SIST EN ISO 19980:2012
(Main)Ophthalmic instruments - Corneal topographers (ISO 19980:2012)
Ophthalmic instruments - Corneal topographers (ISO 19980:2012)
This International Standard 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 International Standard and thus qualifies as a CT according to this International Standard. It also specifies tests and procedures that allow the verification of capabilities of systems that are beyond the minimum requirements for CTs. This International Standard 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 International Standard is applicable to instruments, systems and methods that are intended to measure the surface shape of the cornea of the human eye.
Ophthalmische Instrumente - Hornhauttopographen (ISO 19980:2012)
Diese Internationale Norm gilt für Instrumente, Systeme und Methoden, deren Zweck in der Vermessung der Oberflächenform der Hornhaut eines menschlichen Auges liegt.
ANMERKUNG Diese Messungen sind Messungen der Oberflächenkrümmung in bestimmten Bereichen, drei-dimensio¬nale topographische Messungen der Oberfläche oder andere globale Parameter, die eine Oberfläche charakterisieren.
Diese Internationale Norm gilt nicht für ophthalmische Instrumente, welche in die Klasse der Ophthalmometer fallen.
Diese Internationale Norm legt bestimmte Begriffe fest, die im Zusammenhang mit der Charakterisierung der Hornhautoberfläche eine spezielle Bedeutung haben und für die daher eine einheitliche Festlegung für das gesamte Gebiet der Augenheilkunde und alle mit der Materie befassten Personen naheliegend ist.
Diese Internationale Norm legt die Mindestanforderungen an Instrumente und Systeme, die in die Klasse der Hornhauttopographen fallen, fest. Sie definiert Prüfungen und Verfahren, die belegen, dass ein System oder Instrument der Norm entspricht und so als Hornhauttopograph im Sinne der vorliegenden Internationalen Norm gilt. Sie legt weiterhin Prüfungen und Verfahren fest, die die Überprüfung der Technik von Systemen erlauben, die über den Mindestanforderungen für Hornhauttopographen liegen.
Instruments ophtalmiques - Topographes de la cornée (ISO 19980:2012)
La présente Norme internationale spécifie les exigences minimales relatives aux instruments et systèmes classés parmi les topographes cornéens (TC). Elle spécifie également les essais et modes opératoires permettant de vérifier la conformité d'un système ou d'un instrument à la présente Norme internationale, et de le définir comme étant un TC au sens de la présente Norme internationale. Elle spécifie en outre les essais et modes opératoires permettant de vérifier les aptitudes des systèmes dépassant les exigences minimales relatives aux TC.
La présente Norme internationale définit les termes spécifiques permettant de caractériser la forme de la cornée, de manière à les normaliser dans tout le domaine des soins.
La présente Norme internationale 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.
Elle ne s'applique pas aux instruments ophtalmiques classés parmi les ophtalmomètres.
Oftalmični instrumenti - Topografi roženice (ISO 19980:2012)
Ta mednarodni standard določa minimalne zahteve za instrumente in sisteme, ki spadajo v razred topografov roženice (CT). Določa tudi preskuse in postopke za preverjanje, ali sistem ali instrument izpolnjuje zahteve tega mednarodnega standarda in s tem pogoje za razvrstitev kot topograf roženice v skladu s tem mednarodnim standardom. Poleg tega določa preskuse in postopke, ki omogočajo preverjanje zmogljivosti sistemov, ki presegajo minimalne zahteve glede topografov roženice. Ta mednarodni standard določa izraze, specifične za opis oblike roženice, tako da se lahko standardizirajo za celotno področje okulistike. Ta mednarodni standard se uporablja za instrumente, sisteme in metode, katerih namen je merjenje površinske oblike roženice človeškega očesa.
