ISO 19980:2012
(Main)Ophthalmic instruments — Corneal topographers
Ophthalmic instruments — Corneal topographers
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.
Instruments ophtalmiques — Topographes de la cornée
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.
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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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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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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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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.
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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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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
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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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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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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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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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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
(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):
(8)
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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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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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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.
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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
0 ()
−03,
accuracy ±1 µm
5) toric r = 8,0 mm ± 0,2 mm ≥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 with a micrometer unit.
NOTE 2 According to 2) and 3): an ellipsoid and toric shape can be manufactured by a contact lens company and measured with a
3D-coordinate measuring device.
Table 3 — Tolerance level for test surfaces
Tolerances, if measurements are expressed in terms of radius of curvature, in millimetres
Area
Measuring accuracy Type
Centre diameter Middle diameter Outer diameter
Twice the standard deviation A 0,05 0,03 0,03
Twice the standard deviation B 0,1 0,07 0,07
Tolerances, if measurements are expressed in terms of curvature, in keratometric dioptres
Area
Measuring accuracy Type
Centre diameter Middle diameter Outer diameter
Twice the standard deviation A 0,27 0,16 0,16
Twice the standard deviation B 0,52 0,37 0,37
NOTE Keratometric dioptres are related to the radius of curvature given in millimetres, using the formula: keratometric dioptres
= 337,5/radius of curvature.
5.3 Data collection — Test surfaces
Align the test surface to the instrument in the manner specified by the manufacturer of the system for measuring
human eyes. Measure the surface and save the measured data. At each measured point, the data set consists
of the value of the measured variable and the two-dimensional position of the measurement.
5.4 Analysis of the data
5.4.1 General
The treatment of the corneal topographic data consists of a comparison between the measured values of two
data sets. The structure of the data sets is slightly different for the analysis of accuracy and the analysis of
repeatability, so they will be given separately.
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ISO 19980:2012(E)
5.4.2 Structure of the accuracy data set
For the purpose of accuracy determination, one data set consists of the measured values and measurement
locations from the measurement of a known test surface. The other data set consists of the known values of
the test surface at the locations measured by the instrument and reported as part of the data set. The analysis
of the paired sets of data is done in accordance with 5.4.3.
5.4.3 Analysis of the paired data sets
For each data set pair, a difference in measured values is taken. This gives rise to a data set of difference
values, designated ΔD , for each measured point on the corneal surface. The indices i and j label the two data
ijk
sets used. The index k labels the position of the individual points. The position is specified by two coordinate
values which may be, for instance, the meridian θ and radial position x on which the point lies. The known
values for the test surface are calculated from knowledge of its surface shape and the measured position.
The difference values, ΔD , are next grouped into subsets based on their position values. Each subset is
ijk
associated with one of the measurement zones specified in Table 4 and comprised of those data points whose
positions are within that measurement zone.
Table 4 — Analysis zones for accuracy and repeatability testing
Zone
Central: 1 mm ≤ diameter ≤ 3 mm
Middle: 3 mm < diameter ≤ 6 mm
Outer: diameter > 6 mm
Each subset of difference values is then treated as an ensemble. The mean values, M , and standard
ij
deviations, s , are taken for an ensemble, where
ij
ΔDw=−DD (10)
()
ijk kikjk
n
1
M = ΔD (11)
ij ∑ ijk
n
k=1
n
2
ΔDM−
()
ijk ij
∑
k=1
s = (12)
ij
n−1
where
n is the number of measured points;
i, j are the indices specifying the two data sets;
k is the index specifying the point location;
D is data value at point k (it can be a curvature value, a power value or an elevation value);
ik
M is the ensemble difference mean for the data sets i and j;
ij
s is the standard deviation of the ensemble differences for the data sets i and j;
ij
w is the area weighting value for position k as found using the method given in Annex C.
k
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ISO 19980:2012(E)
5.4.4 Report of accuracy performance
The accuracy performance of a corneal topography system shall be described by reporting the following information:
a) specifications of test surface used;
b) orientation of test surface with respect to the CT axis;
c) mean difference for each zone according to Table 4;
d) twice the standard deviation of differences for each zone according to Table 4.
6 Accompanying documents
The CT shall be accompanied by documents containing instructions for use together with maintenance
procedures and their frequency of application. In particular this information shall contain:
a) name and address of manufacturer;
b) a list of accessories suitable for use with the CT;
c) a reference to this International Standard, i.e. ISO 19980:2012, if the manufacturer claims compliance;
d) any additional documents as specified in 7.9 of I
...
