EN ISO 25178-606:2026

• Abstract:

  This document specifies the design and characteristics of focus variation instruments for areal measurement of surface topography. Because surface profiles can be extracted from areal surface topography data, the methods described in this document are also applicable to profiling measurements as well.; This document applies to focus variation without pattern illumination or with fixed pattern illumination. This document does not cover methods using varying pattern illumination during the measurement.

• Status: Published
• Publication Date: 12-May-2026
• ICS: 17.040.40 - Geometrical Product Specification (GPS)
• Technical Committee: CEN/TC 290 - Dimensional and geometrical product specification and verification
• Current Stage: 6060 - Definitive text made available (DAV) - Publishing
• Start Date: 13-May-2026
• Completion Date: 13-May-2026
• Ref Project: SIST EN ISO 25178-606:2026 - Geometrical product specifications (GPS) - Surface texture: Areal - Part 606: Design and characteristics of non-contact (focus variation) instruments (ISO 25178-606:2026)

• Replaces: EN ISO 25178-606:2015 - Geometrical product specification (GPS) - Surface texture: Areal - Part 606: Nominal characteristics of non-contact (focus variation) instruments (ISO 25178-606:2015)
• Effective Date: 04-Oct-2023
• Consolidated By: ISO 25178-606:2026 - Geometrical product specifications (GPS) — Surface texture: Areal — Part 606: Design and characteristics of non-contact (focus variation) instruments
• Effective Date: 12-Feb-2026

Overview

EN ISO 25178-606:2026 outlines the design and technical characteristics of non-contact focus variation instruments used for areal surface topography measurement. Developed by CEN and ISO, this standard serves as a vital part of the Geometrical Product Specifications (GPS) series, ensuring accurate and reliable measurement of surface texture. The document applies to focus variation methods employing no pattern illumination or fixed pattern illumination, supporting a wide range of industrial and research applications where precise, non-contact surface analysis is essential.

By setting clear requirements for focus variation instruments, EN ISO 25178-606:2026 enhances the consistency and comparability of surface measurements. This makes it easier for users to select, calibrate, and operate these instruments for both areal and profiling measurements, ultimately supporting high-quality manufacturing and research processes.

Key Topics

• Focus Variation Principle
  Defines focus variation as a non-contact optical method for measuring areal surface topography using image sharpness at various focal depths.
• Instrument Components
  Specifies required components, including sensors, optics, illumination systems (with or without fixed pattern), axial scanners, and measurement algorithms.
• Measurement Algorithms
  Discusses approaches for analyzing image sharpness to calculate surface heights, ensuring the conversion of raw image data into accurate 3D surface information.
• Metrological Characteristics
  Outlines the measurement noise, spatial resolution, linearity, and influence of environmental factors and light sources on measurement accuracy.
• Design Features
  Highlights key aspects, such as the use of polarization filters, objective lenses, and the importance of long working distances and high numerical aperture for optimal performance.
• Influence Quantities and Error Sources
  Identifies primary sources of measurement error, including vibrations, instrument noise, optical distortion, and sampling intervals.

Applications

EN ISO 25178-606:2026 is applicable to organizations and labs engaged in:

• Quality Control and Assurance
  Accurately measuring surface characteristics in manufacturing, ensuring compliance with specifications for automotive, aerospace, medical devices, and electronics industries.
• Material and Surface Research
  Precise assessment of coating thickness, roughness, and topography in R&D environments.
• Process Optimization
  Improving machining, fabrication, and finishing processes by monitoring surface features using advanced non-contact metrology.
• Reverse Engineering
  Supporting detailed surface reconstructions for replicating or improving existing products.
• Calibration and Standardization
  Providing a basis for instrument calibration routines and comparison across different devices or measurement environments.

The standard’s focus on non-contact, optical measurement techniques is especially valuable where contact methods are impractical-such as with delicate, soft, or highly geometrically complex surfaces.

Related Standards

For comprehensive surface texture measurement and GPS, the following standards complement EN ISO 25178-606:2026:

• EN ISO 25178-600: Metrological characteristics for areal topography measuring methods
• EN ISO 25178-607: Design and characteristics of coherence scanning interferometry instruments
• ISO 14638: ISO GPS matrix model-overview of the GPS system
• ISO 14253-1: Decision rules for verification and conformity assessment
• ISO 8015: Fundamental rules of GPS
• ISO 25178 series: Complete family of standards on surface texture: areal methods

Adhering to EN ISO 25178-606:2026 ensures harmonization with global best practices in non-contact areal surface topography measurement, supporting innovation, traceability, and competitiveness in modern manufacturing and metrology.

