Ophthalmic instruments - Corneal topographers (ISO 19980:2021)

Ophthalmische Instrumente - Hornhauttopographen (ISO 19980:2021)

Dieses Dokument legt die Mindestanforderungen an Instrumente und Systeme, die in die Klasse der Hornhauttopographen fallen, fest. Sie definiert auch die Prüfungen und Verfahren zum Nachweis, dass ein System oder Instrument dieses Dokument entspricht und somit als Hornhauttopograph im Sinne des vorliegenden Dokuments gilt. Sie legt weiterhin Prüfungen und Verfahren fest, die die Über¬prüfung der Technik 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 daher eine einheitliche Festlegung für das gesamte Gebiet der Augenheilkunde naheliegend ist.
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-dimensio¬nale 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)

Oftalmični instrumenti - Topografi roženice (ISO 19980:2021)

General Information

Status
Published
Technical Committee
Current Stage
6060 - Definitive text made available (DAV) - Publishing
Due Date
16-Jun-2021
Completion Date
16-Jun-2021

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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
THIS DOCUMENT IS A DRAFT CIRCULATED
This document is circulated as received from the committee secretariat.
FOR COMMENT AND APPROVAL. IT IS
THEREFORE SUBJECT TO CHANGE AND MAY
NOT BE REFERRED TO AS AN INTERNATIONAL
STANDARD UNTIL PUBLISHED AS SUCH.
IN ADDITION TO THEIR EVALUATION AS
ISO/CEN PARALLEL PROCESSING
BEING ACCEPTABLE FOR INDUSTRIAL,
TECHNOLOGICAL, COMMERCIAL AND
USER PURPOSES, DRAFT INTERNATIONAL
STANDARDS MAY ON OCCASION HAVE TO
BE CONSIDERED IN THE LIGHT OF THEIR
POTENTIAL TO BECOME STANDARDS TO
WHICH REFERENCE MAY BE MADE IN
Reference number
NATIONAL REGULATIONS.
ISO/DIS 19980:2019(E)
RECIPIENTS OF THIS DRAFT ARE INVITED
TO SUBMIT, WITH THEIR COMMENTS,
NOTIFICATION OF ANY RELEVANT PATENT
RIGHTS OF WHICH THEY ARE AWARE AND TO
PROVIDE SUPPORTING DOCUMENTATION. ISO 2019
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oSIST prEN ISO 19980:2019
ISO/DIS 19980:2019(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2019

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
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Published in Switzerland
ii © ISO 2019 – All rights reserved
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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

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.

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.
iv © ISO 2019 – All rights reserved
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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

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

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

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
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
Prolate ellipse 1 > p > 0 0 < E < 1 0 < e < 1
Sphere 1,0 0,0 0,0
Oblate ellipse p > 1 E < 0 0 < e < 1
See 3.15.

The eccentricity, e, does not distinguish between prolate and oblate orientations of an ellipse (see 3.9 and

Annex A).
3.5
corneal topographer

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

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):

K = (1)
where r is the axial radius of curvature
SEE: Figure 2.
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3.8.1.2
axial curvature

average of the value of the tangential curvature from the

corneal vertex to the meridional point and given by Equation (2):
Kx dx
K = (2)
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.
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

local surface curvature measured in the meridional plane and defined by Equation (3):

∂ Mx /∂x
K = (3)
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

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)
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

conversion value equal to 337,5 used to convert corneal curvature from reciprocal millimetres (mm )

to keratometric dioptres
3.12
keratometric dioptres

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):
p=± (6)
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

distance from a surface point, P, to the axis along the normal to corneal meridian at that point, and

defined by Equation (8):
r = (8)
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

distance from a surface point, P, and the centre of the meridional curvature point, and defined by

Equation (9):
r = (9)
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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