SIST EN ISO 25178-604:2025

SLOVENSKI STANDARD
01-maj-2025
Specifikacija geometrijskih veličin izdelka - Tekstura površine: ploskovna - 604.
del: Konstrukcije in značilnosti nekontaktnih instrumentov (interferometrija s
koherentnim optičnim čitalnikom) (ISO 25178-604:2025)
Geometrical product specifications (GPS) - Surface texture: Areal - Part 604: Design and
characteristics of non-contact (coherence scanning interferometry) instruments (ISO
25178-604:2025)
Geometrische Produktspezifikation (GPS) - Oberflächenbeschaffenheit: Flächenhaft -
Teil 604: Aufbau und Merkmale von berührungslos messenden Geräten (Weißlicht-
Interferometrie) (ISO 25178-604:2025)
Spécification géométrique des produits (GPS) - État de surface: Surfacique - Partie 604:
Conception et caractéristiques des instruments sans contact (à interférométrie par
balayage à cohérence) (ISO 25178-604:2025)
Ta slovenski standard je istoveten z: EN ISO 25178-604:2025
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 25178-604
EUROPEAN STANDARD
NORME EUROPÉENNE
February 2025
EUROPÄISCHE NORM
ICS 17.040.20 Supersedes EN ISO 25178-604:2013
English Version
Geometrical product specifications (GPS) - Surface texture:
Areal - Part 604: Design and characteristics of non-contact
(coherence scanning interferometry) instruments (ISO
25178-604:2025)
Spécification géométrique des produits (GPS) - État de Geometrische Produktspezifikation (GPS) -
surface: Surfacique - Partie 604: Conception et Oberflächenbeschaffenheit: Flächenhaft - Teil 604:
caractéristiques des instruments sans contact (à Aufbau und Merkmale von berührungslos messenden
interférométrie par balayage à cohérence) (ISO 25178- Geräten (Weißlicht-Interferometrie) (ISO 25178-
604:2025) 604:2025)
This European Standard was approved by CEN on 21 February 2025.

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
© 2025 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 25178-604:2025 E
worldwide for CEN national Members.

Contents Page
European foreword . 3

European foreword
This document (EN ISO 25178-604:2025) 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 August 2025, and conflicting national standards shall
be withdrawn at the latest by August 2025.
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 25178-604:2013.
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 25178-604:2025 has been approved by CEN as EN ISO 25178-604:2025 without any
modification.
International
Standard
ISO 25178-604
Second edition
Geometrical product specifications
2025-02
(GPS) — Surface texture: Areal —
Part 604:
Design and characteristics of
non-contact (coherence scanning
interferometry) instruments
Spécification géométrique des produits (GPS) — État de surface:
Surfacique —
Partie 604: Conception et caractéristiques des instruments sans
contact (à interférométrie par balayage à cohérence)
Reference number
ISO 25178-604:2025(en) © ISO 2025

ISO 25178-604:2025(en)
© ISO 2025
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: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii
ISO 25178-604:2025(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) Principles of CSI instruments for areal surface topography measurement . 7
Annex B (informative) Sources of measurement error for CSI instruments .13
Annex C (informative) Relationship to the GPS matrix model .18
Bibliography . 19

iii
ISO 25178-604:2025(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 25178-604:2013), 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 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 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 25178-604:2025(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 chain link F of the chains 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 the specifications made in accordance with this document, unless
otherwise indicated.
For more detailed information on 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 coherence scanning interferometry (CSI)
instrument for the measurement of areal surface topography. Annex A briefly summarizes CSI 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 CSI.
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 CSI instruments. 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 25178-604:2025(en)
Geometrical product specifications (GPS) — Surface
texture: Areal —
Part 604:
Design and characteristics of non-contact (coherence
scanning interferometry) instruments
1 Scope
This document specifies the design and metrological characteristics of coherence scanning interferometry
(CSI) instruments for the areal measurement of surface topography. Because surface profiles can be
extracted from surface topography data, the methods described in this document are also applicable to
profiling measurements.
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 terms and definitions given in ISO 25178-600 and the following 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
coherence scanning interferometry
CSI
surface topography measurement method wherein the localization of interference fringes (3.7) during a scan
of optical path length provides a means to determine a surface topography map
Note 1 to entry: The optical path length difference is the difference in optical path length, including the effect
of geometry and refractive index, between the measurement and reference paths of an interferometer (see
ISO 10934:2020, 3.3.1).
Note 2 to entry: CSI uses a broad illumination spectral bandwidth or the illumination geometry, or both, to localize the
interference fringes.
Note 3 to entry: CSI uses either fringe localization alone or in combination with interference phase (3.8) evaluation,
depending on the surface type, desired surface topography repeatability and software capabilities.
Note 4 to entry: Table 1 provides a list of alternative terms for CSI that are within the scope of this document.