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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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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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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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.
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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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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
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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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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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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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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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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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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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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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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.
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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
0 ()
−03,
accuracy ±1 µm
5) toric r = 8,0 mm ± 0,2 mm ≥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 with a micrometer unit.
NOTE 2 According to 2) and 3): an ellipsoid and toric shape can be manufactured by a contact lens company and measured with a
3D-coordinate measuring device.
Table 3 — Tolerance level for test surfaces
Tolerances, if measurements are expressed in terms of radius of curvature, in millimetres
Area
Measuring accuracy Type
Centre diameter Middle diameter Outer diameter
Twice the standard deviation A 0,05 0,03 0,03
Twice the standard deviation B 0,1 0,07 0,07
Tolerances, if measurements are expressed in terms of curvature, in keratometric dioptres
Area
Measuring accuracy Type
Centre diameter Middle diameter Outer diameter
Twice the standard deviation A 0,27 0,16 0,16
Twice the standard deviation B 0,52 0,37 0,37
NOTE Keratometric dioptres are related to the radius of curvature given in millimetres, using the formula: keratometric dioptres
= 337,5/radius of curvature.
5.3 Data collection — Test surfaces
Align the test surface to the instrument in the manner specified by the manufacturer of the system for measuring
human eyes. Measure the surface and save the measured data. At each measured point, the data set consists
of the value of the measured variable and the two-dimensional position of the measurement.
5.4 Analysis of the data
5.4.1 General
The treatment of the corneal topographic data consists of a comparison between the measured values of two
data sets. The structure of the data sets is slightly different for the analysis of accuracy and the analysis of
repeatability, so they will be given separately.
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ISO 19980:2012(E)
5.4.2 Structure of the accuracy data set
For the purpose of accuracy determination, one data set consists of the measured values and measurement
locations from the measurement of a known test surface. The other data set consists of the known values of
the test surface at the locations measured by the instrument and reported as part of the data set. The analysis
of the paired sets of data is done in accordance with 5.4.3.
5.4.3 Analysis of the paired data sets
For each data set pair, a difference in measured values is taken. This gives rise to a data set of difference
values, designated ΔD , for each measured point on the corneal surface. The indices i and j label the two data
ijk
sets used. The index k labels the position of the individual points. The position is specified by two coordinate
values which may be, for instance, the meridian θ and radial position x on which the point lies. The known
values for the test surface are calculated from knowledge of its surface shape and the measured position.
The difference values, ΔD , are next grouped into subsets based on their position values. Each subset is
ijk
associated with one of the measurement zones specified in Table 4 and comprised of those data points whose
positions are within that measurement zone.
Table 4 — Analysis zones for accuracy and repeatability testing
Zone
Central: 1 mm ≤ diameter ≤ 3 mm
Middle: 3 mm < diameter ≤ 6 mm
Outer: diameter > 6 mm
Each subset of difference values is then treated as an ensemble. The mean values, M , and standard
ij
deviations, s , are taken for an ensemble, where
ij
ΔDw=−DD (10)
()
ijk kikjk
n
1
M = ΔD (11)
ij ∑ ijk
n
k=1
n
2
ΔDM−
()
ijk ij
∑
k=1
s = (12)
ij
n−1
where
n is the number of measured points;
i, j are the indices specifying the two data sets;
k is the index specifying the point location;
D is data value at point k (it can be a curvature value, a power value or an elevation value);
ik
M is the ensemble difference mean for the data sets i and j;
ij
s is the standard deviation of the ensemble differences for the data sets i and j;
ij
w is the area weighting value for position k as found using the method given in Annex C.
k
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ISO 19980:2012(E)
5.4.4 Report of accuracy performance
The accuracy performance of a corneal topography system shall be described by reporting the following information:
a) specifications of test surface used;
b) orientation of test surface with respect to the CT axis;
c) mean difference for each zone according to Table 4;
d) twice the standard deviation of differences for each zone according to Table 4.
6 Accompanying documents
The CT shall be accompanied by documents containing instructions for use together with maintenance
procedures and their frequency of application. In particular this information shall contain:
a) name and address of manufacturer;
b) a list of accessories suitable for use with the
...
