneal topographers (ISO 19980:2021)
Ophthalmic instruments - Corneal topographers (ISO 19980:2021)
This document specifies minimum requirements for instruments and systems that fall into the class of
corneal topographers (CTs). It also specifies tests and procedures to verify that a system or instrument
complies with this document and thus qualifies as a CT according to this document. It also specifies
tests and procedures that allow the verification of capabilities of systems that are beyond the minimum
requirements for CTs.
This document defines terms that are specific to the characterization of the corneal shape so that they
may be standardized throughout the field of vision care.
This document is applicable to instruments, systems and methods that are intended to measure the
surface shape of the cornea of the human eye.
NOTE The measurements can be of the curvature of the surface in local areas, three-dimensional
topographical measurements of the surface or other more global parameters used to characterize the surface.
This document is not applicable to ophthalmic instruments classified as ophthalmometers.
Ophthalmische Instrumente - Hornhauttopographen (ISO 19980:2021)
Dieses Dokument legt die Mindestanforderungen an Instrumente und Systeme, die in die Klasse der Hornhauttopographen (en: corneal topographer, CT) fallen, fest. Es definiert auch die Prüfungen und Verfahren zum Nachweis, dass ein System oder Instrument diesem Dokument entspricht und somit als Hornhauttopograph nach diesem Dokument gilt. Es legt weiterhin Prüfungen und Verfahren fest, die die Überprüfung der Fähigkeiten von Systemen erlauben, die über den Mindestanforderungen für Hornhauttopographen liegen.
Dieses Dokument legt Begriffe fest, die im Zusammenhang mit der Charakterisierung der Hornhaut-oberfläche eine spezielle Bedeutung haben und für die sich daher eine genormte Festlegung für das gesamte Gebiet der Augenheilkunde anbietet.
Dieses Dokument ist anwendbar für Instrumente, Systeme und Prüfverfahren, deren Zweck in der Vermessung der Oberflächenform der Hornhaut eines menschlichen Auges liegt.
ANMERKUNG Diese Messungen können Messungen der Oberflächenkrümmung in bestimmten Bereichen, drei¬dimensionale topographische Messungen der Oberfläche oder anderer, globaler Parameter, die eine Oberfläche charakterisieren, sein.
Dieses Dokument ist nicht anwendbar für ophthalmische Instrumente, die in die Klasse der Ophthalmometer fallen.
Instruments ophtalmiques - Topographes de la cornée (ISO 19980:2021)
Le présent document spécifie les exigences minimales relatives aux instruments et systèmes classés parmi les topographes cornéens (TC). Il spécifie également les essais et modes opératoires permettant de vérifier la conformité d'un système ou d'un instrument au présent document, et de le définir comme étant un TC au sens du présent document. Il spécifie en outre les essais et modes opératoires permettant de vérifier les aptitudes des systèmes dépassant les exigences minimales relatives aux TC.
Le présent document définit les termes spécifiques à la caractérisation de la forme de la cornée, de façon à pouvoir les normaliser dans l'ensemble du domaine des soins ophtalmologiques.
Le présent document concerne les instruments, systèmes et méthodes de mesure de la forme de la cornée de l'œil humain.
NOTE Il peut s'agir de mesurages de la courbure de la surface des zones locales, de mesurages topographiques à trois dimensions de la surface ou d'autres paramètres plus généraux utilisés pour caractériser la surface.
Le présent document ne s'applique pas aux instruments ophtalmiques classés parmi les ophtalmomètres.
Oftalmični instrumenti - Topografi roženice (ISO 19980:2021)
General Information
Relations
Standards Content (Sample)
SLOVENSKI STANDARD
SIST EN ISO 19980:2021
01-september-2021
Nadomešča:
SIST EN ISO 19980:2012
Oftalmični instrumenti - Topografi roženice (ISO 19980:2021)
Ophthalmic instruments - Corneal topographers (ISO 19980:2021)
Ophthalmische Instrumente - Hornhauttopographen (ISO 19980:2021)
Instruments ophtalmiques - Topographes de la cornée (ISO 19980:2021)
Ta slovenski standard je istoveten z: EN ISO 19980:2021
ICS:
11.040.70 Oftalmološka oprema Ophthalmic equipment
SIST EN ISO 19980:2021 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
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SIST EN ISO 19980:2021
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SIST EN ISO 19980:2021
EN ISO 19980
EUROPEAN STANDARD
NORME EUROPÉENNE
June 2021
EUROPÄISCHE NORM
ICS 11.040.70 Supersedes EN ISO 19980:2012
English Version
Ophthalmic instruments - Corneal topographers (ISO
19980:2021)
Instruments ophtalmiques - Topographes de la cornée Ophthalmische Instrumente - Hornhauttopographen
(ISO 19980:2021) (ISO 19980:2021)
This European Standard was approved by CEN on 6 June 2021.