ISO 25178-604:2025(en)
Table 1 — Summary of common alternative terms for CSI
Term Bibliography
Coherence probe microscopy
References [13], [14], [15],
Coherence radar
[16], [17] and [18]
Coherence correlation interferometry
White light interferometry
References [19], [20] and
White light scanning interferometry
[21]
Scanning white light interferometry
Vertical scanning interferometry
References [22] and [23]
Height scanning interferometry
Full-field optical coherence tomography Reference [24]
[SOURCE: ISO 25178-6:2010, 3.3.5, modified — Note 1 to entry has been replaced by Notes 1 to 4 to entry.]
3.2
coherence scanning interferometry scan
CSI scan
mechanical or optical scan which varies the optical length of either the reference path or measurement path
to vary the optical path difference
Note 1 to entry: The imaging optics is nominally parallel to the axial scan axis of the microscope (see
ISO 25178-607:2019, 3.5).
Note 2 to entry: A CSI signal (3.3) can correspond to a sequence of electronic camera detections of intensity values
during a CSI scan (see Annex A).
Note 3 to entry: In CSI, the most common (but not exclusive) scanning means is a physical adjustment of the path
length of an interferometer (see ISO/TR 14999-2).
Note 4 to entry: Mechanical means for performing the CSI scan can be motorized or piezo-electrically driven stages or
others, depending on the instrument design, the linearity and consistency of the CSI scan, or the desired maximum CSI
scan length (3.5).
3.3
coherence scanning interferometry signal
CSI signal
correlogram
white light interferometry signal
intensity data recorded for an individual image point or camera pixel as a function of CSI scan (3.2) position
Note 1 to entry: See Figure 1 for a simulated example CSI signal for an equivalent wavelength (3.12) of 450 nm and
a measurement optical bandwidth of 110 nm at full width half maximum (see ISO 25178-600:2019, 3.3.2) and a low
illumination numerical aperture (see ISO 10934:2020, 3.1.10.4 and ISO 25178-600:2019, 3.3.6).