NORME ISO
INTERNATIONALE 19980
Deuxième édition
2012-04-01
Instruments ophtalmiques —
Topographes de la cornée
Ophthalmic instruments — Corneal topographers
Numéro de référence
ISO 19980:2012(F)
©
ISO 2012
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ISO 19980:2012(F)
DOCUMENT PROTÉGÉ PAR COPYRIGHT
© ISO 2012
Droits de reproduction réservés. Sauf prescription différente, aucune partie de cette publication ne peut être reproduite ni utilisée sous
quelque forme que ce soit et par aucun procédé, électronique ou mécanique, y compris la photocopie et les microfilms, sans l’accord écrit
de l’ISO à l’adresse ci-après ou du comité membre de l’ISO dans le pays du demandeur.
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
Publié en Suisse
ii © ISO 2012 – Tous droits réservés
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ISO 19980:2012(F)
Sommaire Page
Avant-propos .iv
1 Domaine d’application . 1
2 Références normatives . 1
3 Termes et définitions . 1
4 Exigences . 9
4.1 Zone mesurée . 9
4.2 Densité d’échantillonnage du mesurage . 9
4.3 Mesurage et rapport de performances . 9
4.4 Présentation colorée des résultats . 9
5 Méthodes d’essai et dispositifs d’essai . 9
5.1 Essais . 9
5.2 Surfaces d’essai .10
5.3 Collecte de données, surfaces d’essai .12
5.4 Analyse des données .12
6 Documents d’accompagnement .13
7 Marquage .13
Annexe A (informative) Surfaces d’essai des topographes cornéens (TC) .14
Annexe B (normative) Affichages normalisés des topographes cornéens (TC) .16
Annexe C (normative) Calcul des valeurs de pondération de zone.20
Annexe D (normative) Méthodes d’essai de mesure des cornées humaines .22
Bibliographie .23
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ISO 19980:2012(F)
Avant-propos
L’ISO (Organisation internationale de normalisation) est une fédération mondiale d’organismes nationaux de
normalisation (comités membres de l’ISO). L’élaboration des Normes internationales est en général confiée aux
comités techniques de l’ISO. Chaque comité membre intéressé par une étude a le droit de faire partie du comité
technique créé à cet effet. Les organisations internationales, gouvernementales et non gouvernementales,
en liaison avec l’ISO participent également aux travaux. L’ISO collabore étroitement avec la Commission
électrotechnique internationale (CEI) en ce qui concerne la normalisation électrotechnique.
Les Normes internationales sont rédigées conformément aux règles données dans les Directives ISO/CEI, Partie 2.
La tâche principale des comités techniques est d’élaborer les Normes internationales. Les projets de Normes
internationales adoptés par les comités techniques sont soumis aux comités membres pour vote. Leur publication
comme Normes internationales requiert l’approbation de 75 % au moins des comités membres votants.
L’attention est appelée sur le fait que certains des éléments du présent document peuvent faire l’objet de droits
de propriété intellectuelle ou de droits analogues. L’ISO ne saurait être tenue pour responsable de ne pas avoir
identifié de tels droits de propriété et averti de leur existence.
L’ISO 19980 a été élaborée par le comité technique ISO/TC 172, Optique et photonique, sous-comité SC 7,
Optique et instruments ophtalmiques.
Cette deuxième édition annule et remplace la première édition (ISO 19980:2005), qui a fait l’objet d’une
révision technique.
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NORME INTERNATIONALE ISO 19980:2012(F)
Instruments ophtalmiques — Topographes de la cornée
1 Domaine d’application
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.
2 Références normatives
Les documents de référence suivants sont indispensables pour l’application du présent document. Pour les
références datées, seule l’édition citée s’applique. Pour les références non datées, la dernière édition du
document de référence (y compris les éventuels amendements) s’applique.
CEI 60601-1:2005, Appareils électromédicaux — Partie 1: Exigences générales pour la sécurité de base et les
performances essentielles
3 Termes et définitions
Pour les besoins du présent document, les termes et définitions suivants s’appliquent.
3.1
apex cornéen
point de la surface cornéenne où la moyenne de la courbure principale locale est la plus élevée
3.2
excentricité cornéenne
e
c
excentricité, e, de la section conique s’adaptant le mieux au méridien cornéen étudié
NOTE Si le méridien cornéen n’est pas spécifié, l’excentricité cornéenne est celle du méridien cornéen le plus plat
(voir Tableau 1 et Annexe A).