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, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway,
Poland, Portugal, Republic of North Macedonia, 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: Rue de la Science 23, B-1040 Brussels
© 2021 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 19980:2021 E
worldwide for CEN national Members.
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SIST EN ISO 19980:2021
EN ISO 19980:2021 (E)
Contents Page
European foreword . 3
2
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SIST EN ISO 19980:2021
EN ISO 19980:2021 (E)
European foreword
This document (EN ISO 19980:2021) 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 December 2021, and conflicting national standards
shall be withdrawn at the latest by December 2021.
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 19980:2012.
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, France, Germany, Greece, Hungary, Iceland,
Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Republic of
North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and the
United Kingdom.
Endorsement notice
The text of ISO 19980:2021 has been approved by CEN as EN ISO 19980:2021 without any modification.
3
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SIST EN ISO 19980:2021
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SIST EN ISO 19980:2021
INTERNATIONAL ISO
STANDARD 19980
Third edition
2021-06
Ophthalmic instruments — Corneal
topographers
Instruments ophtalmiques — Topographes de la cornée
Reference number
ISO 19980:2021(E)
©
ISO 2021
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SIST EN ISO 19980:2021
ISO 19980:2021(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2021
All rights reserved. Unless otherwise specified, or required in the context of its implementation, 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 written permission. Permission can be requested from either ISO at the address
below or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii © ISO 2021 – All rights reserved
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SIST EN ISO 19980:2021
ISO 19980:2021(E)
Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Requirements . 9
4.1 Area measured . 9
4.2 Measurement sample density . 9
4.3 Measurement and report of performance . 9
4.4 Colour presentation of results . 9
5 Test methods and test devices . 9
5.1 Tests . 9
5.1.1 Accuracy test . 9
5.1.2 Repeatability test . 9
5.2 Test surfaces . 9
5.2.1 Reflection-based and Placido ring corneal topographers . 9
5.2.2 Luminous surface corneal topographer .10
5.2.3 Optical-sectioning corneal topographer.10
5.2.4 Specification of test surfaces .10
5.2.5 Verification of test surfaces .10
5.2.6 Type testing of surfaces .10
5.3 Data collection — Test surfaces .11
5.4 Analysis of the data .11
5.4.1 General.11
5.4.2 Structure of the accuracy data set.12
5.4.3 Analysis of the paired data sets .12
5.4.4 Report of accuracy performance .13
6 Accompanying documents .13
7 Marking .13
Annex A (informative) Test surfaces for corneal topographers (CTs) .14
Annex B (informative) Standardized displays for corneal topographers (CTs) .16
Annex C (normative) Calculation of area-weighting values .19
Annex D (normative) Test methods for measuring human corneas .21
Bibliography .22
© ISO 2021 – All rights reserved iii
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SIST EN ISO 19980:2021
ISO 19980:2021(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 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, in collaboration with the European Committee for
Standardization (CEN) Technical Committee CEN/TC 170, Ophthalmic optics, in accordance with the
Agreement on technical cooperation between ISO and CEN (Vienna Agreement).
This third edition cancels and replaces the second edition (ISO 19980:2012), which has been technically
revised. The main changes compared to the previous edition are as follows:
a) normative references were updated;
b) 5.2.6 regarding requirements for test surfaces and requirement for testing of accuracy was
changed;
c) in 5.4.3, formulae for data analysis have been updated;
d) Table 4 was deleted;
e) document editorially revised.
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 2021 – All rights reserved
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SIST EN ISO 19980:2021
INTERNATIONAL STANDARD ISO 19980:2021(E)
Ophthalmic instruments — Corneal topographers
1 Scope
This document specifies minimum requirements for instruments and systems that fall into the class of
corneal topographers (CTs). It also specifies tests and procedures to verify that a system or instrument
complies with this document and thus qualifies as a CT according to this document. It also specifies
tests and procedures that allow the verification of capabilities of systems that are beyond the minimum
requirements for CTs.
This document defines terms that are specific to the characterization of the corneal shape so that they
may be standardized throughout the field of vision care.
This document is applicable to instruments, systems and methods that are intended to measure the
surface shape of the cornea of the human eye.
NOTE The measurements can be of the curvature of the surface in local areas, three-dimensional
topographical measurements of the surface or other more global parameters used to characterize the surface.
This document is not applicable to ophthalmic instruments classified as ophthalmometers.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
IEC 60601-1:2005 + A1: 2012 + A2: 2020, Medical electrical equipment — Part 1: General requirements for
basic safety and essential performance
3 Terms and definitions
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:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
3.1
corneal apex
location on the corneal surface where the mean of the local principal curvature is greatest
Note 1 to entry: See Figure 1.
3.2
corneal eccentricity
e
c
eccentricity, e, of the conic section that best fits the corneal meridian (3.3) of interest
Note 1 to entry: If the meridian is not specified, the corneal eccentricity is that of the flattest corneal meridian
(see Table 1 and Annex A).