SLOVENSKI STANDARD
01-julij-2026
Specifikacija geometrijskih veličin izdelka (GPS) - Tekstura površine: ploskovna -
606. del: Načrtovanje in značilnosti brezkontaktnih instrumentov (sprememba
ostrine) (ISO 25178-606:2026)
Geometrical product specifications (GPS) - Surface texture: Areal - Part 606: Design and
characteristics of non-contact (focus variation) instruments (ISO 25178-606:2026)
Geometrische Produktspezifikation (GPS) - Oberflächenbeschaffenheit: Flächenhaft -
Teil 606: Aufbau und Merkmale von berührungslos messenden Geräten (Fokusvariation)
(ISO 25178-606:2026)
Spécification géométrique des produits (GPS) - État de surface: Surfacique - Partie 606:
Conception et caractéristiques des instruments sans contact (à variation de focale) (ISO
25178-606:2026)
Ta slovenski standard je istoveten z: EN ISO 25178-606:2026
ICS:
17.040.20 Lastnosti površin Properties of surfaces
17.040.40 Specifikacija geometrijskih Geometrical Product
veličin izdelka (GPS) Specification (GPS)
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EN ISO 25178-606
EUROPEAN STANDARD
NORME EUROPÉENNE
May 2026
EUROPÄISCHE NORM
ICS 17.040.40 Supersedes EN ISO 25178-606:2015
English Version
Geometrical product specifications (GPS) - Surface texture:
Areal - Part 606: Design and characteristics of non-contact
(focus variation) instruments (ISO 25178-606:2026)
Spécification géométrique des produits (GPS) - État de Geometrische Produktspezifikation (GPS) -
surface: Surfacique - Partie 606: Conception et Oberflächenbeschaffenheit: Flächenhaft - Teil 606:
caractéristiques des instruments sans contact (à Aufbau und Merkmale von berührungslos messenden
variation de focale) (ISO 25178-606:2026) Geräten (Fokusvariation) (ISO 25178-606:2026)
This European Standard was approved by CEN on 23 April 2026.

CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this
European Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references
concerning such national standards may be obtained on application to the CEN-CENELEC Management Centre or to any CEN
member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by
translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC Management
Centre has the same status as the official versions.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway,
Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Türkiye and
United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2026 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 25178-606:2026 E
worldwide for CEN national Members.

Contents Page
European foreword . 3

European foreword
This document (EN ISO 25178-606:2026) has been prepared by Technical Committee ISO/TC 213
"Dimensional and geometrical product specifications and verification" in collaboration with Technical
Committee CEN/TC 290 “Dimensional and geometrical product specification and verification” the
secretariat of which is held by AFNOR.
This European Standard shall be given the status of a national standard, either by publication of an
identical text or by endorsement, at the latest by November 2026, and conflicting national standards
shall be withdrawn at the latest by November 2026.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN shall not be held responsible for identifying any or all such patent rights.
This document supersedes EN ISO 25178-606:2015.
Any feedback and questions on this document should be directed to the users’ national standards
body/national committee. A complete listing of these bodies can be found on the CEN website.
According to the CEN-CENELEC Internal Regulations, the national standards organizations of the
following countries are bound to implement this European Standard: Austria, Belgium, Bulgaria,
Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland,
Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Republic of
North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Türkiye and the
United Kingdom.
Endorsement notice
The text of ISO 25178-606:2026 has been approved by CEN as EN ISO 25178-606:2026 without any
modification.
International
Standard
ISO 25178-606
Second edition
Geometrical product specifications
2026-05
(GPS) — Surface texture: Areal —
Part 606:
Design and characteristics of
non-contact (focus variation)
instruments
Spécification géométrique des produits (GPS) — État de surface:
Surfacique —
Partie 606: Conception et caractéristiques des instruments sans
contact (à variation de focale)
Reference number
ISO 25178-606:2026(en) © ISO 2026

ISO 25178-606:2026(en)
© ISO 2026
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: [phone]
Email: [e-mail]
Website: www.iso.org
Published in Switzerland
ii
ISO 25178-606:2026(en)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Instrument requirements . 5
5 Metrological characteristics . 6
6 Design features . 6
7 General information . 6
Annex A (informative) Components of a focus variation instrument . 7
Annex B (informative) Sources of measurement error for focus variation . 14
Annex C (informative) Relationship to the GPS matrix model . 17
Bibliography .18

