oSIST prEN ISO 19980:2026

• Abstract:

  This document specifies minimum requirements for instruments and systems that fall into the class of corneal topographers (CTs). It also specifies tests and procedures to verify that a system or instrument complies with this document and thus qualifies as a CT according to this document. It also specifies tests and procedures that allow the verification of capabilities of systems that are beyond the minimum requirements for CTs.; This document defines terms that are specific to the characterization of the corneal shape so that they may be standardized throughout the field of vision care.; This document is applicable to instruments, systems and methods that are intended to measure the surface shape of the cornea of the human eye.; NOTE The measurements can be of the curvature of the surface in local areas, three-dimensional topographical measurements of the surface or other more global parameters used to characterize the surface.; This document is not applicable to ophthalmic instruments classified as ophthalmometers.

• Status: Not Published
• Public Enquiry End Date: 30-Oct-2026
• ICS: 11.040.70 - Ophthalmic equipment
• Technical Committee: VAZ - Healthcare
• Current Stage: 4020 - Public enquire (PE) (Adopted Project)
• Start Date: 04-Sep-2026
• Due Date: 22-Jan-2027
• Ref Project: prEN ISO 19980 - Ophthalmic instruments - Corneal topographers (ISO/DIS 19980:2026)

• Revises: SIST EN ISO 19980:2021 - Ophthalmic instruments - Corneal topographers (ISO 19980:2021)
• Effective Date: 02-Apr-2025

oSIST prEN ISO 19980:2026 is a SIST standard for ophthalmic instruments called corneal topographers (CTs). It specifies minimum requirements, test methods and reporting rules for instruments and systems that measure the surface shape of the human cornea. In practice, it helps manufacturers, test labs and buyers identify whether a device qualifies as a CT under this document and how its performance is shown.

What does oSIST prEN ISO 19980:2026 specify?

oSIST prEN ISO 19980:2026 applies to instruments, systems and methods intended to measure corneal surface shape in the human eye. The document covers local curvature, three-dimensional topography and other global surface parameters used to characterize the cornea. It also defines terms used for corneal shape so that results and descriptions are consistent across vision care.

The document excludes ophthalmic instruments classified as ophthalmometers. Its structure is straightforward: Clause 4 gives the requirements, Clause 5 gives type-test methods and test devices, Clause 6 covers accompanying documents, and Clause 7 covers marking.

Annex	What it covers
Annex A	Informative examples of test surfaces for corneal topographers
Annex B	Normative standardized displays and colour palettes for corneal topographers
Annex C	Normative calculation of area-weighting values
Annex D	Normative test methods for measuring human corneas

What are the key requirements of oSIST prEN ISO 19980:2026?

Clause 4 sets the basic performance boundaries for a CT. Clause 5 then tells the reader how to test those boundaries with defined surfaces or human eyes, and how to analyse the results.

• Measured area and field of view - Clause 4.1 requires a CT to directly measure at least to a radial perpendicular distance of 3.75 mm from the CT axis when a spherical surface with an 8 mm radius is used. If the manufacturer claims the maximum area covered, that claim has to be reported as the maximum sampled radial distance on that reference surface. In practice, this tells a buyer how much of the cornea the instrument really covers.

• Sampling density - Clause 4.2 requires direct sampling close enough that any point within the measured area has a sample within 0.5 mm. This matters because sparse sampling can miss local shape changes that affect maps, fitting and clinical interpretation.

• Claims about curvature or elevation performance - Clause 4.3 requires any performance claim for curvature or elevation to be tested according to Clause 5 and reported according to Clause 5.4. In practice, a performance statement must be backed by the standardized test route, not by ad hoc measurements.

• Standardized colour presentation - Clause 4.4 and Annex B require the CT to make available the standardized colour palettes and to designate one display as the standardized display. The document also allows additional displays with other parameters. This matters because maps from different systems should be easier to compare.

• Type tests - Clause 5.1 states that the tests are type tests. The document separates an accuracy test, which uses specified test surfaces, from a repeatability test, which is performed in vivo on human eyes and uses Annex D. That distinction matters because bench accuracy and real-eye repeatability are not the same thing.

