Non-destructive testing - Thermographic testing - Active thermography with laser excitation

This document determines the guidelines and the specifications for non-destructive testing using active thermography with laser excitation.
Active thermography with laser excitation is mainly applicable, but not limited to different materials (e.g. composites, metals, ceramics) and to:
-   the detection of surface-breaking discontinuities, particularly cracks;
-   the detection of discontinuities located just below the surface or below coatings with an efficiency that diminishes rapidly with a few mm depth;
-   the detection of disbonds and delamination parallel to the examined surface;
-   the measurement of thermal material properties, like thermal diffusivity;
-   the measurement of coating thickness.
The requirements for the equipment, for the verification of the system, for the surface condition of the part to be tested, for the scanning conditions, for the recording, the processing and the interpretation of the results are specified. Acceptance criteria are not defined.
Active thermography with laser excitation can be applied in industrial production as well as in maintenance and repair (vehicle parts, engine parts, power plant, aerospace, etc.).

Zerstörungsfreie Prüfung - Thermografische Prüfung - Aktive Thermografie mit Laser-Anregung

Dieses Dokument bestimmt die Leitlinien und Spezifikationen für zerstörungsfreie Prüfungen mittels aktiver Thermografie mit Laser Anregung.
Aktive Thermografie mit Laser Anregung ist hauptsächlich, aber nicht ausschließlich auf verschiedene Werkstoffe (z. B. Verbundstoffe, Metalle, Keramik) und bei Folgendem anwendbar:
   Erkennung von zur Oberfläche hin offenen Unregelmäßigkeiten, besonders von Rissen;
   Erkennung von Unregelmäßigkeiten unmittelbar unter der Oberfläche oder unter Beschichtungen mit einem Wirkungsgrad, der bereits in einer Tiefe von wenigen mm schnell abnimmt;
   Erkennung von Ablösungen und Schichtablösung parallel zur untersuchten Oberfläche;
   Messung von thermischen Werkstoffeigenschaften, wie z. B. der thermischen Diffusivität;
   Messung der Beschichtungsdicke.
Es werden die Anforderungen an die Geräte, die Verifizierung des Systems, die Oberflächenbeschaffenheit des zu prüfenden Teils, die Abtastbedingungen, die Aufzeichnung sowie an die Verarbeitung und Auswertung der Ergebnisse festgelegt. Annahmekriterien werden nicht festgelegt.
Aktive Thermografie mit Laser Anregung kann bei der industriellen Fertigung sowie bei der Wartung und Reparaturen (von Autoteilen, Motorteilen, Kraftwerken, in der Luft  und Raumfahrtindustrie usw.) angewendet werden.

Essais non destructifs - Analyse thermographique - Thermographie active avec excitation laser

Le présent document définit les lignes directrices et les spécifications applicables aux essais non destructifs par thermographie active avec excitation laser.
La thermographie active avec excitation laser est principalement utilisée pour différents matériaux (composites, métaux, céramiques par exemple) sans toutefois s’y limiter et pour :
   la détection des discontinuités débouchantes, notamment des fissures ;
   la détection des discontinuités situées juste en dessous de la surface ou sous des revêtements, avec une efficacité qui diminue rapidement sur une profondeur de quelques millimètres ;
   la détection des décollements et délaminations parallèles à la surface examinée ;
   le mesurage des propriétés thermiques des matériaux, telles que la diffusivité thermique ;
   le mesurage de l’épaisseur d’un revêtement.
Les exigences applicables à l’appareillage, à la vérification du système, à l’état de surface de la pièce à contrôler, aux conditions de balayage, à l’enregistrement, au traitement et à l’interprétation des résultats sont spécifiées. Les critères d’acceptation ne sont pas définis.
La thermographie active avec excitation laser peut être utilisée pour la production industrielle, ainsi que pour la maintenance et la réparation (pièces de véhicule, pièces de moteur, centrales électriques, aérospatiales, etc.).