General Information
Relations
Standards Content (Sample)
SLOVENSKI STANDARD
SIST EN ISO 19980:2012
01-september-2012
1DGRPHãþD
SIST EN ISO 19980:2005
2IWDOPLþQLLQVWUXPHQWL7RSRJUDILURåHQLFH ,62
Ophthalmic instruments - Corneal topographers (ISO 19980:2012)
Ophthalmische Instrumente - Hornhauttopographen (ISO 19980:2012)
Instruments ophtalmiques - Topographes de la cornée (ISO 19980:2012)
Ta slovenski standard je istoveten z: EN ISO 19980:2012
ICS:
11.040.70 Oftalmološka oprema Ophthalmic equipment
SIST EN ISO 19980:2012 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
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SIST EN ISO 19980:2012
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SIST EN ISO 19980:2012
EUROPEAN STANDARD
EN ISO 19980
NORME EUROPÉENNE
EUROPÄISCHE NORM
April 2012
ICS 11.040.70 Supersedes EN ISO 19980:2005
English Version
Ophthalmic instruments - Corneal topographers (ISO
19980:2012)
Instruments ophtalmiques - Topographes de la cornée (ISO Ophthalmische Instrumente - Hornhauttopographen (ISO
19980:2012) 19980:2012)
This European Standard was approved by CEN on 31 March 2012.
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, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
Management Centre: Avenue Marnix 17, B-1000 Brussels
© 2012 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 19980:2012: E
worldwide for CEN national Members.
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SIST EN ISO 19980:2012
EN ISO 19980:2012 (E)
Contents Page
Foreword .3
2
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SIST EN ISO 19980:2012
EN ISO 19980:2012 (E)
Foreword
This document (EN ISO 19980:2012) 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 October 2012, and conflicting national standards shall be withdrawn at
the latest by October 2012.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. CEN [and/or CENELEC] shall not be held responsible for identifying any or all such patent rights.
This document supersedes EN ISO 19980:2005.
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, Romania, Slovakia, Slovenia, Spain,
Sweden, Switzerland, Turkey and the United Kingdom.
Endorsement notice
The text of ISO 19980:2012 has been approved by CEN as a EN ISO 19980:2012 without any modification.
3
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SIST EN ISO 19980:2012
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SIST EN ISO 19980:2012
INTERNATIONAL ISO
STANDARD 19980
Second edition
2012-04-01
Ophthalmic instruments — Corneal
topographers
Instruments ophtalmiques — Topographes de la cornée
Reference number
ISO 19980:2012(E)
©
ISO 2012
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SIST EN ISO 19980:2012
ISO 19980:2012(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2012
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means,
electronic or mechanical, including photocopying and microfilm, without permission in writing from either ISO at the address below or ISO’s
member body in the country of the requester.
ISO copyright office
Case postale 56 • CH-1211 Geneva 20
Tel. + 41 22 749 01 11
Fax + 41 22 749 09 47
E-mail copyright@iso.org
Web www.iso.org
Published in Switzerland
ii © ISO 2012 – All rights reserved
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SIST EN ISO 19980:2012
ISO 19980:2012(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
5.2 Test surfaces . 9
5.3 Data collection — Test surfaces . 11
5.4 Analysis of the data . 11
6 Accompanying documents .13
7 Marking .13
Annex A (informative) Test surfaces for corneal topographers (CTs) .14
Annex B (normative) 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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SIST EN ISO 19980:2012
ISO 19980:2012(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.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
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.
ISO 19980 was prepared by Technical Committee ISO/TC 172, Optics and photonics, Subcommittee SC 7,
Ophthalmic optics and instruments.
This second edition cancels and replaces the first edition (ISO 19980:2005), which has been technically revised.
This corrected version of ISO 19980:2012 incorporates the following corrections:
Equations (7) and (8), which were missing, have been added.
iv © ISO 2012 – All rights reserved
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SIST EN ISO 19980:2012
INTERNATIONAL STANDARD ISO 19980:2012(E)
Ophthalmic instruments — Corneal topographers
1 Scope
This International Standard 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 International Standard and thus qualifies as a CT according to this International Standard.
It also specifies tests and procedures that allow the verification of capabilities of systems that are beyond the
minimum requirements for CTs.
