SIST EN IEC 61757-1-4:2026

SLOVENSKI STANDARD
01-april-2026
Optični senzorji - 1-4. del: Merjenje deformacij - Porazdeljeno zaznavanje na
podlagi Rayleighovega sipanja (IEC 61757-1-4:2025)
Fibre optic sensors - Part 1-4: Strain measurement - Distributed sensing based on
Rayleigh scattering (IEC 61757-1-4:2025)
Glasfasersensoren - Teil 1-4: Dehnungsmessung - Verteilte Sensorik auf Basis der
Rayleigh-Streuung (IEC 61757-1-4:2025)
Capteurs fibroniques - Partie 1-4: Mesure de déformation - Détection répartie basée sur
la diffusion de rayleigh (IEC 61757-1-4:2025)
Ta slovenski standard je istoveten z: EN IEC 61757-1-4:2026
ICS:
33.180.99 Druga oprema za optična Other fibre optic equipment
vlakna
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD EN IEC 61757-1-4

NORME EUROPÉENNE
EUROPÄISCHE NORM February 2026
ICS 33.180.99
English Version
Fibre optic sensors - Part 1-4: Strain measurement - Distributed
sensing based on Rayleigh scattering
(IEC 61757-1-4:2025)
Capteurs fibroniques - Partie 1-4: Mesure de déformation - Glasfasersensoren - Teil 1-4: Dehnungsmessung - Verteilte
Détection répartie basée sur la diffusion de Rayleigh Sensorik auf Basis der Rayleigh-Streuung
(IEC 61757-1-4:2025) (IEC 61757-1-4:2025)
This European Standard was approved by CENELEC on 2026-01-20. CENELEC 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 CENELEC 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 CENELEC member into its own language and notified to the CEN-CENELEC Management Centre has the
same status as the official versions.
CENELEC members are the national electrotechnical committees of Austria, Belgium, Bulgaria, Croatia, Cyprus, the Czech Republic,
Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, the
Netherlands, Norway, Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,
Türkiye and the United Kingdom.

European Committee for Electrotechnical Standardization
Comité Européen de Normalisation Electrotechnique
Europäisches Komitee für Elektrotechnische Normung
CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2026 CENELEC All rights of exploitation in any form and by any means reserved worldwide for CENELEC Members.
Ref. No. EN IEC 61757-1-4:2026 E

European foreword
The text of document 86C/1972/CDV, future edition 1 of IEC 61757-1-4, prepared by SC 86C "Fibre
optic systems, sensing and active devices" of IEC/TC 86 "Fibre optics" was submitted to the IEC-
CENELEC parallel vote and approved by CENELEC as EN IEC 61757-1-4:2026.
The following dates are fixed:
• latest date by which the document has to be implemented at national (dop) 2027-02-28
level by publication of an identical national standard or by endorsement
• latest date by which the national standards conflicting with the (dow) 2029-02-28
document have to be withdrawn
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CENELEC shall not be held responsible for identifying any or all such patent rights.
Any feedback and questions on this document should be directed to the users’ national committee. A
complete listing of these bodies can be found on the CENELEC website.
Endorsement notice
The text of the International Standard IEC 61757-1-4:2025 was approved by CENELEC as a
European Standard without any modification.
In the official version, for Bibliography, the following notes have to be added for the standard indicated:
IEC 60869-1 NOTE Approved as EN IEC 60869-1
IEC 60793-2-50 NOTE Approved as EN IEC 60793-2-50
Annex ZA
(normative)
Normative references to international publications
with their corresponding European publications
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.
NOTE 1  Where an International Publication has been modified by common modifications, indicated by (mod),
the relevant EN/HD applies.
NOTE 2  Up-to-date information on the latest versions of the European Standards listed in this annex is available
here: www.cencenelec.eu.
Publication Year Title EN/HD Year
IEC 61757 - Fibre optic sensors - Generic specification EN IEC 61757 -
IEC 61757-1-2 2023 Fibre optic sensors - Part 1-2: Strain EN IEC 61757-1-2 2023
measurement - Distributed sensing based
on Brillouin scattering
IEC 61757-2-2 - Fibre optic sensors - Part 2-2: Temperature EN 61757-2-2 -
measurement - Distributed sensing
IEC 61757-3-2 - Fibre optic sensors - Part 3-2: Acoustic EN IEC 61757-3-2 -
sensing and vibration measurement -
Distributed sensing
ISO/IEC Guide 98-3 - Uncertainty of measurement - Part 3: - -
Guide to the expression of uncertainty in
measurement (GUM:1995)
IEC 61757-1-4 ®
Edition 1.0 2025-12
INTERNATIONAL
STANDARD
Fibre optic sensors -
Part 1-4: Strain measurement - Distributed sensing based on Rayleigh scattering
ICS 33.180.99  ISBN 978-2-8327-0944-3