ISO 25178-604:2025(en)
Key
X CSI scan position expressed in micrometres B interference fringes
Y intensity C phase gap
A modulation envelope (calculated)
Figure 1 — Defined features of a CSI signal
3.4
coherence scanning interferometry scan increment
CSI scan increment
distance travelled by the CSI scan (3.2) between data captures
Note 1 to entry: A data capture can be a single image point or a camera frame.
Note 2 to entry: The CSI scan increment is most often small enough to sample each interference fringe (3.7) at several
points, e.g. four camera frames per fringe, consistent with the Nyquist criterion. Sub-Nyquist sampling is also possible
for higher data acquisition speeds, at the cost of higher measurement noise.
3.5
coherence scanning interferometry scan length
CSI scan length
total range of physical path length traversed by the CSI scan (3.2)
Note 1 to entry: The CSI scan length should normally be sufficiently long so as to capture the desired surface
topography range plus at least a portion of the modulation envelope width.
3.6
coherence scanning interferometry scanning rate
CSI scanning rate
CSI scan speed
speed at which the CSI scan (3.2) is executed
Note 1 to entry: For a linear CSI scan, the CSI scanning rate is the camera frame rate multiplied by the CSI scan
increment (3.4).
3.7
interference fringe
modulating portion of the CSI signal (3.3), related to the interference
effect and generated by the variation of optical path length during the CSI scan (3.2)
Note 1 to entry: The interference fringes are approximately sinusoidal as a function of scan position.
Note 2 to entry: See Figure 1 for an illustration of the interference fringes of a CSI signal.
Note 3 to entry: The term “interferogram” is often used to describe the image of an interference fringe pattern recorded
by a single camera frame (see ISO/TR 14999-2:2019, 6.2). An interference fringe in an interferogram is an attribute of
the interference pattern; whereas an interference fringe in CSI (3.1) refers to an attribute of a scan-dependent signal,
as illustrated in Figure 1.
ISO 25178-604:2025(en)
3.8
interference phase
phase corresponding to the sinusoidal form of the interference fringes
(3.7) in the CSI signal (3.3)
3.9
modulation amplitude
interference fringe visibility
interference fringe contrast
one-half the peak-to-valley variation or equivalent measure of the
amplitude of the interference fringes (3.7)
Note 1 to entry: See ISO/TR 14999-2:2019, 4.1.2 and 5.2.5, for example uses of the terms “visibility” and “contrast” as
synonyms, respectively.
Note 2 to entry: The modulation amplitude of a CSI signal (3.3) varies as a function of scan position.
3.10
modulation envelope
fringe contrast envelope
fringe visibility function
fringe visibility envelope
degree of coherence as a function of CSI scan position
overall variation in the modulation amplitude (3.9) of a CSI signal (3.3) as a function of scan position
Note 1 to entry: See Figure 1 for an illustration of the modulation envelope of a CSI signal (3.3).
Note 2 to entry: The modulation envelope is a consequence of limited optical coherence, which follows from using a
spectrally broadband light source (white light) or a spatially extended light source, or both.
Note 3 to entry: The modulation envelope is calculated as a function of scan position that depends on the data
analysis method.
3.11
coherence scanning interferometry signal processing option
CSI signal processing option
processing selection that determines whether the software makes use of the modulation envelope (3.10), the
interference phase (3.8), a model-based analysis or other approach to interpreting the CSI signal (3.3)
Note 1 to entry: See Clause A.3.
3.12
equivalent wavelength
λ
eq
change in surface topography height which corresponds to the scan
length between two successive interference fringes (3.7) in the CSI signal (3.3) near the maximum value of
the modulation envelope (3.10) of a CSI signal
Note 1 to entry: The equivalent wavelength is a definition in the context of CSI for the measurement optical wavelength,
defined in ISO 25178-600:2019, 3.3.3, as the “effective value of the wavelength of the light used to measure a surface”.
Note 2 to entry: The measurement optical wavelength is affected by conditions such as the light source spectrum,
spectral transmission of the optical components and spectral response of the image sensor array.
Note 3 to entry: The equivalent wavelength can be calculated from factors related to the instrument design, calibrated
experimentally, or determined as part of the CSI signal analysis (see Clause A.3).

ISO 25178-604:2025(en)
3.13
width of the modulation envelope
scan length over which the signal strength represented by the modulation envelope (3.10) is greater than a
defined value
Note 1 to entry: The width of the modulation envelope is quantifiable in different ways, such as the full width at half
maximum (FWHM).
Note 2 to entry: The width of the modulation envelope is related to the coherence length described in
ISO 11145:2018, 3.11.4, and is a function of the light source bandwidth (see ISO 25178-600:2019, 3.3.2), the camera
spectral sensitivity and geometrical factors such as the numerical aperture of the illumination (see ISO 10934:2020,
3.1.10.4 and ISO 25178-600:2019, 3.3.6).
3.14
phase gap
ϕ
G
offset in units of phase at the equivalent wavelength (3.12) between
the CSI scan (3.2) position for the interference fringe (3.7) and the maximum value of the modulation envelope
(3.10) of a CSI signal (3.3)
Note 1 to entry: See Figure 1 for an example CSI signal illustrating the phase gap.
Note 2 to entry: The phase gap is a calculated value that depends on the data analysis method.
Note 3 to entry: The phase gap can vary as a function of optical dispersion in the instrument optics as well as sample
surface characteristics such as surface films (see ISO 25178-600:2019, 3.4.1), local slope and optically non-uniform
materials (see ISO 25178-600:2019, 3.4.6).
3.15
fringe-order error
2π error
error in the identification of the correct 2π phase interval in a
topography map that makes use of the interference phase (3.8) as part of the CSI signal processing option (3.11)
Note 1 to entry: Fringe-order errors are integer multiples of one-half the equivalent wavelength (3.12) in height.
Note 2 to entry: Fringe-order errors can lead to artificial steps within the topography map. On smooth, continuous
surfaces, these artificial steps can sometimes be corrected by using phase unwrapping algorithms (see
ISO/TR 14999-2:2019, 6.6)
4 Instrument requirements
An instrument according to this document shall perform areal surface topography measurements of a
sample surface using CSI. The instrument shall comprise an interferometer (see ISO/TR 14999-2) and means
to perform a CSI scan. The instrument shall acquire camera images captured at scan positions determined
by a CSI scan increment. The data acquisition proceeds at a CSI scanning rate over a CSI scan length. The
CSI signal for a single image point shall comprise interference fringes having an interference phase and
modulation amplitudes shaped by a modulation envelope characterized by the width of the modulation
envelope. The instrument shall convert acquired data to an areal topography using a CSI signal processing
option that uses the interference fringes or modulation envelope, or both. The topography height values
shall be inferred from either the CSI scanning rate or the equivalent wavelength, or both. If the final surface
topography relies on the interference phase, the CSI signal processing option shall take into account the
phase gap when interpreting the interference fringes, so as to avoid fringe-order errors.
Figure 2 shows the information flow between these elements for a CSI microscope, from the real surface to
a scale-limited surface. Example CSI hardware, techniques and error sources are given in Annexes A and B.