• Accuracy test surfaces and tolerances - Clause 5.2 specifies the test surfaces and how they are verified. The test set includes spheres, an ellipsoid of revolution and a toric surface, with requirements that vary by topographer technology such as reflection-based, Placido ring, luminous surface and optical-sectioning CTs. Clause 5.2.4 and Table 3 classify accuracy by type A or type B using root mean square (RMS) difference limits. In practice, this is the main bench route for showing how closely the CT matches a known surface.

• Data analysis and reporting - Clause 5.4 compares two paired data sets and uses mean difference and RMS difference, with area weighting from Annex C where needed. For accuracy, the report must name the test surface, its orientation to the CT axis, the mean difference and twice the RMS value. For repeatability, Annex D requires the number of measured eyes, the mean difference and twice the RMS value. This is what a technical file or evaluation report should contain.

• User information and marking - Clause 6 requires instructions for use plus maintenance procedures and their frequency, together with manufacturer details, model and type, accessories, the standard reference if conformity is claimed, and the performance information from Clause 5.4 and Annex D. Clause 7 requires permanent marking with the manufacturer or supplier name and address and the model name. In practice, this supports traceability, servicing and procurement checks.

What terms does oSIST prEN ISO 19980:2026 define?

• Corneal topographer (CT) - A non-contact instrument or system that measures the shape of the corneal surface.
• Corneal apex - The point on the cornea where the mean local principal curvature is greatest.
• Corneal meridian - The curve formed where the cornea intersects a plane that contains the CT axis.
• Corneal topographer axis (CT axis) - The coordinate axis, usually parallel to the optical axis, used to describe corneal shape.
• Corneal shape factor - A value that describes the conic section best fitting a corneal meridian.
• Keratometric corneal power - The corneal refracting power based on front-surface radius of curvature and keratometric constant.
• Toricity - The difference in principal curvatures at a point or local area on a surface.

Who uses oSIST prEN ISO 19980:2026?

oSIST prEN ISO 19980:2026 is used by manufacturers of ophthalmic instruments, test laboratories, and technical teams that verify CT performance. It is also relevant to quality managers, regulatory or compliance staff, and buyers who need to compare instruments on a defined technical basis.

In practice, these readers use the document to design CTs, choose test surfaces, run accuracy and repeatability tests, prepare technical files, set up standardized displays, and check that accompanying documents and marking are complete. Clinical and vision-care users can also use it to understand what a CT measurement means and what kind of corneal surface data the instrument is intended to provide.

What changed in oSIST prEN ISO 19980:2026 from the previous edition?

oSIST prEN ISO 19980:2026 replaces the third edition, ISO 19980:2021, and the foreword lists the main technical and editorial changes. The document adapts 3.11 and 3.12, adds a requirement to test with all test surfaces for type A corneal topographers in 5.2.6, and replaces standard deviation with RMS as the accuracy measure in Clauses 5.2 and 5.4.

It also removes the surface profile tolerance requirement for test surfaces in 5.2.6, removes references to IEC 60601-1, changes Annex B from informative to normative, removes the HBS columns in Table B.2, and includes additional editorial changes.

What does the oSIST prEN ISO 19980:2026 document contain?

The document contains definitions and figures for corneal shape, including corneal vertex, corneal apex, curvature, radius of curvature and surface geometry. It also includes formulas for shape factor, curvature, elevation, radius of curvature, area weighting and paired-data analysis, which are the core tools used in testing and reporting.

Clause 5 uses tables for test surfaces and tolerances, plus technology-specific surface requirements for reflection-based, Placido ring, luminous surface and optical-sectioning CTs. Annex B provides standardized map settings and colour palettes for curvature and elevation displays, while Annex C gives the weighting method used when sample points do not represent equal surface area.

Annex A explains which test surfaces are useful for different kinds of corneal shape, such as spheres, ellipsoids of revolution and higher-order polynomial surfaces. Annex D sets out the human-cornea repeatability test and the reporting items needed for performance evaluation.