IEC 61757-1-4:2025-12(en)
IEC 61757-1-4:2025 © IEC 2025
CONTENTS
FOREWORD . 3
INTRODUCTION . 5
1 Scope . 6
2 Normative references . 6
3 Terms, definitions, abbreviated terms, and symbols . 6
3.1 Terms and definitions . 6
3.2 Abbreviated terms . 10
3.3 Symbols . 11
4 General test setup for measurement of performance parameters . 11
4.1 General and test setup requirements . 11
4.2 General documentation requirements . 15
5 Measurement procedures for performance parameters . 16
5.1 Strain measurement error . 16
5.1.1 Test procedure and conditions . 16
5.1.2 Parameter calculation and reporting . 16
5.2 Spatial resolution . 16
5.2.1 Test procedure and conditions . 16
5.2.2 Parameter calculation and reporting . 17
5.3 Strain repeatability . 18
5.3.1 Test procedure and conditions . 18
5.3.2 Parameter calculation and reporting . 18
5.4 Spatial strain uncertainty . 19
5.4.1 Test procedure and conditions . 19
5.4.2 Parameter calculation and reporting . 19
5.5 Warm-up time . 19
5.5.1 Test procedure and conditions . 19
5.5.2 Parameter calculation and reporting . 20
5.6 System performance with altered attenuation . 20
5.6.1 General . 20
5.6.2 Long distance measurement . 21
5.6.3 Short distance measurement with high loss . 22
Annex A (informative) Application area of Rayleigh-based distributed strain
measurements . 24
Annex B (informative) Strain measurement using cross correlation of Rayleigh
scattering . 25
Bibliography . 27

Figure 1 – Optical fibre strain profile and related strain sample points. 8
Figure 2 – General test setup for a single-ended configuration. 12
Figure 3 – Measured versus applied strain (typical curve) . 14
Figure 4 – Rayleigh frequency shift as a function of elongation of a single-mode fibre . 14
Figure 5 – Illustration of spatial resolution test results . 17

IEC 61757-1-4:2025 © IEC 2025
Figure 6 – Performance evaluation at distance measurement range . 21
Figure 7 – Performance evaluation at short distance with high loss . 22
Figure B.1 – Strain measurement obtained from two Rayleigh scattering spectra
measured with the OTDR technique . 25

IEC 61757-1-4:2025 © IEC 2025
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
Fibre optic sensors -
Part 1-4: Strain measurement -
Distributed sensing based on Rayleigh scattering

FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote international
co-operation on all questions concerning standardization in the electrical and electronic fields. To this end and
in addition to other activities, IEC publishes International Standards, Technical Specifications, Technical Reports,
Publicly Available Specifications (PAS) and Guides (hereafter referred to as “IEC Publication(s)”). Their
preparation is entrusted to technical committees; any IEC National Committee interested in the subject dealt with
may participate in this preparatory work. International, governmental and non-governmental organizations liaising
with the IEC also participate in this preparation. IEC collaborates closely with the International Organization for
Standardization (ISO) in accordance with conditions determined by agreement between the two organizations.
2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
consensus of opinion on the relevant subjects since each technical committee has representation from all
interested IEC National Committees.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC
Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any
misinterpretation by any end user.
4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
transparently to the maximum extent possible in their national and regional publications. Any divergence between
any IEC Publication and the corresponding national or regional publication shall be clearly indicated in the latter.
5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity
assessment services and, in some areas, access to IEC marks of conformity. IEC is not responsible for any
services carried out by independent certification bodies.
6) All users should ensure that they have the latest edition of this publication.
7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
members of its technical committees and IEC National Committees for any personal injury, property damage or
other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and
expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
Publications.
8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) IEC draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). IEC 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, IEC 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 https://patents.iec.ch. IEC
shall not be held responsible for identifying any or all such patent rights.
IEC 61757-1-4 has been prepared by subcommittee 86C: Fibre optic systems, sensing and
active devices, of IEC technical committee 86: Fibre optics. It is an International Standard.
The text of this International Standard is based on the following documents:
Draft Report on voting
86C/1972/CDV 86C/1995/RVC
Full information on the voting for its approval can be found in the report on voting indicated in
the above table.
The language used for the development of this International Standard is English.
IEC 61757-1-4:2025 © IEC 2025
This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available
at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are
described in greater detail at www.iec.ch/publications.
A list of all parts in the IEC 61757 series, published under the general title Fibre optic sensors,
can be found on the IEC website.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under webstore.iec.ch in the data related to the
specific document. At this date, the document will be
– reconfirmed,
– withdrawn, or
– revised.
IEC 61757-1-4:2025 © IEC 2025
INTRODUCTION
This document is part of the IEC 61757 series, which is dedicated to fibre optic sensors. Generic
specifications for fibre optic sensors are defined in IEC 61757.
The individual parts of the IEC 61757 series are numbered as IEC 61757-M-T, where M denotes
the measurand and T the technology of the fibre optic sensor. The IEC 61757-1-T series is
concerned with strain measurements.