Neporušitvene preiskave - Termografsko preskušanje - Aktivna termografija z laserskim vzbujanjem

Ta dokument določa smernice in specifikacije za neporušitvene preiskave z uporabo aktivne termografije z laserskim vzbujanjem.
Aktivna termografija z laserskim vzbujanjem se med drugim v glavnem uporablja za različne materiale (npr. kompozite, kovino, keramiko) ter za:
–   odkrivanje površinskih prekinitev, predvsem razpok;
–   odkrivanje prekinitev tik pod površino ali pod prevlekami, pri čemer se učinkovitost hitro zmanjšuje z globino nekaj milimetrov;
–   odkrivanje odlepitev in razslojenosti vzporedno s površino, ki se preiskuje;
–   merjenje lastnosti toplotnih materialov, kot je toplotna razprševalnost;
–   merjenje debeline prevleke.
Določene so zahteve za opremo, preverjanje sistema, stanje površine dela, ki se preskuša, pogoje skeniranja, snemanje, obdelavo in interpretacijo rezultatov. Merila sprejemljivosti niso opredeljena.
Aktivno termografijo z laserskim vzbujanjem je mogoče uporabiti v industrijski proizvodnji ter pri vzdrževanju in popravilih (deli vozil, deli motorjev, elektrarne, aeronavtika itd.).

General Information

Status
Published
Public Enquiry End Date
23-Jul-2020
Publication Date
05-Oct-2022
Current Stage
6060 - National Implementation/Publication (Adopted Project)
Start Date
15-Jun-2022
Due Date
20-Aug-2022
Completion Date
06-Oct-2022

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SLOVENSKI STANDARD
SIST EN 17501:2022
01-november-2022
Neporušitvene preiskave - Termografsko preskušanje - Aktivna termografija z
laserskim vzbujanjem
Non-destructive testing - Thermographic testing - Active thermography with laser
excitation
Zerstörungsfreie Prüfung - Thermografische Prüfung - Aktive Thermografie mit Laser-
Anregung
Essais non destructifs - Analyse thermographique - Thermographie active avec
excitation laser
Ta slovenski standard je istoveten z: EN 17501:2022
ICS:
19.100 Neporušitveno preskušanje Non-destructive testing
SIST EN 17501:2022 en,fr,de
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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SIST EN 17501:2022

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SIST EN 17501:2022


EN 17501
EUROPEAN STANDARD

NORME EUROPÉENNE

June 2022
EUROPÄISCHE NORM
ICS 19.100
English Version

Non-destructive testing - Thermographic testing - Active
thermography with laser excitation
Essais non destructifs - Analyse thermographique - Zerstörungsfreie Prüfung - Thermografische Prüfung -
Thermographie active avec excitation laser Aktive Thermografie mit Laser-Anregung
This European Standard was approved by CEN on 20 April 2022.

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, Turkey 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
© 2022 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN 17501:2022 E
worldwide for CEN national Members.

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SIST EN 17501:2022
EN 17501:2022 (E)
Contents Page
European foreword . 4
1 Scope . 5
2 Normative references . 5
3 Terms and definitions . 5
4 Qualification and certification of personnel . 6
5 Principle of laser thermography and experimental setup . 6
5.1 General . 6
5.2 Typical configurations of excitation . 8
5.2.1 General . 8
5.2.2 Laser thermography in static configuration (without relative movement) . 8
5.2.3 Laser thermography in dynamic configuration (with relative movement) . 8
5.2.4 Laser thermography with different temporal excitations . 8
5.2.5 Laser thermography with different spatial excitations . 8
5.3 Laser and laser optics requirements . 8
5.3.1 Laser irradiance and wavelength . 8
5.3.2 Spatial illumination shapes . 9
5.3.3 Switchable laser for lock-in thermography and other temporal techniques . 10
5.3.4 Safety . 10
5.4 Scanning system requirement . 11
5.4.1 General . 11
5.4.2 Test object position and orientation . 11
5.4.3 Movement of the test object . 12
5.4.4 Movement of the whole measurement system . 12
5.4.5 Movement of the laser beam through optics . 12
5.4.6 Movement of the laser beam and IR camera through optics . 12
5.4.7 Setup stability . 12
5.5 Specifications of the IR camera . 12
5.6 Data processing and analysis techniques . 14
5.6.1 General . 14
5.6.2 Spot with relative movement . 14
5.6.3 Line with relative movement . 17
5.7 Data processing for crack characterization. 17
5.7.1 General . 17
5.7.2 Static pulsed laser spot . 17
5.7.3 Continuously scanned laser spot . 19
5.7.4 Continuously scanned laser line . 19
5.8 Data processing and analysis techniques for the determination of lateral thermal
diffusivity . 20
5.9 Data processing and analysis techniques for emissivity correction . 20
5.10 Data processing and analysis techniques for coating thickness control . 20
6 Reference test specimens . 20
7 Calibration, validation and performance of testing . 20
8 Evaluation, classification and registration of thermographic indications . 21
2