3.3
méridien cornéen
q
courbe résultant de l’intersection de la surface cornéenne et d’un plan contenant l’axe du topographe cornéen
NOTE 1 Un méridien est identifié par l’angle θ du plan qui le crée avec l’horizontale (voir l’ISO 8429).
NOTE 2 La valeur θ d’un méridien complet est comprise entre 0° et 180°.
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ISO 19980:2012(F)
3.3.1
semi-méridien cornéen
partie d’un méridien complet qui s’étend de l’axe du topographe cornéen vers la périphérie, dans une direction
NOTE La valeur θ d’un semi-méridien est comprise entre 0° et 360°.
3.4
facteur de forme cornéen
E
valeur qui spécifie l’asphéricité et le type (allongé ou aplati) de la section conique qui s’adapte le mieux à un
méridien cornéen
NOTE 1 Sauf spécification contraire, ce facteur fait référence au méridien présentant la courbure la plus faible (méridien
le plus plat) (voir Tableau 1 et Annexe A).
NOTE 2 Bien que l’amplitude de E soit égale au carré de l’excentricité et qu’elle doive donc être toujours positive, le
signe de E est une convention visant à signifier si une ellipse prend une orientation allongée ou aplatie.
NOTE 3 La valeur négative de E est définie par l’ISO 10110-12 comme étant la constante conique désignée par le
symbole K. La valeur négative de E est également appelée asphéricité et a donné le symbole Q.
Tableau 1 — Descripteurs de section conique
a
Section conique Valeur de p Valeur de E Valeur de e
Hyperbole p < 0 E > 1 e > 1
Parabole 0,0 1,0 1,0
b
Ellipse allongée 1 > p > 0 0 < E < 1 0 < e < 1
Sphère 1,0 0,0 0,0
b
Ellipse aplatie p > 1 E < 0 0 < e < 1
a
Voir 3.15.
b
L’excentricité, e, ne fait pas la distinction entre les orientations allongées ou aplaties d’une
ellipse (voir 3.9 et Annexe A).
3.5
topographe cornéen
TC
instrument ou système permettant de mesurer la forme d’une surface cornéenne sans entrer en contact
avec celle-ci
NOTE Un topographe cornéen qui utilise un système vidéo et un système de traitement des images pour mesurer la
surface cornéenne par analyse de l’image reflétée créée par la surface cornéenne d’une cible lumineuse est également
appelé vidéo-kératographe.
3.5.1
topographe cornéen à sectionnement optique
topographe cornéen qui mesure la surface cornéenne en analysant plusieurs de ses sections optiques
3.5.2
topographe cornéen à anneau de Placido
topographe cornéen permettant de mesurer la surface cornéenne en analysant l’image reflétée de la cible d’un
anneau de Placido créée par la surface cornéenne
3.5.3
topographe cornéen fondé sur la réflexion
topographe cornéen permettant de mesurer la surface cornéenne à l’aide de la lumière reflétée sur l’interface
air/film lacrymal précornéen
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ISO 19980:2012(F)
3.5.4
topographe cornéen à surface lumineuse
topographe cornéen permettant de mesurer la surface cornéenne par rétrodiffusion lumineuse à partir d’une
cible projetée sur le film lacrymal précornéen ou la surface du tissu antérieur cornéen
NOTE La rétrodiffusion lumineuse est en général introduite dans ces substances claires d’un point de vue optique
en ajoutant un matériau fluorescent dans le film lacrymal précornéen. Une cible peut comporter une fente, une fente
d’exploration de lumière ou un autre motif lumineux de projection. D’autres méthodes sont possibles.
3.6
axe du topographe cornéen
axe TC
ligne parallèle à l’axe optique de l’instrument avec lequel elle coïncide souvent, servant d’axe de coordonnées
permettant de décrire et de définir la forme de la cornée
3.7
sommet cornéen
point de tangence entre un plan perpendiculaire à l’axe du topographe cornéen et la surface cornéenne
Voir Figure 1.
Légende
1 sommet cornéen
2 apex
3 rayon de courbure au niveau de l’apex
4 centre du point de courbure du méridien
5 section transversale de la surface cornéenne
6 plan perpendiculaire à l’axe TC
7 axe TC
Figure 1 — Illustration des sommets et apex cornéens
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ISO 19980:2012(F)
3.8 Courbure
NOTE Pour les besoins de la présente Norme internationale, l’unité de courbure utilisée est le millimètre réciproque.