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SIST EN ISO 19980:2021
ISO 19980:2021(E)
3.3
corneal meridian
θ
curve created by the intersection of the corneal surface and a plane that contains the corneal
topographer axis
Note 1 to entry: A meridian is identified by the angle θ, that the plane creating it makes to the horizontal (see
ISO 8429).
Note 2 to entry: The value of θ, for a full meridian, ranges from 0° to 180°.
3.4
corneal shape factor
E
value that specifies the type of conic section that best fits a corneal meridian (3.3), given by Formula (1):
Ep=−1 (1)
where
p is the value that specifies a conic section such as a circle, ellipse, hyperbola, or parabola
value p is given by Formula (2):
2
a
p=± (2)
2
b
where
a and b are the semi-diameters of the axes of the conic section;
+ indicates a circle or ellipse;
− indicates a hyperbola
a conic section is specified by Formula (3):
2 2
z x
±=1 (3)
2 2
b a
value E also is the square of the eccentricity (3.9) of the conic section, given by Formula (4):
2
Ee= (4)
Note 1 to entry: Unless otherwise specified, E refers to the meridian with least curvature (flattest meridian). See
Table 1 and Annex A.
Note 2 to entry: Although the magnitude of E is equal to the square of the eccentricity and so is always positive,
the sign of E is a convention to signify whether an ellipse takes a prolate or oblate orientation.
Note 3 to entry: The negative value of E is defined by ISO 10110-12 as the conic constant designated by the symbol
K. The negative value of E has also been called asphericity and given the symbol Q.
Note 4 to entry:
Note 5 to entry:
2 © ISO 2021 – All rights reserved
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SIST EN ISO 19980:2021
ISO 19980:2021(E)
Table 1 — Conic section descriptors
a
Conic section Value of p Value of E Value of e
Hyperbola p < 0 E > 1 e > 1
Parabola 0,0 1,0 1,0
b
Prolate ellipse 0 < p < 1 1 > E > 0 1 > e >0
Circle 1,0 0,0 0,0
b
Oblate ellipse p > 1 E < 0 0 < e < 1
a
See 3.4.
b
Eccentricity, e, does not distinguish between prolate and oblate orientations of an ellipse (see 3.9 and
Annex A).
3.5
corneal topographer
CT
instrument or system that measures the shape of corneal surface in a non-contact manner
Note 1 to entry: A corneal topographer that uses a video camera system and video image processing to measure
the corneal surface by analysing the reflected image created by the corneal surface of a luminous target is also
referred to as a videokeratograph.
3.5.1
optical-sectioning corneal topographer
corneal topographer (3.5) that measures the corneal surface by analysing multiple optical sections of
that surface
3.5.2
Placido ring corneal topographer
corneal topographer (3.5) that measures the corneal surface by analysing the reflected image of a
Placido ring target created by the corneal surface
3.5.3
reflection-based corneal topographer
corneal topographer (3.5) that measures the corneal surface using light reflected from the air/pre-
corneal tear film interface
3.5.4
luminous surface corneal topographer
corneal topographer (3.5) that measures the corneal surface using light back-scattered from a target
projected onto the pre-corneal tear film or the corneal anterior tissue surface
Note 1 to entry: Back-scattering is usually introduced in these optically clear substances by the addition of a
fluorescent material into the pre-corneal tear film. A target may include a slit or scanning slit of light or another
projecting pattern of light. Other methods are possible.
3.6
corneal topographer axis
CT axis
line parallel to the optical axis of the instrument and often coincident with it, that serves as one of the
coordinate axes used to describe and define the corneal shape
3.7
corneal vertex
point of tangency of a plane perpendicular to the corneal topographer axis (3.6) with the corneal surface
Note 1 to entry: See Figure 1.
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SIST EN ISO 19980:2021
ISO 19980:2021(E)
Key
1 corneal vertex
2 corneal apex
3 radius of curvature at the corneal apex
4 centre of meridional curvature point
5 cross-section of the corneal surface
6 plane perpendicular to the CT axis
7 CT axis
Figure 1 — Illustration of the corneal vertex and the corneal apex
3.8 Curvature
3.8.1 Axial curvature
Note 1 to entry Axial curvature is expressed in reciprocal millimetres.
3.8.1.1
axial curvature
sagittal curvature
K
a
reciprocal of the distance from a point on a surface
to the corneal topographer axis (3.6) along the corneal meridian (3.3) normal at the point and given by
Formula (5):
1
K = (5)
a
r
a
where r is the axial radius of curvature
a
Note 1 to entry: See Figure 2.
