ISO 19980:2021 Ophthalmic instruments — Corneal topographers

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Ophthalmic instruments — Corneal topographers

1 Scope

This document specifies minimum requirements for instruments and systems that fall into the class of corneal topographers (CTs). It also specifies tests and procedures to verify that a system or instrument complies with this document and thus qualifies as a CT according to this document. It also specifies tests and procedures that allow the verification of capabilities of systems that are beyond the minimum requirements for CTs.

This document defines terms that are specific to the characterization of the corneal shape so that they may be standardized throughout the field of vision care.

This document is applicable to instruments, systems and methods that are intended to measure the surface shape of the cornea of the human eye.

NOTE The measurements can be of the curvature of the surface in local areas, three-dimensional topographical measurements of the surface or other more global parameters used to characterize the surface.

This document is not applicable to ophthalmic instruments classified as ophthalmometers.

2 Normative references

The following documents are referred to in the text in such a way that some or all of their content constitutes requirements of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.

IEC 60601-1:2005 + A1: 2012 + A2: 2020, Medical electrical equipment — Part 1: General requirements for basic safety and essential performance

3 Terms a nd definiti ons

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 http:// 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).

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):

Ep=−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

a

p=± (2)

2

b

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):

2 2

z x

±=1 (3)

2 2

b a

value E also is the square of the eccentricity (3.9) of the conic section, given by Formula (4):

2

Ee= (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.

Note 4 to entry: Note 5 to entry:

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