SLOVENSKI STANDARD
01-oktober-2026
Oftalmični instrumenti - Topografi roženice (ISO/DIS 19980:2026)
Ophthalmic instruments - Corneal topographers (ISO/DIS 19980:2026)
Ophthalmische Instrumente - Hornhauttopographen (ISO/DIS 19980:2026)
Instruments ophtalmiques - Topographes cornéens (ISO/DIS 19980:2026)
Ta slovenski standard je istoveten z: prEN ISO 19980
ICS:
11.040.70 Oftalmološka oprema Ophthalmic equipment
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

DRAFT
International
Standard
ISO/DIS 19980
ISO/TC 172/SC 7
Ophthalmic instruments — Corneal
Secretariat: DIN
topographers
Voting begins on:
Instruments ophtalmiques — Topographes de la cornée
2026-08-10
Voting terminates on:
ICS: 11.040.70
2026-11-02
THIS DOCUMENT IS A DRAFT CIRCULATED
FOR COMMENTS AND APPROVAL. IT
IS THEREFORE SUBJECT TO CHANGE
AND MAY NOT BE REFERRED TO AS AN
INTERNATIONAL STANDARD UNTIL
PUBLISHED AS SUCH.
This document has not been edited by the ISO Central Secretariat.
IN ADDITION TO THEIR EVALUATION AS
BEING ACCEPTABLE FOR INDUSTRIAL,
TECHNOLOGICAL, COMMERCIAL AND
USER PURPOSES, DRAFT INTERNATIONAL
STANDARDS MAY ON OCCASION HAVE TO
ISO/CEN PARALLEL PROCESSING
BE CONSIDERED IN THE LIGHT OF THEIR
POTENTIAL TO BECOME STANDARDS TO
WHICH REFERENCE MAY BE MADE IN
NATIONAL REGULATIONS.
RECIPIENTS OF THIS DRAFT ARE INVITED
TO SUBMIT, WITH THEIR COMMENTS,
NOTIFICATION OF ANY RELEVANT PATENT
RIGHTS OF WHICH THEY ARE AWARE AND TO
PROVIDE SUPPORTING DOCUMENTATION.
Reference number
ISO/DIS 19980:2026(en)
DRAFT
ISO/DIS 19980:2026(en)
International
Standard
ISO/DIS 19980
ISO/TC 172/SC 7
Ophthalmic instruments — Corneal
Secretariat: DIN
topographers
Voting begins on:
Instruments ophtalmiques — Topographes de la cornée
ICS: 11.040.70 Voting terminates on:
THIS DOCUMENT IS A DRAFT CIRCULATED
FOR COMMENTS AND APPROVAL. IT
IS THEREFORE SUBJECT TO CHANGE
AND MAY NOT BE REFERRED TO AS AN
INTERNATIONAL STANDARD UNTIL
PUBLISHED AS SUCH.
This document has not been edited by the ISO Central Secretariat.
IN ADDITION TO THEIR EVALUATION AS
BEING ACCEPTABLE FOR INDUSTRIAL,
© ISO 2026
TECHNOLOGICAL, COMMERCIAL AND
USER PURPOSES, DRAFT INTERNATIONAL
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
STANDARDS MAY ON OCCASION HAVE TO
ISO/CEN PARALLEL PROCESSING
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
BE CONSIDERED IN THE LIGHT OF THEIR
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
POTENTIAL TO BECOME STANDARDS TO
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or ISO’s member body in the country of the requester.
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RECIPIENTS OF THIS DRAFT ARE INVITED
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TO SUBMIT, WITH THEIR COMMENTS,
CH-1214 Vernier, Geneva
NOTIFICATION OF ANY RELEVANT PATENT
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Website: www.iso.org
Published in Switzerland Reference number
ISO/DIS 19980:2026(en)
ii
ISO/DIS 19980:2026(en)
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 .9
4.3 Measurement and report of performance .9
4.4 Colour presentation of results .9
5 Test methods and test devices . 9
5.1 Tests .9
5.1.1 General .9
5.1.2 Accuracy test .9
5.1.3 Repeatability test .9
5.2 Accuracy test specifications .9
5.2.1 Test surface geometry specification .9
5.2.2 Test surface requirements according to topographer technology .10
5.2.3 Verification and calibration of test surfaces .10
5.2.4 Accuracy test tolerances .11
5.3 Data collection — Test surfaces .11
5.4 Analysis of the data . 12
5.4.1 General . 12
5.4.2 Structure of the accuracy data set . 12
5.4.3 Analysis of the paired data sets . 12
5.4.4 Report of accuracy performance . 13
6 Accompanying documents . .13
7 Marking . .13
Annex A (informative) Test surfaces for corneal topographers (CTs) . 14
Annex B (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

