ISO/FDIS 25178-606

ISO/FDIS 25178-606:2025(en)
ISO /TC 213/WG 16
2025-09-26
Secretariat: BSI
Date: 2026-01-09
Geometrical product specifications (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)

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FDIS stage
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Published in Switzerland
iii
ISO/DIS FDIS 25178-606:20242026(en)
Contents
Foreword . v
Introduction . vii
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Instrument requirements . 7
5 Metrological characteristics . 9
6 Design features . 9
7 General information . 9
Annex A (informative) Components of a focus variation instrument . 10
Annex B (informative) Sources of measurement error for focus variation . 17
Annex C (informative) Relationship to the GPS matrix model . 21
Bibliography . 23

© ISO 2024 - 2026 – All rights reserved
iv
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, which summarizes normative features and
characteristics;
— — 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;
v
ISO/DIS FDIS 25178-606:20242026(en)
— — 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.
Field Code Changed
© ISO 2024 - 2026 – All rights reserved
vi
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 CAnnex 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.
vii
DRAFT International Standard ISO/FDIS 25178-606:2025(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 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.
3.2 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(3.1)) method
3.3 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 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 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(3.3)) position
3.6 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(3.3))
position
Note 1 to entry: See Figure 1Figure 1.
Note 2 to entry: The surface is located at the axial scan position of the maximum of the focus information curve (see "c"
in Figure 1Figure 1).).
Note 3 to entry: To improve the resolution in axial scan direction, the maximum of a fitted curve can be used instead of
"c" in Figure 1Figure 1.
© ISO 2024 2026 – All rights reserved
Key
a axial scan position
b focus information
c highest focus information from the captured axial scan positions
X axial scan position
Y focus information
1 highest focus information from the captured axial scan positions
Figure 1 — Focus information curve
3.7 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.
© ISO 2025 2026 – All rights reserved
3.8 3.8
angular range of illumination
α
angular range from which the sample is illuminated
Note 1 to entry: See Figure 2Figure 2.
Note 2 to entry: The angular range of illumination affects the maximum measurable slope on optically smooth surfaces.
3.8.1 3.8.1
angular range of coaxial illumination
αI
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 αI is often related to the angular range of detection, see Figure 3Figure 3.
Note 3 to entry: In normal cases, the value αI can be derived from the numerical aperture of the objective.
Note 4 to entry: When special illumination sources are used (ring light, external light sources, etc., see Figure 2Figure 2))
the angular range of illumination (3.8(3.8)) can be much larger than αI.
3.8.2 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 2Figure 2.
3.8.3 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 2Figure 2.
© ISO 2024 2026 – All rights reserved
Key
L lens of optical system
RL ring light
α angular range of coaxial illumination
I
βImin minimum incident angle of ring light illumination
βImax maximum incident angle of ring light illumination
S sample
L lens of optical system
RL ring light
α angular range of coaxial illumination
I
βImin minimum incident angle of ring light illumination
βImax maximum incident angle of ring light illumination
S sample
Figure 2 — Angular range of illumination
3.9 3.9
angular range of detection
αD
angular range of light rays that can be gathered by the objective
© ISO 2025 2026 – All rights reserved
Note 1 to entry: See Figure 3Figure 3.

Key
L lens of optical system
RL ring light
αD angular range of detection
S sample
L lens of optical system
RL ring light
αD angular range of detection
S sample
Figure 3 — Angular range of detection
3.10 3.10
axial scan length
total range travelled by the focus variation microscope axial scan (3.3(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”.]
© ISO 2024 2026 – All rights reserved
3.11 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(3.13)) or analysers (3.12(3.12))
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 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 4Figure 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 AAnnex A and Annex BAnnex B.
© ISO 2025 2026 – All rights reserved
Illumination
system
Measured
Optical Focus
Real Image
electro-
Variation
Objective imaging Camera
surface stack magnetic
algorithm
system
surface
Spatial
filtering
Scale-
limited
surface
Key
Measurand
operator with intended modification

operator without intended modification

measurand
operator with intended modification
© ISO 2024 2026 – All rights reserved
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 2025 2026 – All rights reserved
Annex A
(informative)
Components of a focus variation instrument
A.1 Introduction
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 [8]] to [24[24].].
A.2 Typical configuration
Figure A.1Figure A.1 illustrates the typical configuration of a focus variation instrument.
© ISO 2024 2026 – All rights reserved
Key
1 electronic camera 9 light beam
2 optical components 10 analyser
3 white light source 11 polarizer (optional)
4 illumination beam splitter 12
optical axis (alternate long and short dash
line)
5 objective 13 ring light (optional)
6 sample 14 illumination pattern (optional)
7 axial scan (along optical axis) 15 reflected light
© ISO 2025 2026 – All rights reserved
8 focus information curve
1 electronic camera
2 optical components
3 white light source
4 illumination beam splitter
5 objective
6 sample
7 axial scan (along optical axis)
8 focus information curve
9 light beam
10 analyser
11 polarizer (optional)
12 optical axis (alternate long and short dash line)
13 ring light (optional)
14 illumination pattern (optional)
15 reflected light
Figure A.1— — Schematic diagram of a typical focus variation instrument
A.3 Operation principle
Focus variation combines the small depth of focus of an optical system with an axial scan along the optical axis
to provide topographical information from the variation of focus. In the following, the operating principle is
demonstrated for a typical focus variation instrument schematically shown in Figure A.1Figure A.1. The main
component of the system is the optical instrument containing various lenses that can be equipped with
different objectives allowing measurements with different resolution. With a beam splitting mirror, light
emerging from a white light source is inserted into the optical path of the system and focused onto the sample
through the objective. Depending on the topography of the sample, the light is scattered into several directions
as soon as it hits the sample through the objective. If the topography shows diffuse reflective properties, the
light is scattered strongly into all directions. In the case of specular reflection, the light is reflected mainly into
one direction. All rays emerging from the sample and hitting the objective lens are collected in the optics and
gathered by a light sensitive sensor behind the beam splitting mirror. Due to the small depth of field of the
optics, only small regions of the object are sharply imaged. To perform a complete detection of the surface
with full depth of field, the optics is moved along the optical axis while continuously capturing data from the
surface. Each region of the object is sharply focused at one of the axial positions of the scanner. Algorithms
convert the acquired sensor data into 3D information with full depth of the field. This is achieved by analysing
the variation of focus along the axial axis.
In addition to the scanned height da
...