IEC 61757-1-4:2025 © IEC 2025
1 Scope
This part of IEC 61757 defines the terminology, structure, and measurement methods of
distributed fibre optic sensors for absolute strain measurements based on spectral correlation
analysis of Rayleigh backscattering signatures in single-mode fibres, where the fibre is the
distributed strain measurement element in a measurement range from about 10 m to tens of
km. This document also applies to hybrid sensor systems that combine the advantages of
Brillouin and Rayleigh backscattering effects to obtain optimal measurement quality.
This document also specifies the most important features and performance parameters of these
distributed fibre optic strain sensors and defines procedures for measuring these features and
parameters.
This part of IEC 61757 does not apply to point measurements or to dynamic strain
measurements. Distributed strain measurements using Brillouin scattering in single-mode fibres
are covered in IEC 61757-1-2.
The most relevant applications of this strain measurement technique are listed in Annex A,
while Annex B provides a short description of the underlying measurement principle.
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 61757, Fibre optic sensors - Generic specification
IEC 61757-1-2:2023, Fibre optic sensors - Part1-2: Strain measurement - Distributed sensing
based on Brillouin scattering.
IEC 61757-2-2, Fibre optic sensors - Part 2-2: Temperature measurement - Distributed sensing
IEC 61757-3-2, Fibre optic sensors - Part 3-2: Acoustic sensing and vibration measurement -
Distributed sensing
ISO/IEC Guide 98-3, Uncertainty of measurement - Part 3: Guide to the expression of
uncertainty in measurement (GUM:1995)
3 Terms, definitions, abbreviated terms, and symbols
3.1 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 61757, IEC 61757‑2‑2,
IEC 61757-3-2 and the following apply.
ISO and IEC maintain terminology databases for use in standardization at the following
addresses:
– IEC Electropedia: available at https://www.electropedia.org/
– ISO Online browsing platform: available at https://www.iso.org/obp
NOTE For the following definitions, the relevant test procedures and parameters are defined in Clause 4.
IEC 61757-1-4:2025 © IEC 2025
3.1.1
distributed fibre optic strain sensing system
DSS
measurement set-up consisting of a distributed fibre optic sensor connected to an interrogation
unit, including processor, data archive, and user interface, which provides a spatially resolved
strain measurement
[SOURCE: IEC 61757-1-2:2023, 3.1.1]
3.1.2
distance measurement range
maximum distance from the DSS interrogation unit output connector along the fibre optic sensor
within which the DSS measures strain with specified measurement performance under defined
conditions
Note 1 to entry: Defined conditions are spatial resolution (3.1.8), spatial strain uncertainty (3.1.9), and
measurement time (3.1.5).
Note 2 to entry: This supporting parameter is closely related to the total accumulated optical loss (one way)
tolerated by the interrogation unit without affecting specified measurement performance. In test cases used to prove
or verify the reported specifications, the total fibre length is equal to or greater than the specified distance
measurement range, for the tolerated total accumulated optical loss.
Note 3 to entry: The distance measurement range is usually expressed in m or km.
[SOURCE: IEC 61757-1-2:2023, 3.1.2, modified – Note 4 to entry deleted.]
3.1.3
strained spot
ΔL
length of fibre optic sensor that experiences a small elongation (δL), which causes strain that
is significantly bigger than the strain repeatability of the interrogation unit and which is
confirmed by a reference strain measurement
Note 1 to entry: The applied strain ε is equal to (δL/ΔL).
Note 2 to entry: It is useful to define strain in με, where 1 με corresponds to a δL of 1 μm over a ΔL of 1 m.
[SOURCE: IEC 61757-1-2:2023, 3.1.3]
3.1.4
location
L
optical distance from the DSS interrogation unit output connector to a desired strain sample
point along the fibre optic sensor
Note 1 to entry: The farthest location from the DSS interrogation unit output connector for the particular test is
quantified as L and is often chosen to be the same as the distance measurement range for purposes of
F,long
comparing the measurement results with quoted specifications.
Note 2 to entry: The location is usually expressed in m or km.
[SOURCE: IEC 61757-1-2:2023, 3.1.4]
IEC 61757-1-4:2025 © IEC 2025
3.1.5
measurement time
time between independent strain measurements when making successive measurements on a
single fibre optic sensor
Note 1 to entry: Equivalently, it is the time interval between successive strain trace timestamps under these
conditions.
Note 2 to entry: This parameter includes acquisition time and processing time for the measured data. This
parameter is typically selectable by the user in some limited fashion. Multiple independent strain measurements can
be averaged together to provide an overall measurement time.
[SOURCE: IEC 61757-1-2:2023, 3.1.5]
3.1.6
point defect
local deviation of a fibre optic sensor from its nominal optical and mechanical properties
occurring at a single location, or over a length substantially less than the DSS spatial resolution
Note 1 to entry: The definition of a point defect encompasses a wide range of situations, which can produce similar
effects on the strain trace. Examples include:
– a point loss, like a bad fibre splice,
– a reflectance (or return loss), as can be introduced by a fibre connector,
– a localized region of high loss, like a bend or kink in the fibre, and
– a physical discontinuity in the fibre, like a splice between two fibres of different core diameters.
[SOURCE: IEC 61757-1-2:2023, 3.1.6]
3.1.7
sample spacing
distance between two consecutive strain sample points in a single strain trace
Note 1 to entry: Sample spacing can be a user-selectable parameter in the interrogation unit.
Note 2 to entry: The sample spacing is usually expressed in mm.
Note 3 to entry: See Figure 1.
Note 4 to entry: This parameter is also called “gage pitch”.