iii
ISO/DIS 19980:2026(en)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out through
ISO technical committees. Each member body interested in a subject for which a technical committee
has been established has the right to be represented on that committee. International organizations,
governmental and non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely
with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are described
in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types
of ISO documents should be noted. This document was drafted in accordance with the editorial rules of the
ISO/IEC Directives, Part 2 (see www.iso.org/directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of any patent
rights identified during the development of the document will be in the Introduction and/or on the ISO list of
patent declarations received (see www.iso.org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and expressions
related to conformity assessment, as well as information about ISO's adherence to the World Trade
Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 172, Optics and photonics, Subcommittee SC
7, Ophthalmic optics and instruments, in collaboration with the European Committee for Standardization
(CEN) Technical Committee CEN/TC 170, Ophthalmic optics, in accordance with the Agreement on technical
cooperation between ISO and CEN (Vienna Agreement).
This fourth edition cancels and replaces the third edition (ISO 19980:2021), which has been technically
revised. The main changes compared to the previous edition are as follows:
— Adapted 3.11 and 3.12.
— Added the requirement to test with all test surfaces for type A corneal topographers in 5.2.6.
— Replaced standard deviation with RMS as measure of accuracy in 5.2 and 5.4.
— Removed surface profile tolerance requirement for test surfaces in 5.2.6.
— Removed references to IEC 60601-1.
— Corrected Annex B status from informative to normative (editorial).
— Removed the HBS columns in Table B.2.
— Editorial changes.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.

iv
DRAFT International Standard ISO/DIS 19980:2026(en)
Ophthalmic instruments — Corneal topographers
1 Scope
This document specifies minimum requirements for instruments and systems that fall into the class of
corneal topographers (CTs). It also specifies tests and procedures to verify that a system or instrument
complies with this document and thus qualifies as a CT according to this document. It also specifies tests and
procedures that allow the verification of capabilities of systems that are beyond the minimum requirements
for CTs.
This document defines terms that are specific to the characterization of the corneal shape so that they may
be standardized throughout the field of vision care.
This document is applicable to instruments, systems and methods that are intended to measure the surface
shape of the cornea of the human eye.
NOTE The measurements can be of the curvature of the surface in local areas, three-dimensional topographical
measurements of the surface or other more global parameters used to characterize the surface.
This document is not applicable to ophthalmic instruments classified as ophthalmometers.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
corneal apex
location on the corneal surface where the mean of the local principal curvature is greatest
Note 1 to entry: See Figure 1.
3.2
corneal eccentricity
e
c
eccentricity, e, of the conic section that best fits the corneal meridian (3.3) of interest
Note 1 to entry: If the meridian is not specified, the corneal eccentricity is that of the flattest corneal meridian (see
Table 1 and Annex A).
ISO/DIS 19980:2026(en)
3.3
corneal meridian
θ
curve created by the intersection of the corneal surface and a plane that contains the corneal topographer
axis
Note 1 to entry: A meridian is identified by the angle θ, that the plane creating it makes to the horizontal (see ISO 8429).
Note 2 to entry: The value of θ, for a full meridian, ranges from 0° to 180°.
3.4
corneal shape factor
E
value that specifies the type of conic section that best fits a corneal meridian (3.3), given by Formula (1):
(1)
where, p is the value that specifies a conic section such as a circle, ellipse, hyperbola, or parabola.
Value p is given by Formula (2):
(2)
where
a and b are the semi-diameters of the axes of the conic section;
+ indicates a circle or ellipse;
− indicates a hyperbola.
A conic section is specified by Formula (3):
(3)
Value E also is the square of the eccentricity (3.9) of the conic section, given by Formula (4):
(4)
Note 1 to entry: Unless otherwise specified, E refers to the meridian with least curvature (flattest meridian). See
Table 1 and Annex A.
Note 2 to entry: Although the magnitude of E is equal to the square of the eccentricity and so is always positive, the
sign of E is a convention to signify whether an ellipse takes a prolate or oblate orientation.
Note 3 to entry: The negative value of E is defined by ISO 10110-12 as the conic constant designated by the symbol K.
The negative value of E has also been called asphericity and given the symbol Q.
Table 1 — Conic section descriptors
a
Conic section Value of p Value of E Value of e
Hyperbola p < 0 E > 1 e > 1
Parabola 0,0 1,0 1,0
b
Prolate ellipse 0 < p < 1 1 > E > 0 1 > e >0
Circle 1,0 0,0 0,0
b
Oblate ellipse p > 1 E < 0 0 < e < 1
a
See 3.4.
b
Eccentricity, e, does not distinguish between prolate and oblate orientations of an ellipse (see 3.9 and Annex A).