4 © ISO 2021 – All rights reserved
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SIST EN ISO 19980:2021
ISO 19980:2021(E)
3.8.1.2
axial curvature
K
a
average of the value of the tangential curvature from the
corneal vertex to the meridional point and given by Formula (6):
x
p
Kx dx
()
m
∫
0
K = (6)
a
x
p
where
x is the radial position variable on the meridian;
x is the radial position at which K is evaluated;
p a
K is the meridional curvature.
m
Key
1 normal to meridian at point P
2 P, a point on the meridian where curvature is to be found
3 centre of meridional curvature point
4 intersection normal — CT axis
5 meridian (a cross-section of the corneal surface)
6 CT axis
Figure 2 — Illustration of axial curvature, K , axial radius of curvature, r , meridional
a a
curvature, K , and meridional radius of curvature, r
m m
3.8.2
Gaussian curvature
product of the two principal normal curvature values at a surface location
Note 1 to entry: Gaussian curvature is expressed in reciprocal square millimetres.
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SIST EN ISO 19980:2021
ISO 19980:2021(E)
3.8.3
meridional curvature
tangential curvature
K
m
local surface curvature measured in the meridional plane and defined by Formula (7):
22
∂ Mx /∂x
()
K = (7)
m
3
2
2
1+∂Mx /∂x
[]()
{}
where M (x) is a function giving the elevation of the meridian at any perpendicular distance, x, from the
corneal topographer axis (3.6)
Note 1 to entry: The meridional plane includes the surface point and the chosen axis. The meridional normal is
a line passing through the surface point perpendicular to the tangent to the meridional curve at that point and
lying in the meridional plane.
Note 2 to entry: Meridional curvature is in general not a normal curvature. It is the curvature of the corneal
meridian at a point on a surface.
Note 3 to entry: See Figure 2.
3.8.4
normal curvature
curvature at a point on the surface of the curve created by the intersection of the surface with any
plane containing the normal to the surface at that point
3.8.4.1
mean curvature
arithmetic average of the principal curvatures at a point on the surface
3.8.4.2
principal curvature
maximum or minimum curvature at a point on the surface
3.9
eccentricity
e
value descriptive of a conic section and the rate of curvature change away from the apex of the curve,
i.e. how quickly the curvature flattens or steepens away from the apex of the surface
Note 1 to entry: Eccentricity ranges from zero to positive infinity for the group of conic sections. In order to
signify use of an oblate ellipse, e is sometimes given a negative sign that is not used in computations. Otherwise,
use of the prolate ellipse is assumed. See Table 1.
3.10
elevation
distance between a corneal surface and a defined reference surface, measured in a defined direction
from a specified position
3.10.1
axial elevation
elevation as measured from a selected point on the corneal surface in a direction parallel to the corneal
topographer axis (3.6)
3.10.2
normal elevation
elevation as measured from a selected point on the corneal surface in a direction along the normal to
the corneal surface at that point
6 © ISO 2021 – All rights reserved
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SIST EN ISO 19980:2021
ISO 19980:2021(E)
3.10.3
reference normal elevation
elevation as measured from a selected point on the corneal surface in a direction along the normal to
the reference surface
3.11
corneal power
refracting power of the cornea based on the radius of curvature of the front surface
Note 1 to entry: Corneal power is expressed in keratometric dioptres (3.12).
3.12
keratometric dioptres
unit of corneal power based on the radius of curvature of the front surface and the keratometric
constant, 337,5, using Formula (8):
keratometric dioptres = 337,5/radius of curvature (8)
Note 1 to entry: The radius of curvature is expressed in millimetres.
3.13
surface normal
line passing through a surface point of the surface perpendicular to the plane tangent to the surface at
that point
3.14
ellipsoid of revolution
surface of revolution that results when the generating arc is non-circular
3.15
radius of curvature
reciprocal of the curvature
Note 1 to entry: The radius of curvature is expressed in millimetres.
3.15.1
axial radius of curvature
sagittal radius of curvature
r
a
distance from a surface point, P, to the axis along the normal to corneal meridian at that point, and
defined by Formula (9):
x
r = (9)
a
sin(ϕ x)
where
x is the perpendicular distance from the axis to the meridian point, in millimetres;
φ(x) is the angle between the axis and the meridian normal at point x.
Note 1 to entry: See Figure 2.
© ISO 2021 – All rights reserved 7
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SIST EN ISO 19980:2021
ISO 19980:2021(E)
3.15.2
meridional radius of curvature
tangential radius of curvature
r
m
distance from a surface point, P, and the centre of the meridional curvature point, and defined by
Formula (10):
1
r = (10)
m
K
m
Note 1 to entry: See Figure 2.
3.16 Surface
3.16.1
aspheric surface
non-spherical surface
surface with at least one principal meridian that is non-circular in cross-section, i.e., e ≠ 0
3.16.2
atoric surface
surface having mutually perpendicular principal meridians of unequal curvature where at least one
principal meridian is non-circular in cross-section
Note 1 to entry: Atoric surfaces are symmetrical with respect to both principal meridians.