ISO 25178-604:2025(en)
Key
measurand
operator with intended modification
operator without intended modification
Figure 2 — Information flow concept diagram for CSI
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 described in ISO 25178-600 shall be considered in the design.
Annex A provides examples of specific design features of CSI instruments.
7 General information
The relationship between this document and the GPS matrix model is given in Annex C.

ISO 25178-604:2025(en)
Annex A
(informative)
Principles of CSI instruments for areal surface topography
measurement
A.1 General
CSI is a mature technology and there are substantial resources in existing ISO documents listed in the
[25][26]
bibliography and in the published literature regarding instrument design and theory of operation.
[27][28]
This annex is a summary with the goal of clarifying terms and definitions as well as some of the
influence quantities that contribute to the metrological characteristics of CSI.
A.2 Instrument design
Figure A.1 provides an illustration of an example CSI microscope system based on a Michelson interferometer.
[2][16][29][30][31]
The scanning mechanism G imparts a controlled variation of optical path length by means of an axial
scan of the sample part along the z-axis direction (see ISO 25178-607:2019, 3.5). The sample surface lies
nominally within the plane, consistent with the coordinate system defined in ISO 25178-600:2019, 3.1.2, and
is imaged to the electronic camera. Superimposed onto the image of the sample is an interference pattern or
interferogram resulting from the coherent superposition of the light from the sample surface and from the
reference surface (see ISO/TR 14999-2:2019, 6.2.1).
CSI instruments configured as microscopes often have interchangeable interference objectives (see
ISO 25178-603) in place of conventional microscope objectives (see ISO 10934:2020, 3.1.106). These
[25][26][27][32][33][34][35]
objectives have built-in beam splitters and reference surfaces. CSI microscopes can
have the scanning mechanism as part of the part support or integrated into the mount for the interference
objective.
ISO 25178-604:2025(en)
Key
A light source F workpiece
B illumination aperture stop G scanning mechanism
C condensing lens H imaging aperture stop
D beamsplitter I camera lens
E reference mirror J camera
ζ scanning motion K tube lens
Figure A.1 — Schematic diagram of a Michelson-type CSI microscope
The measurement principle is to determine the surface height at each point on the sample surface by analysis
of multiple interference patterns recorded as CSI signals for each image point acquired during a sequence
of controlled CSI scan positions. Figure A.2 shows the camera image at different scan positions during a
data acquisition scan, illustrating the changes in the appearance of the fringes that correspond to horizontal
slices through the object surface topography. Note that elements related to the camera and imaging system
that were shown in Figure A.1 are also part of the assembly but are not shown in Figure A.2.

ISO 25178-604:2025(en)
Key
A objective lens E scanning mechanism
B interferometer beamsplitter F image at beginning of the scan
C reference mirror G image at midpoint of the scan
D workpiece H image at end of the scan
ζ scanning motion
NOTE Figure A.2 shows a conceptual drawing of data acquisition for a CSI microscope equipped with a Mirau-
type interference objective lens.
Figure A.2 — Conceptual drawing of data acquisition for a CSI microscope
A characteristic of most CSI systems is that the reported areal surface topography is everywhere in focus,
even if the topography variations are much greater than the depth of field of the objective lens. This
characteristic assumes that the objectives are adjusted such that the position of best focus and the peak of
[32]
the modulation envelope are coincident. For microscope systems that scan the interference objective, the
objective lenses are of the infinite conjugate type, meaning that a point on the object is imaged at infinity
(see ISO 9335:2012, 4.4.2.3, and ISO/TR 14999-1:2005, 4.1).
Light sources for CSI are most commonly spatially incoherent and spectrally broadband or white
light, exemplified by incandescent lamps or white-light or broadband light-emitting diodes (see
ISO 10934:2020, 3.1.73). CSI instruments are also realized with light sources in the blue, green, red or infrared
wavelengths. The light source can include interchangeable filters for adjusting the illumination spectrum
(see ISO 10934:2020, 3.1.38.7). For dynamically moving objects, such as oscillating microstructures, the
[37][38][39]
light source can be flashed at high speed to stroboscopically freeze the object motion.
For a CSI microscope, the light source is often imaged into the objective pupil in the epi-illumination
Köhler geometry (see ISO 10934:2020, 3.1.73.2 and 3.1.73.3). Many instruments have adjustable light stops
for controlling the size of the illumination field as well as the illumination aperture (see ISO 10934:2020,
[25]
3.1.10.4). In Figure A.1, an illumination aperture stop B controls the numerical aperture (NA) of the
illumination, while an imaging aperture stop H controls the imaging NA.