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SIST EN 17501:2022
EN 17501:2022 (E)
9 Test report . 21
Annex A (informative) List of influential parameters for the NDT qualification of laser
thermographic system . 23
A.1 General . 23
A.2 Input data group parameters . 23
A.2.1 Component and his environment: . 23
A.2.2 Discontinuities: . 23
A.3 NDT laser TT System (procedure parameters) . 24
A.3.1 IR Camera and optics . 24
A.3.2 Laser . 25
A.3.3 Scanning system and set-up . 25
A.3.4 Calibration Blocks . 25
A.3.5 Data processing and analysis . 25
Annex B (informative) Reference blocks . 27
B.1 Test specimen containing an artificial surface breaking notch . 27
B.2 Test specimen containing a natural crack . 28
B.3 Test specimen containing natural cracks – reference block no. 1 for magnetic
particle testing according to EN ISO 9934-2 . 29
B.4 Test specimen containing artificial subsurface notches . 29
Bibliography . 31

3

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SIST EN 17501:2022
EN 17501:2022 (E)
European foreword
This document (EN 17501:2022) has been prepared by Technical Committee CEN/TC 138 “Non-
destructive testing”, 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 December 2022, and conflicting national standards shall
be withdrawn at the latest by December 2022.
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.
Any feedback and questions on this document should be directed to the users’ national standards body.
A complete listing of these bodies can be found on the CEN website.
According to the CEN-CENELEC Internal Regulations, the national standards organisations 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, Turkey and the United
Kingdom.
4

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SIST EN 17501:2022
EN 17501:2022 (E)
1 Scope
This document specifies a method and establishes guidelines for non-destructive testing using active
thermography with laser excitation.
Active thermography with laser excitation is mainly applicable, but not limited, to different materials (e.g.
composites, metals, ceramics) and to:
— the detection of surface-breaking discontinuities, particularly cracks;
— the detection of discontinuities located just below the surface or below coatings with an efficiency
that diminishes rapidly with a few mm depth;
— the detection of disbonds and delamination parallel to the examined surface;
— the measurement of thermal material properties, like thermal diffusivity;
— the measurement of coating thickness.
The requirements for the equipment, for the verification of the system, for the surface condition of the
test object, for the scanning conditions, for the recording, the processing and the interpretation of the
results are specified. This document does not apply to the definition of acceptance criteria.
Active thermography with laser excitation can be applied in industrial production as well as in
maintenance and repair (vehicle parts, engine parts, power plant, aerospace, etc.).
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.
EN 12464-1, Light and lighting - Lighting of work places - Part 1: Indoor work places
EN 16714-1, Non-destructive testing - Thermographic testing - Part 1: General principles
EN 16714-2, Non-destructive testing - Thermographic testing - Part 2: Equipment
EN 16714-3, Non-destructive testing - Thermographic testing - Part 3: Terms and definitions
EN 17119, Non-destructive testing - Thermographic testing - Active thermography
EN ISO 9712, Non-destructive testing - Qualification and certification of NDT personnel (ISO 9712)
3 Terms and definitions
For the purposes of this document, the terms and definitions given in EN 16714-3 and EN 17119 and the
following apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https://www.iso.org/obp
— IEC Electropedia: available at https://www.electropedia.org/
5

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SIST EN 17501:2022
EN 17501:2022 (E)
3.1
laser
light amplification by stimulated emission of radiation used as light source for thermal excitation in laser
thermography
3.2
scanning system
system which provides relative movement of the shaped laser beam, the surface of the test object and/or
the IR camera
3.3
flying spot technique
scanning of the shaped laser beam on the surface of the test object
3.4
spot diameter
full width at half maximum of the irradiation profile of the laser beam at the target plane
3.5
scanning speed
relative velocity of the laser spot at the target plane
3.6
illumination pattern
spatial distribution of the laser irradiation at the target plane
4 Qualification and certification of personnel
The competence of the test personnel using this document shall be demonstrated according to
EN ISO 9712 or an equivalent formalized system and the following:
— the relevant standards, rules, specifications, test instructions and description of the methods;
— the type of equipment and its operation;
— the mounting, design, structure and operation of the test objects;
The test personnel shall have sufficient knowledge about the test object and about the possible diagnostic
findings.
5 Principle of laser thermography and experimental setup
5.1 General
Laser thermography is a technique of active thermography. A very basic experimental setup is given in
Figure 1. The absorption of laser radiation at the surface generates a heat flow into the test object (also
called photothermal effect). The illumination pattern of the laser can be designed to deliver a focused
spot, a line, or an area (e.g. square or circle). The presence of a discontinuity inside or on the surface of
the test object alters the heat flow in a specific way. As a result, the shape and symmetry of the induced
heat flow pattern allows the testing to be optimized to different classes of discontinuities, like, e.g. planar
and volume defects or linear cracks or measurement of material properties that influence the diffusion
of heat. The resulting temporal evolution of the thermal radiation distribution on the surface during or
after laser illumination is acquired by an infrared (IR) camera, and converted to a signal that can be
analysed by different signal processing algorithms. One advantage of applying a laser as thermal energy
6