3.16.3
oblate surface
surface whose curvature increases as the location on the surface moves from a central position to a
peripheral position in all meridians
3.16.4
prolate surface
surface whose curvature decreases as the location on the surface moves from a central position to a
peripheral position in all meridians
3.16.5
reference surface
surface, that can be described in an exact, preferably mathematical fashion, used as a reference from
which distance measurements are made to the measured corneal surface, and for which, in addition to
the mathematical description, the positional relationship to the corneal surface is specified
Note 1 to entry: For instance, a reference surface might be described as a sphere that is the best least-squares fit
to the measured corneal surface. Similarly, a plane could serve as a reference surface.
3.16.6
toric surface
surface for which the principal curvatures are unequal and for which principal meridians are circular
sections
Note 1 to entry: Such surfaces are said to exhibit central astigmatism.
3.17
toricity
difference in principal curvatures at a specified point or local area on a surface
8 © ISO 2021 – All rights reserved
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SIST EN ISO 19980:2021
ISO 19980:2021(E)
4 Requirements
4.1 Area measured
When measuring a spherical surface with a radius of curvature of 8 mm, a corneal topographer (CT)
shall directly measure locations on the surface normal whose radial perpendicular distance from the
corneal topographer axis (CT axis) is at least 3,75 mm. If the maximum area covered by a CT is claimed,
it shall be reported as the maximum radial perpendicular distance from the CT axis sampled on this
8 mm-radius spherical surface.
4.2 Measurement sample density
Within the area defined by the requirement of 4.1, the surface shall be directly sampled in sufficient
locations so that any surface location within the area has a sample taken within 0,5 mm of it.
4.3 Measurement and report of performance
If the performance of a CT for the measurement of either curvature or elevation is claimed or reported,
the testing shall be done in accordance with 5.1, 5.2 and 5.3 and the analysis and reporting of results
shall be performed in accordance with 5.4.
4.4 Colour presentation of results
The CT shall present the results according to the colour p
...
SLOVENSKI STANDARD
oSIST prEN ISO 19980:2019
01-december-2019
Oftalmični instrumenti - Topografi roženice (ISO/DIS 19980:2019)
Ophthalmic instruments - Corneal topographers (ISO/DIS 19980:2019)
Ophthalmische Instrumente - Hornhauttopographen (ISO/DIS 19980:2019)
Instruments ophtalmiques - Topographes de la cornée (ISO/DIS 19980:2019)
Ta slovenski standard je istoveten z: prEN ISO 19980
ICS:
11.040.70 Oftalmološka oprema Ophthalmic equipment
oSIST prEN ISO 19980:2019 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
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oSIST prEN ISO 19980:2019
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oSIST prEN ISO 19980:2019
DRAFT INTERNATIONAL STANDARD
ISO/DIS 19980
ISO/TC 172/SC 7 Secretariat: DIN
Voting begins on: Voting terminates on:
2019-09-19 2019-12-12
Ophthalmic instruments — Corneal topographers
Instruments ophtalmiques — Topographes de la cornée
ICS: 11.040.70
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NATIONAL REGULATIONS.
ISO/DIS 19980:2019(E)
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PROVIDE SUPPORTING DOCUMENTATION. ISO 2019
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Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Requirements . 9
4.1 Area measured . 9
4.2 Measurement sample density . 9
4.3 Measurement and report of performance . 9
4.4 Colour presentation of results . 9
5 Test methods and test devices .10
5.1 Tests .10
5.1.1 Accuracy test .10
5.1.2 Repeatability test .10
5.2 Test surfaces .10
5.2.1 Reflection-based systems .10
5.2.2 Luminous surface systems .10
5.2.3 Optical-sectioning systems .10
5.2.4 Specification of test surfaces .10
5.2.5 Verification of test surfaces .11
5.2.6 Type testing of surfaces .11
5.3 Data collection — Test surfaces .12
5.4 Analysis of the data .12
5.4.1 General.12
5.4.2 Structure of the accuracy data set.12
5.4.3 Analysis of the paired data sets .12
5.4.4 Report of accuracy performance .13
6 Accompanying documents .13
7 Marking .13
Annex A (informative) Test surfaces for corneal topographers (CTs) .14
Annex B (informative) Standardized displays for corneal topographers (CTs) .16
Annex C (normative) Calculation of area-weighting values .19
Annex D (normative) Test methods for measuring human corneas .21
Bibliography .22
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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
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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).
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expressions related to conformity assessment, as well as information about ISO's adherence to the
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.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 19980:2012), which has been technically
revised.
The main changes compared to the previous edition are as follows:
a) Normative reference updated;
b) 5.2.6 regarding requirements for test surfaces and requirement for testing of accuracy changed;
c) in 5.4.3 equations for data analysis updated;
d) table 4 deleted;
e) document editorially revised.
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.