ISO 25178-604:2025(en)
Depending on the measurement task, it can be useful to have a small diameter aperture stop (component
B in Figure A.1), or equivalently, a small light source, such that the CSI instrument illuminates the sample
with an almost parallel beam. A small illumination NA facilitates a large working distance, shadowing is
avoided and deep surface features can be investigated. However, a small illumination NA reduces the lateral
resolution compared to completely filling the entrance pupil of the objective.
Cameras for the visible wavelengths can be of the charge-coupled device (CCD) or complementary metal-
oxide semiconductor (CMOS type), with a format ranging from 300 000 pixels to over 4 million pixels. The
sampling interval as described in ISO 25178-600:2019, 3.1.17 is determined by the camera format and the
optical magnification (see Clause B.10).
The topographic lateral resolution defined in ISO 25178-600 summarizes the net effect of the camera,
optics and data processing on the ability of the instrument to resolve closely spaced topographical features
on the surface (see also Clauses B.9 and B.10). The net effect of the camera, optics and data processing
on the topographic lateral resolution can be determined in accordance with ISO 25178-700. For example,
the instrument transfer function (ITF) defined in ISO 25178-600:2019, 3.1.19 as a curve, describes an
instrument’s height response as a function of the spatial frequency of the surface topography. Another
approach is given by using the topography fidelity, defined in ISO 25178-600:2019, 3.1.26. The lateral
resolution and the ITF can vary with specific surfaces structures comprising steep slopes, sharp edges or
high aspect ratios. Further information on the ITF can be found in References [2], [40], [41] and [42].
Adjustments upwards or downwards of the position of the objective or a sample stage (not shown in
Figure A.1) brings the test surface into focus (see ISO 10934:2020, 3.1.65). Part setup can require a nominal
adjustment of both focus and tip and tilt, although automation can complete some or all these steps (for
example, see "autofocus" as defined in ISO 10934:2020, 3.2.4).
The CSI scan length is often between 10 μm and 400 μm for piezoelectric scanners. For motorized scanners,
[31]
the scan length can be 70 mm or more. In that CSI measures surface heights by referencing to CSI scan
[25]
position, knowledge of the scanner position is important to the overall accuracy of the CSI instrument.
Although less common, it is feasible to move the reference mirror, beam splitter or some other combination
of optical elements to perform a CSI scan. With a moving reference mirror, the depth of field determines the
range of surface heights accommodated by the instrument.
A.3 CSI theory of operation
As illustrated in Figure 1, a CSI signal is characterized by oscillating interference fringes and an overall
modulation envelope. Depending on the data analysis mode and the instrument design, surface topography
measurements are based on the location of the modulation envelope at each image point during the CSI scan
or the interference phase, or both.
An approximate mathematical model of a CSI signal I(x, y) is shown in Formula (A.1):
 
4π zx , y −ζ
()(()
2 2
 
Ix ,,yI= xy +Ix ,,yz exp − xy −ζσ/2 cos +φ (A.1)
() () () ()()
 
DC AC G
 
λ
 
eq
 
where
I (x, y) is the background intensity;
DC
I (x, y) is the interference fringe intensity;
AC
z(x, y) surface height for an individual image point;
ζ
is the scan position.
In systems with low illumination and NA (see ISO 10934:2020, 3.1.10.4, and ISO 25178-607), the equivalent
wavelength λ is close to the mean spectral emission wavelength of the light source. The phase gap φ is
eq G
the distance expressed in terms of interference phase between the modulation peak position and the central
bright fringe of the interference pattern. The standard deviation σ is for a Gaussian modelling of the