ISO/DIS 19980:2026(en)
3.5
corneal topographer
CT
instrument or system that measures the shape of corneal surface in a non-contact manner
Note 1 to entry: A corneal topographer that uses a video camera system and video image processing to measure the
corneal surface by analysing the reflected image created by the corneal surface of a luminous target is also referred to
as a videokeratograph.
3.5.1
optical-sectioning corneal topographer
corneal topographer (3.5) that measures the corneal surface by analysing multiple optical sections of that
surface
3.5.2
Placido ring corneal topographer
corneal topographer (3.5) that measures the corneal surface by analysing the reflected image of a Placido
ring target created by the corneal surface
3.5.3
reflection-based corneal topographer
corneal topographer (3.5) that measures the corneal surface using light reflected from the air/pre-corneal
tear film interface
3.5.4
luminous surface corneal topographer
corneal topographer (3.5) that measures the corneal surface using light back-scattered from a target
projected onto the pre-corneal tear film or the corneal anterior tissue surface
Note 1 to entry: Back-scattering is usually introduced in these optically clear substances by the addition of a fluorescent
material into the pre-corneal tear film. A target may include a slit or scanning slit of light or another projecting pattern
of light. Other methods are possible.
3.6
corneal topographer axis
CT axis
line parallel to the optical axis of the instrument and often coincident with it, that serves as one of the
coordinate axes used to describe and define the corneal shape
3.7
corneal vertex
point of tangency of a plane perpendicular to the corneal topographeraxis (3.6) with the corneal surface
Note 1 to entry: See Figure 1.

ISO/DIS 19980:2026(en)
Key
1 corneal vertex
2 corneal apex
3 radius of curvature at the corneal apex
4 centre of meridional curvature point
5 cross-section of the corneal surface
6 plane perpendicular to the CT axis
7 CT axis
Figure 1 — Illustration of the corneal vertex and the corneal apex
3.8
Curvature
3.8.1
Axial curvature
Note 1 to entry: Axial curvature is expressed in reciprocal millimetres.
3.8.1.1
axial curvature
sagittal curvature
K
a
〈calculated using the axial radius of curvature〉 reciprocal of the distance from a point on a surface to the
corneal topographer axis (3.6) along the corneal meridian (3.3) normal at the point and given by Formula (5):
(5)
where r is the axial radius of curvature
a
Note 1 to entry: See Figure 2.

ISO/DIS 19980:2026(en)
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 Formula (6):
(6)
where
x is the radial position variable on the meridian;
x is the radial position at which K is evaluated;
p a
K is the meridional curvature.
m
Key
1 normal to meridian at point P
2 P, a point on the meridian where curvature is to be found
3 centre of meridional curvature point
4 intersection normal — CT axis
5 meridian (a cross-section of the corneal surface)
6 CT axis
Figure 2 — Illustration of axial curvature, K , axial radius of curvature, r , meridional curvature,
a a
K , and meridional radius of curvature, r
m m
3.8.2
Gaussian curvature
product of the two principal normal curvature values at a surface location
Note 1 to entry: Gaussian curvature is expressed in reciprocal square millimetres.