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DRAFT INTERNATIONAL STANDARD ISO/DIS 19980:2019(E)
Ophthalmic instruments — Corneal topographers
1 Scope
This document specifies minimum requirements for instruments and systems that fall into the class of
corneal topographers (CTs). It also specifies tests and procedures to verify that a system or instrument
complies with this document and thus qualifies as a CT according to this document. It also specifies
tests and procedures that allow the verification of capabilities of systems that are beyond the minimum
requirements for CTs.
This document defines terms that are specific to the characterization of the corneal shape so that they
may be standardized throughout the field of vision care.
This document is applicable to instruments, systems and methods that are intended to measure the
surface shape of the cornea of the human eye.
NOTE The measurements can be of the curvature of the surface in local areas, three-dimensional
topographical measurements of the surface or other more global parameters used to characterize the surface.
This document is not applicable to ophthalmic instruments classified as ophthalmometers.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
IEC 60601-1:2005, + A1: 2012, Medical electrical equipment — Part 1: General requirements for basic safety
and essential performance
3 Terms and definitions
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:
— ISO Online browsing platform: available at http: //www .iso .org/obp
— IEC Electropedia: available at http: //www .electropedia .org/
3.1
corneal apex
location on the corneal surface where the mean of the local principal curvature is greatest
3.2
corneal eccentricity
e
c
eccentricity, e, of the conic section that best fits the corneal meridian of interest
Note 1 to entry: If the meridian is not specified, the corneal eccentricity is that of the flattest corneal meridian
(see Table 1 and Annex A).
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3.3
corneal meridian
θ
curve created by the intersection of the corneal surface and a plane that contains the corneal
topographer axis
Note 1 to entry: A meridian is identified by the angle θ, that the plane creating it makes to the horizontal (see
ISO 8429).
Note 2 to entry: The value of θ, for a full meridian, ranges from 0 ° to 180 °.
3.3.1
corneal semi-meridian
portion of a full meridian extending from the CT axis toward the periphery in one direction
Note 1 to entry: The value of θ for a semi-meridian ranges from 0 ° to 360 °.
3.4
corneal shape factor
E
value that specifies the asphericity and type (prolate or oblate) of the conic section that best fits a
corneal meridian
Note 1 to entry: Unless otherwise specified, it refers to the meridian with least curvature (flattest meridian). See
Table 1 and Annex A.
Note 2 to entry: Although the magnitude of E is equal to the square of the eccentricity and so must always be
positive, the sign of E is a convention to signify whether an ellipse takes a prolate or oblate orientation.
Note 3 to entry: The negative value of E is defined by ISO 10110-12 as the conic constant designated by the symbol
K. The negative value of E has also been called asphericity and given the symbol Q.
Table 1 — Conic section descriptors
a
Conic section Value of p Value of E Value of e
Hyperbola p < 0 E > 1 e > 1
Parabola 0,0 1,0 1,0
b
Prolate ellipse 1 > p > 0 0 < E < 1 0 < e < 1
Sphere 1,0 0,0 0,0
b
Oblate ellipse p > 1 E < 0 0 < e < 1
a
See 3.15.
b
The eccentricity, e, does not distinguish between prolate and oblate orientations of an ellipse (see 3.9 and
Annex A).
3.5
corneal topographer
CT
instrument or system that measures the shape of corneal surface in a non-contact manner
Note 1 to entry: A corneal topographer that uses a video camera system and video image processing to measure
the corneal surface by analysing the reflected image created by the corneal surface of a luminous target is also
referred to as a videokeratograph.
3.5.1
optical-sectioning corneal topographer
corneal topographer that measures the corneal surface by analysing multiple optical sections of
that surface
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3.5.2
Placido ring corneal topographer
corneal topographer that measures the corneal surface by analysing the reflected image of a Placido
ring target created by the corneal surface
3.5.3
reflection-based corneal topographer
corneal topographer that measures the corneal surface using light reflected from the air/pre-corneal
tear film interface
3.5.4
luminous surface corneal topographer
corneal topographer that measures the corneal surface using light back-scattered from a target
projected onto the pre-corneal tear film or the corneal anterior tissue surface
Note 1 to entry: Back-scattering is usually introduced in these optically clear substances by the addition of a
fluorescent material into the pre-corneal tear film. A target may include a slit or scanning slit of light or another
projecting pattern of light. Other methods are possible.
3.6
corneal topographer axis
CT axis
line parallel to the optical axis of the instrument and often coincident with it, that serves as one of the
coordinate axes used to describe and define the corneal shape
3.7
corneal vertex
point of tangency of a plane perpendicular to the corneal topographer axis with the corneal surface See
Figure 1.
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Key
1 corneal vertex
2 apex
3 radius of curvature at the apex
4 centre of meridional curvature point
5 cross-section of the corneal surface
6 plane perpendicular to the CT axis
7 CT axis
Figure 1 — Illustration of the corneal vertex and the apex
3.8 Curvature
Note 1 to entry For the purposes of this document, the unit of curvature is reciprocal millimetre.