This International Standard 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 International Standard 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.
It is not applicable to ophthalmic instruments classified as ophthalmometers.
2 Normative references
The following referenced documents are indispensable for the application 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, 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.
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 If the meridian is not specified, the corneal eccentricity is that of the flattest corneal meridian (see Table 1
and Annex A).
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 A meridian is identified by the angle θ, that the plane creating it makes to the horizontal (see ISO 8429).
NOTE 2 The value of θ, for a full meridian, ranges from 0° to 180° .
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SIST EN ISO 19980:2012
ISO 19980:2012(E)
3.3.1
corneal semi-meridian
portion of a full meridian extending from the CT axis toward the periphery in one direction
NOTE 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 Unless otherwise specified, it refers to the meridian with least curvature (flattest meridian). See Table 1 and Annex A.
NOTE 2 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 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 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
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
2 © ISO 2012 – All rights reserved
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SIST EN ISO 19980:2012
ISO 19980:2012(E)
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 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.
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
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SIST EN ISO 19980:2012
ISO 19980:2012(E)
3.8 Curvature
NOTE For the purposes of this International Standard, the unit of curvature is reciprocal millimetre.
3.8.1 Axial curvature
3.8.1.1
axial curvature
sagittal curvature
K
a
〈calculated using the axial radius of 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):
1
K = (1)
a
r
a
where r is the axial radius of curvature
a
See Figure 2.
3.8.1.2
axial curvature
K
a
〈calculated using the meridional curvature〉 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
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SIST EN ISO 19980:2012
ISO 19980:2012(E)
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 ,
a a
meridional 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 Gaussian curvature is expressed in reciprocal square millimetres.
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 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
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SIST EN ISO 19980:2012
ISO 19980:2012(E)
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 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
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
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SIST EN ISO 19980:2012
ISO 19980:2012(E)
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
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 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 φ x
()
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SIST EN ISO 19980:2012
ISO 19980:2012(E)
where
x
is the perpendicular distance from the axis to the meridian point, in millimetres;
f(x) is the angle between the axis and the meridian normal at point 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 Atoric surfaces are symmetrical with respect to both principal meridians.
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 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 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
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SIST EN ISO 19980:2012
ISO 19980:2012(E)
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 pallet presented in Annex B.
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. 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.
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SIST EN ISO 19980:2012
ISO 19980:2012(E)
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.
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 Specifications for various test surfaces that have been judged to be useful for assessing the performance of
CTs are given in Annex A.
5.2.5 Verification of test surfaces
Conformity to the specifications of 5.2.4 for test surfaces used in accordance with 5.3 shall be verified within
the limits specified in 5.2.4. Verification of elevation may be done either:
a) by direct measure of the surface using profilometry with a precision of at least twice the tolerance, at a
sample density of at least that specified for the instrument in 4.2,
or
b) by transference methods using a verified master surface and a measurement device of sufficient precision that
measurement differences of the master surface may be used to correct measured values of the tested surface.
Verification of curvature may be done either:
— by mathematical calculation from verified elevation values,
or
— by direct physical measurement of the curvature using a method that has a precision of twice the specified
tolerance limits.
5.2.6 Type testing of surfaces
Five test surfaces as defined in Table 2 should be type-tested with every CT.
The CT should be marked A or B according to the achieved tolerance level (see Table 3) valid for the five test
surfaces mentioned in Table 2.
10 © ISO 2012 – All rights reserved
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SIST EN ISO 19980:2012
ISO 19980:2012(E)
Table 2 — Test surfaces for type testing
Surface Radius of curvature e Diameter
1) sphere ≥10 mm
+00,
65, 0 mm
()
−02,
accuracy ±1 µm
2) sphere ≥10 mm
+00,
80, 0 mm
()
−02,
accuracy ±1 µm
3) sphere ≥10 mm
+00,
95, 0 mm
()
−02,
accuracy ±1 µm
4) ellipsoid of revolution 0,6 ± 0,1 ≥10 mm
+00,
r = 78, 0 mm
...
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