SIST EN ISO 19980:2005

SLOVENSKI STANDARD
SIST EN ISO 19980:2005
01-december-2005
2þHVQDRSWLND±7RSRJUDILURåHQLFH,62
Ophthalmic instruments - Corneal topographers (ISO 19980:2005)
Ophthalmische Instrumente - Hornhauttopographen (ISO 19980:2005)
Instruments ophtalmiques - Topographes de la cornée (ISO 19980:2005)
Ta slovenski standard je istoveten z: EN ISO 19980:2005
ICS:
11.040.70 Oftalmološka oprema Ophthalmic equipment
SIST EN ISO 19980:2005 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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SIST EN ISO 19980:2005

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SIST EN ISO 19980:2005
EUROPEAN STANDARD
EN ISO 19980
NORME EUROPÉENNE
EUROPÄISCHE NORM
August 2005
ICS 11.040.70

English Version
Ophthalmic instruments - Corneal topographers (ISO
19980:2005)
Instruments ophtalmiques - Topographes de la cornée (ISO Ophthalmische Instrumente - Hornhauttopographen (ISO
19980:2005) 19980:2005)
This European Standard was approved by CEN on 1 August 2005.
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 Central Secretariat 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 Central Secretariat has the same status as the official
versions.
CEN members are the national standards bodies of Austria, Belgium, Cyprus, Czech Republic, Denmark, Estonia, Finland, France,
Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Slovakia,
Slovenia, Spain, Sweden, Switzerland and United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
Management Centre: rue de Stassart, 36  B-1050 Brussels
© 2005 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 19980:2005: E
worldwide for CEN national Members.

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SIST EN ISO 19980:2005

EN ISO 19980:2005 (E)





Foreword


This document (EN ISO 19980:2005) has been prepared by Technical Committee ISO/TC 172
"Optics and optical instruments" 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 February 2006, and conflicting national
standards shall be withdrawn at the latest by February 2006.

According to the CEN/CENELEC Internal Regulations, the national standards organizations of
the following countries are bound to implement this European Standard: Austria, Belgium,
Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary,
Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland,
Portugal, Slovakia, Slovenia, Spain, Sweden, Switzerland and United Kingdom.



Endorsement notice

The text of ISO 19980:2005 has been approved by CEN as EN ISO 19980:2005 without any
modifications.

2

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SIST EN ISO 19980:2005


INTERNATIONAL ISO
STANDARD 19980
First edition
2005-08-15


Ophthalmic instruments — Corneal
topographers
Instruments ophtalmiques — Topographes de la cornée





Reference number
ISO 19980:2005(E)
©
ISO 2005

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SIST EN ISO 19980:2005
ISO 19980:2005(E)
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©  ISO 2005
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Published in Switzerland

ii © ISO 2005 – All rights reserved

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SIST EN ISO 19980:2005
ISO 19980:2005(E)
Contents Page
Foreword. iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions. 1
4 Requirements . 8
4.1 Area measured . 8
4.2 Measurement sample density. 8
4.3 Measurement and report of performance . 8
4.4 Colour presentation of results . 8
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. 14
Annex B (normative) Standardized displays for corneal topographers. 18
Annex C (normative) Calculation of area-weighting values . 21
Annex D (normative) Test methods for measuring human corneas. 23
Bibliography . 24

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SIST EN ISO 19980:2005
ISO 19980:2005(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.

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SIST EN ISO 19980:2005
INTERNATIONAL STANDARD ISO 19980:2005(E)