3.8.1 Axial curvature
3.8.1.1
axial curvature
sagittal curvature
K
a
reciprocal of the distance from a point on a surface to
the corneal topographer axis along the corneal meridian normal at the point and given by Equation (1):
1
K = (1)
a
r
a
where r is the axial radius of curvature
a
SEE: Figure 2.
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3.8.1.2
axial curvature
K
a
average of the value of the tangential curvature from the
corneal vertex to the meridional point and given by Equation (2):
x
p
Kx dx
()
m
∫
0
K = (2)
a
x
p
where
x is the radial position variable on the meridian;
x is the radial position at which K is evaluated;
p a
K is the meridional curvature.
m
Key
1 normal to meridian at point P
2 P, a point on the meridian where curvature is to be found
3 centre of meridional curvature point
4 intersection normal — CT axis
5 meridian (a cross-section of the corneal surface)
6 CT axis
Figure 2 — Illustration of axial curvature, K , axial radius of curvature, r , meridional
a a
curvature, K , and meridional radius of curvature, r
m m
3.8.2
Gaussian curvature
product of the two principal normal curvature values at a surface location
Note 1 to entry: Gaussian curvature is expressed in reciprocal square millimetres.
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3.8.3
meridional curvature
tangential curvature
K
m
local surface curvature measured in the meridional plane and defined by Equation (3):
22
∂ Mx /∂x
()
K = (3)
m
3
2
2
1+∂Mx /∂x
[]()
{}
where M (x) is a function giving the elevation of the meridian at any perpendicular distance, x, from the
corneal topographer axis
Note 1 to entry: Meridional curvature is in general not a normal curvature. It is the curvature of the corneal
meridian at a point on a surface.
See Figure 2.
3.8.4
normal curvature
curvature at a point on the surface of the curve created by the intersection of the surface with any
plane containing the normal to the surface at that point
3.8.4.1
mean curvature
arithmetic average of the principal curvatures at a point on the surface
3.8.4.2
principal curvature
maximum or minimum curvature at a point on the surface
3.9
eccentricity
e
value descriptive of a conic section and the rate of curvature change away from the apex of the curve,
i.e. how quickly the curvature flattens or steepens away from the apex of the surface
Note 1 to entry: Eccentricity ranges from zero to positive infinity for the group of conic sections:
— circle (e = 0);
— ellipse (0 < e < 1);
— parabola (e = 1);
— hyperbola (e > 1)
2
Ee= (4)
In order to signify use of an oblate curve of the ellipse, e is sometimes given a negative sign that is not
used in computations. Otherwise, use of the prolate curve of the ellipse is assumed.
3.10
elevation
distance between a corneal surface and a defined reference surface, measured in a defined direction
from a specified position
3.10.1
axial elevation
elevation as measured from a selected point on the corneal surface in a direction parallel to the corneal
topographer axis
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3.10.2
normal elevation
elevation as measured from a selected point on the corneal surface in a direction along the normal to
the corneal surface at that point
3.10.3
reference normal elevation
elevation as measured from a selected point on the corneal surface in a direction along the normal to
the reference surface
3.11
keratometric constant
−1
conversion value equal to 337,5 used to convert corneal curvature from reciprocal millimetres (mm )
to keratometric dioptres
3.12
keratometric dioptres
−1
value of curvature, expressed in reciprocal millimetres (mm ), multiplied by the keratometric
constant, 337,5
3.13
meridional plane
plane that includes the surface point and the chosen axis
3.14 Normal
3.14.1
surface normal
line passing through a surface point of the surface perpendicular to the plane tangent to the surface at
that point
3.14.2
meridional normal
line passing through a surface point of the surface, perpendicular to the tangent to the meridional curve
at that point and lying in the plane creating the meridian
3.15
p-value
number that specifies a conic section such as an ellipse, a hyperbola or a parabola, with the conic section
given in Equation (5):
2 2
z x
±=1 (5)
2 2
b a
and the p-value defined by Equation (6):
2
a
p=± (6)
2
b
Ep=−1 (7)
where
a and b are constants;
+ indicates an ellipse;
− indicates a hyperbola
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See Table 1.
3.16
Placido ring target
target consisting of multiple concentric rings, where each individual ring lies in a plane but the rings
are not, in general, coplanar
3.17
radius of curvature
reciprocal of the curvature
Note 1 to entry: For the purpose of this International Standard, the radius of curvature is expressed in
millimetres.
3.17.1
axial radius of curvature
sagittal radius of curvature
r
a
distance from a surface point, P, to the axis along the normal to corneal meridian at that point, and
defined by Equation (8):
x
r = (8)
a
sin∅ x
()
where
x is the perpendicular distance from the axis to the meridian point, in millimetres;
is the angle between the axis and the meridian normal at point x.