Figure 1 – Optical fibre strain profile and related strain sample points
[SOURCE: IEC 61757-1-2:2023, 3.1.7]
IEC 61757-1-4:2025 © IEC 2025
3.1.8
spatial resolution
smallest length of strain-affected fibre optic sensor for which a DSS can measure and confirm
the reference strain of a defined strained spot within the specified strain measurement error of
the DSS
Note 1 to entry: The spatial resolution is usually expressed in mm.
[SOURCE: IEC 61757-1-2:2023, 3.1.8]
3.1.9
spatial strain uncertainty
uncertainty of the location of strain data in a single strain trace, expressed by twice the standard
deviation of a specified number of adjacent strain sample points, with the fibre optic sensor held
at constant strain and temperature
Note 1 to entry: Due to a potential cross-sensitivity of DSS to temperature, it can be necessary to stabilize the
temperature of the fibre optic sensor.
Note 2 to entry: The spatial strain uncertainty is usually expressed in units of με and noted as a tolerance
(e.g. ±xx με), where 1 με corresponds to a δL of 1 μm over a ΔL of 1 m.
[SOURCE: IEC 61757-1-2:2023, 3.1.9]
3.1.10
strain measurement error
maximum difference between a centred and uniformly weighted moving average of the
measured strain and a reference strain for all data points of the fibre optic sensor over the full
operating temperature range and all acquisition times
Note 1 to entry: Single value (worst case) is expressed like a tolerance in units of με (e.g. ±xx με).
Note 2 to entry: The number of elements used for the moving average is defined in this document. In practical
applications, other methods of smoothing might be applicable.
[SOURCE: IEC 61757-1-2:2023, 3.1.11]
3.1.11
strain repeatability
precision of strain data based on repeated strain traces at a given location expressed by twice
the standard deviation of corresponding strain sample points in each strain trace, with the fibre
optic sensor held at constant strain and temperature
Note 1 to entry: The strain repeatability is expressed like a tolerance in units of με (e.g. ±xx με).
[SOURCE: IEC 61757-1-2:2023, 3.1.12]
3.1.12
strain sample point
single point at a known location along a fibre optic sensor, where a strain value is to be
measured
Note 1 to entry: Due to signal averaging effects, the measured value represents the strain along a very small section
of the fibre optic sensor that includes the strain sample point.
Note 2 to entry: See Figure 1.
[SOURCE: IEC 61757-1-2:2023, 3.1.13]
IEC 61757-1-4:2025 © IEC 2025
3.1.13
strain trace
set of strain sample points distributed along a fibre optic sensor and spaced by the sample
spacing
Note 1 to entry: All sample points are associated with a common time of measurement, often called "trace
timestamp". The measured values represent the strain during a time period that includes the timestamp.
Note 2 to entry: All sample points in a strain trace are measured values produced by the DSS, and not interpolated
or smoothed values produced by subsequent processing outside the interrogation unit.
[SOURCE: IEC 61757-1-2:2023, 3.1.14]
3.1.14
total fibre length
L
F,tot
distance from the DSS interrogation unit output connector to the far end of the fibre optic sensor
Note 1 to entry: The far end of the fibre optic sensor can be either a purposely cut or a terminated end of the fibre,
physically located far from the interrogation unit (in a single-ended configuration).
Note 2 to entry: This parameter is either equal to or greater than the distance measurement range and usually
expressed in m or km.
[SOURCE: IEC 61757-1-2:2023, 3.1.15]
3.1.15
warm-up time
duration between the instant after which the power supply of the DSS interrogation unit is
energized and the instant when the interrogation unit may be used as specified by the
manufacturer
Note 1 to entry: Warm-up time is usually expressed in seconds or minutes.
Note 2 to entry: The warm-up time helps to upload software and to stabilize operating temperatures of optical and
electronic components.
[SOURCE: IEC 61757-1-2:2023, 3.1.16]
3.1.16
cross correlation coefficient