iii
ISO 25178-606:2026(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 document 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).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent
rights in respect thereof. As of the date of publication of this document, ISO had not received notice of (a)
patent(s) which may be required to implement this document. However, implementers are cautioned that
this may not represent the latest information, which may be obtained from the patent database available at
www.iso.org/patents. ISO shall not be held responsible for identifying any or all such patent rights.
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 213, Dimensional and geometrical product
specifications and verification, in collaboration with the European Committee for Standardization (CEN)
Technical Committee CEN/TC 290, Dimensional and geometrical product specification and verification, in
accordance with the Agreement on technical cooperation between ISO and CEN (Vienna Agreement).
This second edition cancels and replaces the first edition (ISO 25178:606:2015), which has been technically
revised.
The main changes are as follows:
— removal of the terms and definitions now specified in ISO 25178-600;
— revision of all terms and definitions for clarity and consistency with other ISO standards documents;
— addition of Clause 4 for instrument requirements;
— addition of Clause 5 on metrological characteristics;
— addition of Clause 6 on design features, which clarifies the types of instruments relevant to this document;
— addition of an information flow concept diagram in Clause 4;
— revision of Annex A describing the principles of instruments addressed by this document;
— addition of Annex B on metrological characteristics and influence quantities, replacement of the
normative table of influence quantities with an informative description of common error sources and
how these relate to the metrological characteristics in ISO 25178-600.
A list of all parts in the ISO 25178 series can be found on the ISO website.
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
ISO 25178-606:2026(en)
Introduction
This document is a geometrical product specification (GPS) standard and is to be regarded as a general GPS
standard (see ISO 14638). It influences the chain link F of the chain of standards on profile and areal surface
texture.
The ISO GPS matrix model given in ISO 14638 gives an overview of the ISO GPS system of which this
document is a part. The fundamental rules of ISO GPS given in ISO 8015 apply to this document and the
default decision rules given in ISO 14253-1 apply to specifications made in accordance with this document,
unless otherwise indicated.
For more detailed information of the relation of this document to other standards and the GPS matrix model,
see Annex C.
This document includes terms and definitions relevant to the focus variation instruments for the
measurement of areal surface topography. Annex A briefly summarizes focus variation instruments and
methods to clarify the definitions and to provide a foundation for Annex B, which describes common sources
of uncertainty and their relation to the metrological characteristics of focus variation.
NOTE Portions of this document, particularly the informative sections, describe patented systems and methods.
This information is provided only to assist users in understanding the operating principles of focus variation. This
document is not intended to establish priority for any intellectual property, nor does it imply a license to proprietary
technologies described herein.

v
International Standard ISO 25178-606:2026(en)
Geometrical product specifications (GPS) — Surface texture:
Areal —
Part 606:
Design and characteristics of non-contact (focus variation)
instruments
1 Scope
This document specifies the design and characteristics of focus variation instruments for areal measurement
of surface topography. Because surface profiles can be extracted from areal surface topography data, the
methods described in this document are also applicable to profiling measurements as well.
This document applies to focus variation without pattern illumination or with fixed pattern illumination.
This document does not cover methods using varying pattern illumination during the measurement.
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.
ISO 25178-600:2019, Geometrical product specifications (GPS) — Surface texture: Areal — Part 600:
Metrological characteristics for areal topography measuring methods
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology 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
focus variation
FV
measurement method whereby the sharpness of a series of surface images that is acquired during an axial
scan in an optical instrument is used to measure the surface topography
Note 1 to entry: In this document, surface image is an image of a surface obtained by capturing the reflected light from
the surface with or without a fixed pattern illumination.
Note 2 to entry: In this document, sharpness is a quantity of the surface image calculated by the neighbourhood
information indicating best focus. Other names for sharpness are for example contrast.
Note 3 to entry: Focus variation without fixed pattern illumination only works on optically rough surfaces (see
ISO 25178-600:2019, 3.4.5). If fixed pattern illumination is used, optically smooth surfaces (see ISO 25178-600:2019,
3.4.4) can also be measured.
ISO 25178-606:2026(en)
3.2
focus variation sensor
device that converts the height of points on the surface into signals during measurement using the focus
variation (3.1) method
3.3
axial scan
mechanical or optical displacement between the sample under inspection and the imaging optics
Note 1 to entry: The optical axis of the imaging optics is nominally parallel to the axial scan axis of the microscope.
[SOURCE: ISO 25178-607:2019, 3.5, modified — “imaging optics” replaced by “optical axis of the imaging
optics” in Note 1 to entry.]
3.4
focus variation measurement algorithm
algorithm for analysing the variation of focus in order to calculate the scan positions where each point is
best in focus
3.5
focus information
measure to quantify the degree of focus based on image sharpness at a specific lateral position in the surface
image and at a specific axial scan (3.3) position
3.6
focus information curve
signal recorded for a specific lateral position of the surface image as a function of the axial scan (3.3) position
Note 1 to entry: See Figure 1.
Note 2 to entry: The surface is located at the axial scan position of the maximum of the focus information curve (see
"1" in Figure 1).
Note 3 to entry: To improve the resolution in axial scan direction, the maximum of a fitted curve can be used instead
of "1" in Figure 1.
Key
X axial scan position
Y focus information
1 highest focus information from the captured axial scan positions
Figure 1 — Focus information curve