3.8.1 Courbure axiale
3.8.1.1
courbure axiale
courbure sagittale
K
a
〈calculée à l’aide du rayon de courbure axial〉 réciproque de la distance entre un point d’une surface et l’axe TC
le long de la normale du méridien cornéen au niveau du point et donnée par l’Équation (1):
1
K = (1)
a
r
a
où r est le rayon de courbure axial
a
Voir Figure 2.
3.8.1.2
courbure axiale
K
a
〈calculée à l’aide de la courbure méridienne〉 moyenne de la valeur de la courbure tangentielle entre le sommet
cornéen et le point méridien, donnée par l’Équation (2):
x
p
Kx dx
()
m
∫
0
K = (2)
a
x
p
où
x est la position radiale variable sur le méridien;
x est la position radiale à laquelle K est évaluée;
p a
K est la courbure méridienne.
m
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ISO 19980:2012(F)
Légende
1 normale au méridien au point P
2 P, point du méridien sur lequel doit se trouver la courbure
3 centre du point de courbure du méridien
4 normale d’intersection — axe TC
5 méridien (section transversale de la surface cornéenne)
6 axe TC
Figure 2 — Illustration de la courbure axiale, K , du rayon de courbure axial, r ,
a a
de la courbure méridienne, K , et du rayon de courbure méridien, r
m m
3.8.2
courbure gaussienne
produit des deux principales valeurs de courbure normale à un endroit de la surface
NOTE La courbure gaussienne est exprimée en millimètres carrés réciproques.
3.8.3
courbure méridienne
courbure tangentielle
K
m
courbure de surface locale mesurée dans le plan méridien et défini par l’Équation (3):
22
∂ Mx / ∂x
()
K = (3)
m
3
2 2
1+∂Mx / ∂x
()
{}
où M(x) est une fonction donnant l’élévation du méridien à une distance perpendiculaire, x, par rapport à l’axe
du topographe cornéen
NOTE En règle générale, la courbure méridienne n’est pas une courbure normale. Il s’agit de la courbure du méridien
cornéen en un point d’une surface.
Voir Figure 2.
3.8.4
courbure normale
courbure en un point de la surface de la courbe créée par l’intersection de la surface avec un plan contenant
la normale à la surface en ce point
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ISO 19980:2012(F)
3.8.4.1
courbure moyenne
moyenne arithmétique des courbures principales en un point de la surface
3.8.4.2
courbure principale
courbure maximale ou minimale en un point de la surface
3.9
excentricité
e
valeur descriptive d’une section conique et du taux de changement de courbure par rapport à l’apex de la
courbe, soit la vitesse d’aplatissement ou de raidissement de la courbe par rapport à l’apex de la surface
NOTE L’excentricité du groupe de sections coniques suivant est comprise entre zéro et l’infini positif:
— cercle (e = 0);
— ellipse (0 < e < 1);
— parabole (e = 1);
— hyperbole (e > 1)
2
Ee= (4)
Pour signifier l’utilisation d’une courbe aplatie de l’ellipse, e, est parfois précédé d’un signe négatif qui n’est pas pris en
compte dans les calculs. Sinon, la courbe allongée de l’ellipse est supposée être utilisée.
3.10
élévation
distance entre une surface cornéenne et une surface de référence définie, mesurée dans une direction définie
par rapport à une position spécifiée
3.10.1
élévation axiale
élévation mesurée à partir d’un point sélectionné de la surface cornéenne dans une direction parallèle à l’axe
du topographe cornéen
3.10.2
élévation normale
élévation mesurée à partir d’un point sélectionné de la surface cornéenne le long de la normale à la surface
cornéenne en ce point
3.10.3
élévation normale de référence
élévation mesurée à partir d’un point sélectionné de la surface cornéenne le long de la normale à la
surface de référence
3.11
constante kératométrique
valeur de conversion égale à 337,5 utilisée pour convertir la courbure cornéenne exprimée en millimètres
−1
réciproques (mm ) en dioptres kératométriques
3.12
dioptres kératométriques
−1
valeur de courbure, exprimée en millimètres réciproques (mm ), multipliée par la constante kératométrique 337,5
3.13
plan méridien
plan contenant le point de surface et l’axe choisi
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ISO 19980:2012(F)
3.14 Normale
3.14.1
normale à la surface
ligne passant par un point de la surface perpendiculaire au plan tangent à la surface en ce point
3.14.2
normale méridienne
droite passant par un point de la surface, perpendiculaire à la tangente à la courbe méridienne en ce point et
se trouvant dans le plan créant le méridien
3.15
valeur p
nombre spécifiant une section conique (par exemple une ellipse, une hyperbole ou une parabole) donnée par
l’Équation (5):
2 2
z x
±=1 (5)
2 2
b a
la valeur p étant définie par l’Équation (6):
2
a
p =± (6)
2
b
E = 1 − p (7)
où
a et b sont des constantes;
+ indique une ellipse;
− indique une hyperbole.