ISO 25178-604:2025(en)
modulation envelope. The width and shape of the modulation envelope relates to spectral and geometric
[19]
factors that limit both the temporal and spatial coherence. More exact models of CSI signals have a more
[43]
complicated form.
The general strategy for data analysis is to measure the surface topography by identifying a feature of the
modulation envelope, such as the peak, the centroid or the position of stationary phase. As an example, in
Formula (A.1), the maximum value of the modulation envelope is for z=ζ ; therefore, the scan position for
which the modulation envelope reaches its peak value is a measure of surface height. A topography
measurement using the modulation envelope is sometimes referred to as the “coherence profile” or
[44]
“coherence map”, and can be the final reported areal surface topography for optically rough surfaces (see
ISO 25178-600:2019, 3.4.5).
A common signal processing option is to refine the topography by analysis of the interference fringes, e.g.
using zero crossings, sinusoidal fitting or the equivalent of phase shifting interferometry (see ISO 25178-603)
using data collected near the modulation envelope peak. The initial surface topography measurement using
the coherence information determines the appropriate 2π interval for the phase evaluation. A measurement
using interference phase is often considered to be more precise or as having a lower measurement noise than
the coherence map, but is typically restricted to optically smooth surfaces (see ISO 25178-600:2019, 3.4.4).
A wide range of data acquisition strategies and data processing methods are employed in modern
instruments to make use of the modulation envelope alone or together with the interference fringes in
the CSI signal described by Formula (A.1). These acquisition strategies and algorithms include electronic
[45][46]
envelope detection, correlation of the experimental CSI signal with a theoretical or calibrated
[47] [44][48]
complex kernel, and Fourier analysis of the frequency content of the CSI signal. Summary details of
algorithms and data processing methods are provided in References [25], [26], [27] and [28].
A.4 CSI for transparent films profiling
CSI has the ability to separate multiple reflections from semi-transparent film structures on surfaces so as
[15][49]
to measure the surface topography over and under these films. From Figure A.3, it is apparent that for
a single-layer film (see ISO 25178-600:2019, 3.4.3) that is thicker than the width of the modulation envelope,
there are two clearly identifiable modulation envelopes corresponding to surface reflections from the film
boundaries. An approach to generating surface topography maps over films is to identify the right-most
signal as the top-surface signal, as shown in Figure A.3. If the refracting properties of the film are known,
the substrate or other secondary surfaces below the top surface can be mapped for height by analysis of
the signals that follow the top-surface signal, yielding additional information such as 3D film thickness. CSI
instruments may have an adjustable illumination NA, to optimize the signal strength and improve accuracy
[23][50][51]
when viewing through films.
The thinnest film for which the two signals shown in Figure A.3 can be considered separable is related to the
width of the modulation envelope. This lower limit on film thickness is defined informally as the axial response
[52] [20]
or axial resolution of the CSI instrument, in analogy with optical coherence tomography, optical
[53]
microscopy (see ISO 10934:2020, 3.1.128.5) and confocal microscopy (see ISO 25178-607:2019, C.2).

ISO 25178-604:2025(en)
Key
X CSI scan position, in µm A substrate signal
Y intensity B top-surface signal
Figure A.3 — Example CSI signal for a single-layer, partially transparent surface film
A.5 Model-based CSI
Signal modelling allows CSI instruments in many cases to measure surface characteristics beyond
[50]
the limits of the signal processing methods in Clauses A.3 and A.4. As an example, for thin films (see
ISO 25178-600:2019, 3.4.2), the separate signals shown in Figure A.3 can overlap such that it is difficult to
clearly separate them. In this case, an approach is to model the expected signal for a range of film thickness.
A search through a library of such theoretical signals for a match to an experimental result provides the
areal surface topography in the presence of the film, as well as additional dimensional properties of the film
[43][55]
layer.
The same strategy of modelling the CSI signal can be used to determine areal surface topography in
the presence of surface features that are closer together than the topographic spatial resolution (see
ISO 25178-600:2019, 3.1.20). Modelling methods include diffraction calculations with variable parameters
[56][57][58][59]
that include feature height and spacing.

ISO 25178-604:2025(en)
Annex B
(informative)
Sources of measurement error for CSI instruments
B.1 Metrological characteristics and influence quantities
ISO 25178-600:2019, 3.1.28 defines a specific set of metrological characteristics for areal surface topography
measuring instruments. These metrological characteristics capture influence quantities, factors that
can influence a measurement result and can be propagated through an appropriate measurement model
to evaluate measurement uncertainty. See ISO
...