∅()x
See Figure 2.
3.17.2
meridional radius of curvature
tangential radius of curvature
r
m
distance from a surface point, P, and the centre of the meridional curvature point, and defined by
Equation (9):
1
r = (9)
m
K
m
See Figure 2.
3.18 Surface
3.18.1
aspheric surface
non-spherical surface
surface with at least one principal meridian that is non-circular in cross-section
3.18.2
atoric surface
surface having mutually perpendicular principal meridians of unequal curvature where at least one
principal meridian is non-circular in cross-section
Note 1 to entry: Atoric surfaces are symmetrical with respect to both principal meridians.
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3.18.3
oblate surface
surface whose curvature increases as the location on the surface moves from a central position to a
peripheral position in all meridians
3.18.4
prolate surface
surface whose curvature decreases as the location on the surface moves from a central position to a
peripheral position in all meridians
3.18.5
reference surface
surface, that can be described in an exact, preferably mathematical fashion, used as a reference from
which distance measurements are made to the measured corneal surface, and for which, in addition to
the mathematical description, the positional relationship to the corneal surface is specified
Note 1 to entry: For instance, a reference surface might be described as a sphere that is the best least-squares fit
to the measured corneal surface. Similarly, a plane could serve as a reference surface.
3.18.6
toric surface
surface for which the principal curvatures are unequal and for which principal meridians are circular
sections
Note 1 to entry: Such surfaces are said to exhibit central astigmatism.
3.19
toricity
difference in principal curvatures at a specified point or local area on a surface
3.20
transverse plane
plane perpendicular to the meridional plane that includes the normal to the surface point
4 Requirements
4.1 Area measured
When measuring a spherical surface with a radius of curvature of 8 mm, a CT shall directly measure
locations on the surface whose radial perpendicular distance from the CT axis is at least 3,75 mm. If the
maximum area covered by a CT is claimed, it shall be reported as the maximum radial perpendicular
distance from the CT axis sampled on this 8 mm-radius spherical surface.
4.2 Measurement sample density
Within the area defined by the requirement of 4.1, the surface shall be directly sampled in sufficient
locations so that any surface location within the area has a sample taken within 0,5 mm of it.
4.3 Measurement and report of performance
If the performance of a CT for the measurement of either curvature or elevation is claimed or reported,
the testing shall be done in accordance with 5.1, 5.2 and 5.3 and the analysis and reporting of results
shall be performed in accordance with 5.4.
4.4 Colour presentation of results
The CT shall present the results according to the colour palette presented in Annex B.
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5 Test methods and test devices
5.1 Tests
5.1.1 Accuracy test
An accuracy test shall be conducted by measuring a test surface specified in 5.2 using the method
specified in 5.3 and analysing the measured data using the method specified in 5.4.
NOTE An accuracy test tests the ability of a corneal topography system to measure the absolute surface
curvature of a known surface at known locations.
5.1.2 Repeatability test
A repeatability test shall be conducted in order to determine the topographer’s performance in relation
to human interface factors such as eye movements, accuracy and speed of alignment of the instrument
on the eye and the time taken to complete a measurement.
This test shall be conducted in vivo on human eyes. See Annex D.
5.2 Test surfaces
5.2.1 Reflection-based systems
The test surfaces shall be constructed of glass or of optical-grade plastic such as polymethylmethacrylate.
The surfaces shall be optically smooth. The back of the surfaces shall be blackened to avoid unwanted
reflections.
5.2.2 Luminous surface systems
The test surfaces shall be constructed of optical-grade plastic such as polymethylmethacrylate,
impregnated with fluorescent molecules. The surfaces shall be optically smooth. Unwanted reflections
shall be eliminated.
5.2.3 Optical-sectioning systems
The test surfaces shall be constructed of glass or of optical-grade plastic such as polymethylmethacrylate.
If desired, the bulk material from which the surface is formed may be altered to produce a limited amount
of bulk optical scattering to assist in the measuring process. The surfaces shall be optically smooth.
Test surfaces used to establish measurement repeatability may be constructed as meniscus shells.
5.2.4 Specification of test surfaces
The curvature and elevation values of a test surface shall be given in the form of continuous
mathematical expressions along with the specification of the appropriate coordinate system for these
expressions. This ensures that the values for curvature or elevation can be obtained for any given
position on the surface and that this can be done if there is a specified translation or rotation of the
given coordinate system.
This requirement is essential since, when in use, as required in 5.3 and 5.4, the position coordinates
needed to find the parameter values will result from measurements made by the corneal topography
system under test and can therefore take any value within the range of the instrument.
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Specification of the test surface shall include tolerance limits on curvature, expressed as a tolerance on
the radius of curvature given in millimetres, and tolerance limits on elevation given in micrometres.
NOTE Specificat
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