Voir Tableau 1.
3.16
cible d’un anneau de Placido
cible composée de plusieurs anneaux concentriques dans laquelle chaque anneau individuel se trouve dans
un plan, les anneaux n’étant en général pas coplanaires
3.17
rayon de courbure
réciproque de la courbure
NOTE Pour les besoins de la présente Norme internationale, le rayon de courbure est exprimé en millimètres.
3.17.1
rayon de courbure axial
rayon de courbure sagittal
r
a
distance entre un point d’une surface, P, et l’axe le long de la normale au méridien cornéen en ce point et
définie par l’Équation (8):
x
r = (8)
a
sinφ x
()
où
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ISO 19980:2012(F)
x est la distance perpendiculaire entre l’axe et le point méridien, en millimètres;
f(x) est l’angle entre l’axe et la normale méridienne au point x.
Voir Figure 2.
3.17.2
rayon de courbure méridien
rayon de courbure tangentiel
r
m
distance entre un point d’une surface, P, et le centre du point de courbure du méridien, et définie par l’Équation (9):
1
r = (9)
m
K
m
Voir Figure 2.
3.18 Surface
3.18.1
surface asphérique
surface non sphérique
surface dont au moins un méridien principal n’est pas circulaire dans la section transversale
3.18.2
surface atorique
surface comportant des méridiens principaux mutuellement perpendiculaires et de courbure inégale dont au
moins un méridien principal n’est pas circulaire dans la section transversale
NOTE Les surfaces atoriques sont symétriques par rapport aux deux méridiens principaux.
3.18.3
surface aplatie
surface dont la courbure augmente au fur et à mesure du déplacement de la surface du centre à la périphérie
dans tous les méridiens
3.18.4
surface allongée
surface dont la courbure diminue au fur et à mesure du déplacement de la surface du centre à la périphérie
dans tous les méridiens
3.18.5
surface de référence
surface qui peut être décrite de manière exacte, de préférence mathématique, faisant office de référence pour
mesurer la distance par rapport à la surface cornéenne mesurée et dont, outre la description mathématique,
la relation de position par rapport à la surface cornéenne est spécifiée
NOTE Par exemple, une surface de référence peut être décrite comme étant une sphère qui est le meilleur ajustement
par les moindres carrés à la surface cornéenne mesurée. De même, un plan peut servir de surface de référence.
3.18.6
surface torique
surface dont les courbures principales ne sont pas égales et dont les méridiens principaux sont des
sections circulaires
NOTE Ces surfaces sont censées produire un astigmatisme central.
3.19
toricité
différence de courbures principales en un point spécifié ou une zone locale spécifiée d’une surface
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ISO 19980:2012(F)
3.20
plan transversal
plan perpendiculaire au plan méridien contenant la normale au point de la surface
4 Exigences
4.1 Zone mesurée
Lors du mesurage d’une surface sphérique présentant un rayon de courbure de 8 mm, un TC doit directement
mesurer les emplacements sur la surface dont la distance perpendiculaire radiale par rapport à l’axe TC
est d’au moins 3,75 mm. Si la zone maximale couverte par un TC est demandée, elle doit être reportée
comme étant la distance perpendiculaire radiale maximale par rapport à l’axe TC échantillonné sur une surface
sphérique présentant un rayon de 8 mm.
4.2 Densité d’échantillonnage du mesurage
Dans la zone limitée par l’exigence de 4.1, la surface doit être directement échantillonnée dans un nombre
suffisant d’emplacements de sorte qu’un emplacement de surface à l’intérieur de la zone contienne un
échantillon prélevé dans 0,5 mm de celle-ci.
4.3 Mesurage et rapport de performances
Si les performances de mesure de la courbure ou de l’élévation offertes par un TC sont demandées ou
reportées, l’essai doit être réalisé conformément à 5.1, 5.2 et 5.3, l’analyse et la génération du rapport des
résultats devant être réalisés conformément à 5.4.