ISO 25178-606:2026(en)
3.7
focus variation illumination system
source of light with a predefined spectral and spatial distribution
Note 1 to entry: Possible light sources are coaxial illumination, ring light and external light sources.
Note 2 to entry: A variant of coaxial illumination is the use of a fixed pattern projected onto a sample surface. For
example, this pattern can be a chessboard.
3.8
angular range of illumination
α
angular range from which the sample is illuminated
Note 1 to entry: See Figure 2.
Note 2 to entry: The angular range of illumination affects the maximum measurable slope on optically smooth
surfaces.
3.8.1
angular range of coaxial illumination
α
I
angular range from which the sample is coaxially illuminated
Note 1 to entry: The value α can be influenced by the choice of the objective.
I
Note 2 to entry: The value α is often related to the angular range of detection, see Figure 3.
I
Note 3 to entry: In normal cases, the value α can be derived from the numerical aperture of the objective.
I
Note 4 to entry: When special illumination sources are used (ring light, external light sources, etc., see Figure 2) the
angular range of illumination (3.8) can be much larger than α .
I
3.8.2
minimum incident angle of ring light illumination
β
Imin
angle between the optical axis and the light rays of the ring light that illuminate the sample closest to the
optical axis
Note 1 to entry: See Figure 2.
3.8.3
maximum incident angle of ring light illumination
β
Imax
angle between the optical axis and the light rays of the ring light that illuminate the sample farthest to the
optical axis
Note 1 to entry: See Figure 2.

ISO 25178-606:2026(en)
Key
L lens of optical system
RL ring light
α angular range of coaxial illumination
I
β minimum incident angle of ring light illumination
Imin
β maximum incident angle of ring light illumination
Imax
S sample
Figure 2 — Angular range of illumination
3.9
angular range of detection
α
D
angular range of light rays that can be gathered by the objective
Note 1 to entry: See Figure 3.
Key
L lens of optical system
RL ring light
α angular range of detection
D
S sample
Figure 3 — Angular range of detection

ISO 25178-606:2026(en)
3.10
axial scan length
total range travelled by the focus variation microscope axial scan (3.3), usually the total displacement
between the sample and the microscope’s objective translated along its optical axis during data acquisition
Note 1 to entry: This parameter can be limited by the overall range of the axial scanner, but is generally a parameter
chosen by the operator taking account of the height range of the surface topography.
[SOURCE: ISO 25178-607:2019, 3.6, modified —“confocal microscope axial scan” has been replaced by “focus
variation microscope axial scan”.]
3.11
polarization mode
method which allows one to filter out light waves in certain polarization states by using special optical
elements called polarizers (3.13) or analysers (3.12)
3.12
analyser
optical element used to filter the rays of the light after they have been reflected from the sample and
gathered by the objective before reaching the electronic camera
3.13
polarizer
optical element used to polarize the rays of the light source before they are transmitted to the sample
4 Instrument requirements
An instrument shall perform areal surface topography measurements of a sample surface using focus
variation. The instrument shall comprise a focus variation sensor, optics, focus variation illumination
system, and an axial scanner. The instrument shall acquire data by illuminating the surface with an angular
range of illumination and capturing data from angular range of detection at several axial positions along the
optical axis by performing an axial scan over an axial scan length. To improve the performance of specular
reflection the application of polarization mode can help. Therefore, a polarizer and an analyser are used.
The instrument shall convert acquired data to an areal topography using a focus variation measurement
algorithm which calculates the focus information to get the focus information curve.
Figure 4 shows the information flow between these elements for a FV instrument, from the real surface
to a scale-limited surface. Example FV hardware, techniques and error sources are given in Annex A and
Annex B.
ISO 25178-606:2026(en)
Key
measurand
operator with intended modification
operator without intended modification
Figure 4 — Information flow concept diagram for focus variation instruments
5 Metrological characteristics
The standard metrological characteristics for areal surface texture measuring instruments specified in
ISO 25178-600 shall be considered when designing and calibrating the instrument.
Annex B describes sources of measurement error that can influence the calibration result.
6 Design features
Standard design features specified in ISO 25178-600 shall be considered in the design.
Annex A provides examples of specific design features of FV instruments.
7 General information
The relation between this document and the GPS matrix model is given in Annex C.

ISO 25178-606:2026(en)
Annex A
(informative)
Components of a focus variation instrument
A.1 General
This annex is a short overview of the focus variation principle and the components of a focus variation
instrument. Further literature can be found in References [8] to [24].
A.2 Typical c
...