ISO/DIS 19980:2026(en)
3.8.3
meridional curvature
tangential curvature
K
m
local surface curvature measured in the meridional plane and defined by Formula (7):
(7)
where M (x) is a function giving the elevation of the meridian at any perpendicular distance, x, from the
corneal topographer axis (3.6)
Note 1 to entry: The meridional plane includes the surface point and the chosen axis. The meridional normal is a line
passing through the surface point perpendicular to the tangent to the meridional curve at that point and lying in the
meridional plane.
Note 2 to entry: Meridional curvature is in general not a normal curvature. It is the curvature of the corneal meridian
at a point on a surface.
Note 3 to entry: See Figure 2.
3.8.4
normal curvature
curvature at a point on the surface of the curve created by the intersection of the surface with any plane
containing the normal to the surface at that point
3.8.4.1
mean curvature
arithmetic average of the principal curvatures at a point on the surface
3.8.4.2
principal curvature
maximum or minimum curvature at a point on the surface
3.9
eccentricity
e
value descriptive of a conic section and the rate of curvature change away from the apex of the curve, i.e.
how quickly the curvature flattens or steepens away from the apex of the surface
Note 1 to entry: Eccentricity ranges from zero to positive infinity for the group of conic sections. In order to signify
use of an oblate ellipse, e is sometimes given a negative sign that is not used in computations. Otherwise, use of the
prolate ellipse is assumed. See Table 1.
3.10
elevation
distance between a corneal surface and a defined reference surface, measured in a defined direction from a
specified position
3.10.1
axial elevation
elevation as measured from a selected point on the corneal surface in a direction parallel to the corneal
topographer axis (3.6)
3.10.2
normal elevation
elevation as measured from a selected point on the corneal surface in a direction along the normal to the
corneal surface at that point
ISO/DIS 19980:2026(en)
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 corneal power
refracting power of the cornea based on the radius of curvature of the front surface and the keratometric
constant, using Formula (8):
(8)
Note 1 to entry: Corneal power is expressed in keratometric dioptres (3.12).
Note 2 to entry: The radius of curvature is expressed in millimetres.
3.12
keratometric dioptres
unit of keratometric corneal power (3.11)
Note 1 to entry: Use D as unit symbol for keratometric dioptres.
3.13
surface normal
line passing through a surface point of the surface perpendicular to the plane tangent to the surface at that
point
3.14
ellipsoid of revolution
surface of revolution that results when the generating arc is ellipse
3.15
radius of curvature
reciprocal of the curvature
Note 1 to entry: The radius of curvature is expressed in millimetres.
3.15.1
axial radius of curvature
sagittal radius of curvature
r
a
distance from a surface point, P, to the axis along the normal to corneal meridian at that point, and defined
by Formula (9):
(9)
where
x is the perpendicular distance from the axis to the meridian point, in millimetres;
φ(x) is the angle between the axis and the meridian normal at point x
Note 1 to entry: See Figure 2.
3.15.2
meridional radius of curvature
tangential radius of curvature
r
m
distance from a surface point, P, and the centre of the meridional curvature point, and defined by
Formula (10):
(10)
ISO/DIS 19980:2026(en)
Note 1 to entry: See Figure 2.
3.16
Surface
3.16.1
aspheric surface
non-spherical surface
surface with at least one principal meridian that is non-circular in cross-section, i.e., e ≠ 0
3.16.2
atoric surface
surface having mutually perpendicular principal meridians of unequal curvature where at least one
principal meridian is non-circular in cross-section
Note 1 to entry: Atoric surfaces are symmetrical with respect to both principal meridians.
3.16.3
oblate surface
surface whose curvature increases as the location on the surface moves from a central position to a
peripheral position in all meridians
3.16.4
prolate surface
surface whose curvature decreases as the location on the surface moves from a central position to a
peripheral position in all meridians
3.16.5
reference surface
surface, that can be described in an exact, preferably mathematical fashion, used as a reference from
which distance measurements are made to the measured corneal surface, and for which, in addition to the
mathematical description, the positional relationship to the corneal surface is specified
Note 1 to entry: For instance, a reference surface might be described as a sphere that is the best least-squares fit to the
measured corneal surface. Similarly, a plane could serve as a reference surface.
3.16.6
toric surface
surface for which the principal curvatures are unequal and for which principal meridians are circular
sections
Note 1 to entry: Such surfaces are said to exhibit central astigmatism.
3.17
toricity
difference in principal curvatures at a specified point or local area on a surface
4 Requirements
4.1 Area measured
When measuring a spherical surface with a radius of curvature of 8 mm, a corneal topographer (CT) shall
directly measure locations on the surface normal whose radial perpendicular distance from the corneal
topographer axis (CT axis) is at least 3,75 mm. If the maximum area covered by a CT is claimed, it shall
be reported as the maximum radial perpendicular distance from the CT axis sampled on this 8 mm-radius
spherical surface.
ISO/DIS 19980:2026(en)
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 withi
...