Ophthalmic instruments — Corneal topographers
1 Scope
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.
This International Standard defines certain terms that are specific to the characterization of the corneal shape
so that they may be standardized throughout the field of vision care and have common meaning for all those
who have occasion to participate in this area.
This International Standard specifies minimum requirements for instruments and systems that fall into the
class of corneal topographers. It specifies tests and procedures that will verify that a system or instrument
complies with the standard and so qualifies as a corneal topographer in the meaning of this International
Standard. It specifies certain tests and procedures that will allow the verification of capabilities of systems that
are beyond the minimum required for corneal topographers.
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 60601-1:1988, Medical electrical equipment — Part 1: General requirements for safety
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
eccentricity e (3.9) of the conic section which 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).
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SIST EN ISO 19980:2005
ISO 19980:2005(E)
3.3
corneal meridian
θ
curve created by the intersection of corneal surface and a plane which contains the corneal topographer (CT)
axis
NOTE 1 A meridian is identified by the angle, θ, that the plane creating it makes to the horizontal as described by
ISO 8429.
NOTE 2 The value of θ, for a full meridian, takes values 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 The value of θ for a semi-meridian takes values from 0° to 360°.
3.4
corneal shape factor
E
value which specifies the asphericity and type (prolate or oblate) of conic section which 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 that of the square of the eccentricity and so must always be positive definite,
the sign of E is a convention to signify if 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 E value of e
Value of p
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
instrument or system which measures the shape of corneal surface in a non-contact manner
NOTE A corneal topographer which 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 which measures the corneal surface by analysing multiple optical sections of that surface
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SIST EN ISO 19980:2005
ISO 19980:2005(E)
3.5.2
Placido ring corneal topographer
corneal topographer which 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 which measures the corneal surface using light reflected from the air – pre-corneal tear
film interface
3.5.4
luminous surface corneal topographer
corneal topographer which 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, which 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 CT 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 19980:2005
ISO 19980:2005(E)
3.8 Curvature
−1
NOTE For the purposes of this document, the unit of curvature is mm .
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 CT
axis along the corneal meridian normal at the point (see Figure 2) and given by the equation:
1
K = (1)
a
r
a
where r is the axial radius of curvature
a
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 the equation:
x
p
K xx d
()
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 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 curvature K , and
a a m
meridional radius of curvature r
m
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SIST EN ISO 19980:2005
ISO 19980:2005(E)
3.8.2
Gaussian curvature
product of the two principal normal curvature values at a surface location
NOTE Gaussian curvature has units of reciprocal millimetres squared.
3.8.3
meridional curvature
tangential curvature
K
m
local surface curvature measured in the meridional plane and defined by the equation:
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
CT axis (see Figure 2)
NOTE Meridional curvature is in general not a normal curvature. It is the curvature of the corneal meridian at a point
of a surface.
3.8.4
normal curvature
curvature at a point of 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 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
E = e (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
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SIST EN ISO 19980:2005
ISO 19980:2005(E)
3.10.1
axial elevation
elevation as measured from a selected point on the corneal surface in a direction parallel to the CT 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 the 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 inverse millimetres (mm ) to
keratometric dioptres
3.12
keratometric dioptres
−1
value of curvature, expressed in inverse millimetres (mm ), multiplied by the keratometric constant, 337,5
3.13
meridional plane
plane which 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 (see Table 1), with the
conic section given in the form:
22
zx
±= 1 (5)
22
ba
and the p-value defined by:
2
a
p =± (6)
2
b
E=−1 p (7)
where
a and b are constants;
+ indicates an ellipse;
− indicates a hyperbola
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SIST EN ISO 19980:2005
ISO 19980:2005(E)
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 The units of radius of curvature, for the purpose of this document, are 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 (see Figure 2),
and defined by the equation:
x
r = (8)
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.
3.17.2
meridional radius of curvature
tangential radius of curvature
r
m
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
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SIST EN ISO 19980:2005
ISO 19980:2005(E)
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, which 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 which 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
3.20
transverse plane
plane perpendicular to the meridional plane which 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 corneal topographer shall directly
measure locations on the surface whose radial perpendicular distance from the corneal topographer axis is at
least 3,75 mm. If the maximum area covered by a corneal topographer is claimed, it shall be reported as the
maximum radial perpendicular distance from the corneal topographer axis sampled on this 8 mm radius
spherical surface.
4.2 Measurement sample density
Within the area bounded 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 corneal topographer 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 corneal topographer shall present the results according to the colour presentation definition described in
Annex B.
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SIST EN ISO 19980:2005
ISO 19980:2005(E)
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 value 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 of 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. The back of the
surfaces shall be blackened to avoid unwanted reflections.
Test surfaces for use in establishing the repeatability of measurements 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 necessary as in use, in accordance with the requirements of 5.3 and 5.4, the position
coordinates needed to find the parameter values will result from measurements by the corneal topography
system under test and so can take any value within the range of the instrument.
The specification of test surface shall include tolerance limits on curvature, expressed as a tolerance on radius
of curvature given in millimetres and tolerance limits on elevation given in micrometres.
NOTE Specifications for various test surfaces which have been judged to be useful for the assessment of the
performance of corneal topographers are given in Annex A.
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SIST EN ISO 19980:2005
ISO 19980:2005(E)
5.2.5 Verification of test surfaces
Test surfaces used in accordance with 5.3 shall be verified to conform to their specification given in
accordance with 5.2.4 within the limits specified in accordance with 5.2.4. Verification of elevation may be
done either
a) by direct measure of the surface using profilimetry of precision at least twice that of the tolerance at a
sample density at least that specified for the instrument by 4.2, or
b) by transference methods using a verified master surface and a measurement device of sufficient
precision so that measurement differences of the master surface may be used to correct measured
values of tested surface.
Verification of curvature may be done either
 by mathematical calculation from verified elevation values, or
 by direct physical measurement of curvature with a method of precision twice that of the specified
tolerance limits.
5.2.6 Type testing of surfaces
Three test surfaces as defined in Table 2 should be type tested with every corneal topographer (CT).
The CT should be marked as A or B according to the achieved tolerance level (see Table 3) which is valid for
the three test surfaces mentioned in Table 2.
Table 2 — Test surfaces for type testing
Surface Parameter e Diameter
1) sphere
8,00 mm to 0,2 mm/+0,0 mm W 10 mm
accuracy ± 1 µm
2) ellipsoid of revolution radius of curvature: 0,6 to 0,1 W 10 mm
r = 7,8 mm to 0,3 mm/+0,0 mm
0
accuracy ± 1 µm
3) toric r = 8,0 mm ± 0,2 mm W 10 mm
1
r > r
2 1
r − r = 0,4 ± 0,07 mm
1 2
accuracy ± 1 µm
NOTE 1 According to 1): control measurement possible w
...