4.4 Présentation colorée des résultats
Le TC doit présenter les résultats conformément à la palette présentée dans l’Annexe B.
5 Méthodes d’essai et dispositifs d’essai
5.1 Essais
5.1.1 Essai d’exactitude
Un essai d’exactitude doit être réalisé en mesurant une surface d’essai spécifiée en 5.2 à l’aide de la méthode
indiquée en 5.3, puis en analysant les données mesurées à l’aide de la méthode décrite en 5.4. Un essai
d’exactitude permet de soumettre à essai l’aptitude d’un système de topographie cornéenne à mesurer la
courbure absolue d’une surface connue en des emplacements connus.
5.1.2 Essai de répétabilité
Un essai de répétabilité doit être réalisé afin de déterminer les performances liées aux facteurs d’interface
humaine du topographe (par exemple le mouvement des yeux, l’exactitude et la vitesse d’alignement de
l’instrument sur l’œil) ainsi que la durée complète de mesure.
Cet essai doit être réalisé in vivo sur les yeux. Voir Annexe D.
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ISO 19980:2012(F)
5.2 Surfaces d’essai
5.2.1 Systèmes fondés sur la réflexion
Les surfaces d’essai doivent être en verre ou en plastique de qualité optique (par exemple polyméthacrylate
de méthyle). Les surfaces doivent doit être lisses du point de vue optique. L’arrière des surfaces doit être noirci
pour éviter les réflexions indésirables.
5.2.2 Systèmes à surface lumineuse
Les surfaces d’essai doivent être en plastique de qualité optique (par exemple polyméthacrylate de méthyle)
imprégné de molécules fluorescentes. Les surfaces doivent doit être lisses du point de vue optique. Les
réflexions indésirables doivent être éliminées.
5.2.3 Systèmes à sectionnement optique
Les surfaces d’essai doivent être en verre ou en plastique de qualité optique (par exemple polyméthacrylate
de méthyle). Le cas échéant, le matériau de base dont est formée la surface peut être altéré pour produire une
quantité limitée de dispersion optique globale afin de faciliter le processus de mesure. Les surfaces doivent
être lisses du point de vue optique.
Les surfaces d’essai utilisées pour établir la répétabilité de mesure doivent être conçues sous la forme d’un ménisque.
5.2.4 Spécification des surfaces d’essai
Les valeurs de courbure et d’élévation d’une surface d’essai doivent être données sous la forme d’expressions
mathématiques continues, en spécifiant le système de coordonnées approprié de ces expressions. Il s’agit de
s’assurer que les valeurs de courbure ou d’élévation peuvent être obtenues quelle que soit la position sur la
surface, cela étant possible si la translation ou la rotation du système de coordonnées donné est spécifiée. Cette
exigence est indispensable étant donné que pendant l’utilisation, conformément aux exigences de 5.3 et 5.4,
les coordonnées de position nécessaires à la recherche des valeurs de paramètre résultent des mesurages
réalisés par le système de topographie cornéenne soumis à essai, et peuvent donc prendre n’importe quelle
valeur dans la plage de l’instrument.
La spécification de la surface d’essai doit inclure les limites de tolérance de la courbure, exprimées comme
étant la tolérance du rayon de courbure, en millimètres, et les limites de tolérance de l’élévation, en micromètres.
NOTE Les spécifications relatives aux différentes surfaces d’essai jugées utiles pour l’évaluation des performances
des topographes cornéens sont données dans l’Annexe A.
5.2.5 Vérification des surfaces d’essai
La conformité aux spécifications données en 5.2.4 des surfaces d’essai utilisées selon 5.3 doit être vérifiée
dans les limites spécifiées en 5.2.4. L’élévation peut être vérifiée
a) par mesurage direct de la surface par profilométrie offrant une précision au moins deux fois supérieure à celle
de la tolérance, à une densité d’échantillon représentant au moins celle spécifiée pour l’instrument en 4.2,
ou
b) par des méthodes de transfert utilisant une surface étalon et un dispositif de mesure offrant une précision
suffisante pour que les différences de mesure de la surface étalon permettent de corriger les valeurs
mesurées de la surface soumise à essai.
La courbure peut être vérifiée
— par des calculs mathématiques utilisant les valeurs d’élévation vérifiées,
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ISO 19980:2012(F)
ou
— par une mesurage physique direct de la courbure selon une méthode offrant une précision deux fois
supérieure à celle des limites de tolérance spécifiées.