value showing similarity between two Rayleigh scattering spectra at the same optical fibre
section
Note 1 to entry: The cross correlation coefficient indicates the quality of the measurement. The possible range of the
cross correlation coefficient is generally from 0 to +1,0, where larger values indicate better correlation.
Note 2 to entry: In the optical time domain reflectometry method (OTDR method), the Rayleigh scattering spectrum
is observed by the intensity as a function of wavelength, while changing the wavelength of the launched signals. In
the optical frequency domain reflectometry method (OFDR method), Rayleigh scattering is observed by phase shifts
of Fourier transformed waveforms.
3.2 Abbreviated terms
DSS distributed fibre optic strain sensing system
FAT factory acceptance test
LVDT linear variable differential transformer
OFDR optical frequency domain reflectometry
OTDR optical time domain reflectometry
TW-COTDR tuneable-wavelength coherent optical time domain reflectometry
VOA variable optical attenuator
IEC 61757-1-4:2025 © IEC 2025
3.3 Symbols
A cross-sectional area
E Young’s modulus
F force
L optical distance from the output connector to a desired strain sample
point
L long fibre lengths
F,long
L optional fibre length
F,opt
L short fibre length
F,short
L total fibre length
F,tot
ΔL length of fibre optic sensor to be strained (strained spot)
δL small change in length of ΔL
N number of traces
n number of data points
S standard deviation
T temperature
maximal DSS operating temperature
T
high
T minimal DSS operating temperature
low
T typical DSS operating temperature
op
T ambient operating temperature of the strain test section
STC
ε strain
ε strain repeatability
rep
ε spatial strain uncertainty
unc
σ stress
4 General test setup for measurement of performance parameters
4.1 General and test setup requirements
A general test setup for single-ended configurations is schematically shown in Figure 2. The
aim of this setup is to provide a common base for determining the measurement specifications
while minimizing complexity, cost, reconfiguration requirements, and test execution time.
Temperature stabilisation is used to avoid possible crosstalk from temperature variations.
IEC 61757-1-4:2025 © IEC 2025
Key
1 temperature-controlled encasement (e.g. temperature chamber)
2 DSS interrogation unit
3 DSS interrogation unit output connector
4 fibre fusion splice
5 optional variable optical attenuator
6 optional long fibre length L (normal spool)
F,opt
7 temperature-controlled environment for stable ambient conditions
8 long fibre length L (loose and strain-free wound)
F,long
9 strain test section with temperature-controlled environment for stable ambient conditions
10 fixed fibre clamping unit
11 movable fibre clamping unit
12 short fibre length L (loose wound), longer than 5 times the spatial resolution
F,short
13 fibre termination
Figure 2 – General test setup for a single-ended configuration
The temperature-controlled encasement containing the interrogation unit of the distributed fibre
optic strain sensing system (DSS) shall provide a steady temperature, for an extended period
of time, within the temperature operating range (T ≤ T ≤ T ) of the device under test.
low op high
Commercial off-the-shelf temperature chambers should be used for determining the
performance parameters. Minimum requirements for such a device are:
– minimal and maximal temperature settings shall exceed the minimal and maximal operating
temperatures of the interrogation unit under test;
– temperature variation in time (steady state) shall be less than (±0,5 °C);
– temperature homogeneity in encasement volume shall be less than (±1,5 °C).
IEC 61757-1-4:2025 © IEC 2025
For the optical power adjustment, a calibrated optical attenuator or an optical attenuator that
can be self-calibrated shall be used. If a variable optical attenuator is used, the optical
attenuator should
– be calibrated for the wavelength of operation (or self-calibrated with a power meter);
– feature a variable attenuation ran
...