5.2.6 Essai de type des surfaces
Il convient de soumettre à essai le type de cinq surfaces d’essai (définies dans le Tableau 2) avec chaque TC.
Il convient de marquer le TC par A ou B selon le niveau de tolérance (voir Tableau 3) valide pour les cinq
surfaces d’essai mentionnées dans le Tableau 2.
Tableau 2 — Surfaces d’essai pour l’essai de type
Surface Rayon de courbure e Diamètre
1) sphère ≥10 mm
+00,
65, 0 mm
( )
−02,
exactitude ±1 µm
2) sphère ≥10 mm
+00,
80, 0 mm
( )
−02,
exactitude ±1 µm
3) sphère ≥10 mm
+00,
95, 0 mm
( )
−02,
exactitude ±1 µm
4) ellipsoïde de 0,6 ± 0,1 ≥10 mm
+00,
r = 78, 0 mm
0 ( )
−03,
révolution
exactitude ±1 µm
5) torique r = 8,0 mm ± 0,2 mm ≥10 mm
1
r < r
2 1
r – r = 0,4 ± 0,07 mm
1 2
exactitude ±1 µm
NOTE 1 Conformément à 1): mesure de contrôle possible avec unité de micromètre.
NOTE 2 Conformément à 2) et 3): une forme ellipsoïde et torique peut être fabriquée par une entreprise de lentilles de contact et
mesurée avec un dispositif de mesure à trois dimensions.
Tableau 3 — Niveau de tolérance des surfaces d’essai
Tolérances si les mesurages sont exprimés en termes de rayon de courbure, en millimètres
Surface
Exactitude de mesure Type
Diamètre central Diamètre médian Diamètre extérieur
Deux fois l’écart-type A 0,05 0,03 0,03
Deux fois l’écart-type B 0,1 0,07 0,07
Tolérances si les mesurages sont exprimés en termes de rayon de courbure, en dioptres kératométriques
Zone
Exactitude de mesure Type
Diamètre central Diamètre médian Diamètre extérieur
Deux fois l’écart-type A 0,27 0,16 0,16
Deux fois l’écart-type B 0,52 0,37 0,37
NOTE Les dioptres kératométriques dépendent du rayon de courbure donné en millimètres par la formule kératométriques
= 337,5/rayon de courbure.
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ISO 19980:2012(F)
5.3 Collecte de données, surfaces d’essai
Aligner la surface d’essai à l’instrument, comme indiqué par le fabricant du système de mesure des yeux
humains. Mesurer la surface et sauvegarder les données mesurées. À chaque point mesuré, l’ensemble de
données est composé de la valeur de la variable mesurée et de la position à deux dimensions de la mesure.
5.4 Analyse des données
5.4.1 Généralités
Le traitement des données de topographie cornéenne consiste à comparer les valeurs mesurées de deux
ensembles de données. La structure des ensembles de données est sensiblement différente pour l’analyse de
l’exactitude et l’analyse de la répétabilité, lesquelles doivent donc être données séparément.
5.4.2 Structure de l’ensemble de données d’exactitude
Pour déterminer l’exactitude, un ensemble de données est composé des valeurs mesurées et des emplacements
de mesure obtenus suite au mesurage d’une surface d’essai connue. L’autre ensemble de données est composé
des valeurs connues de la surface d’essai aux emplacements mesurés par l’instrument, et reportés comme
faisant partie intégrante de l’ensemble de données. L’analyse des ensembles appariés de données est réalisée
conformément à 5.4.3.
5.4.3 Analyse des ensembles de données appariés
Pour chaque paire d’ensembles de données, une différence de valeurs mesurées est prévue. Cela donne
lieu à un ensemble de données de valeurs différentielles, appelées ΔD , pour chaque point mesuré sur la
ijk
surface cornéenne. Les indices i et j indiquent les deux ensembles de données utilisés. L’indice k indique la
position des points individuels. La position est spécifiée par deux valeurs de coordonnées qui peuvent être, par
exemple, le méridien q et la position radiale x sur lesquels se trouve le point. Les valeurs connues de la surface
d’essai sont calculées à partir de la forme de sa surface et de la position mesurée.
Les valeurs différentielles, ΔD , sont ensuite regroupées en sous-ensembles, en fonction de leurs valeurs de
ijk
position. Chaque sous-ensemble est associé à l’une des zones de mesure spécifiées dans le Tableau 4 et est
composé des poin
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