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SIST EN 17501:2022
EN 17501:2022 (E)
source is that a large spatial distance between source and test object surface can be set (of typically some
centimetres up to several metres). A further advantage when utilizing a laser is given by its narrow
emission spectrum, which can be clearly separated from the IR camera detection range. This
characteristic facilitates the reflection configuration without using additional filters.
Typically, high-power lasers with an output power of several watts to kilowatts are used. Due to focusing
2
to, e.g. a spot or a line, a high irradiance (MW to GW per m ) can be achieved. In order to test the entire
test object surface, the illumination shall be scanned over the test object surface by a relative movement
between test object, laser, and/or IR camera. As laser sources, continuous as well as pulsed laser systems
can be used.
Since lasers typically allow very high modulation rates (kHz-range is easily achievable), different
temporal excitation and testing modes can be implemented.
The quality of the thermographic signal depends on a number of experimental setup and test object
parameters. In the following a list of preferable prerequisites to improve signal quality (i.e. signal to noise
ratio and contrast to noise ratio of detected discontinuity) is given:
— high laser power and/or irradiance;
— high and spatially homogenous spectral absorptance of the test object at the laser wavelength;
— high and spatially homogenous spectral emissivity of the test object at the IR camera wavelength;
— high temporal and mechanical stability between the devices moving relatively, i.e. the laser, the
scanning system and the IR camera;
— high responsivity and low NETD of the IR camera.
If, for instance, the absorptance or emissivity of the test object surface is not spatially homogenous, this
leads to inhomogeneously distributed thermal radiation which can be misinterpreted as a defect
indication. In this case an additional coating of the surface might be applied.

Key
1 scanning system 5 laser beam
2 test object 6 IR camera
3 discontinuity 7 thermal radiation
4 laser
Figure 1 — Schematic of laser thermography with movement of the test object
7

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SIST EN 17501:2022
EN 17501:2022 (E)
5.2 Typical configurations of excitation
5.2.1 General
According to EN 17119, laser thermography can be performed in static as well as in dynamic
configuration and with different types of temporal and spatial excitation.
5.2.2 Laser thermography in static configuration (without relative movement)
Laser thermography can be realized without any relative movement between illumination, test object
and IR camera. In this case, one or many thermogram(s) of the whole scene are acquired at the time(s) of
interest. The thermogram or thermogram sequence acquired can then be processed to indicate if any
discontinuity is present in the test object. In particular, pulse and lock-in thermographic testing
techniques that do not rely on a relative movement and that apply either a very short optical impulse or
a periodic optical excitation can be implemented using a laser as optical energy source.
5.2.3 Laser thermography in dynamic configuration (with relative movement)
Laser thermography can be realized with relative movement between illumination, test object and IR
camera. In this case, the IR camera is set to view either the area where the illumination is present, or an
area where the illumination is no longer present but where its thermal impact is still visible (time-lag).
The thermogram or thermogram sequence acquired can then be processed to verify if any discontinuity
is present in the test object.
5.2.4 Laser thermography with different temporal excitations
Depending on the specific laser used, different temporal excitation modes can be implemented. In
particular, if a very short laser impulse is used, this resembles the pulse thermographic testing technique
using optical sources. Another known temporal technique is lock-in thermographic testing using an
optical source whose irradiance is periodically modulated, e.g. by a sine, a triangle or a rectangular
function. Laser sources allow for continuous wave (cw) operation and can be modulated up to very high
modulation rates (>10 kHz-range). Hence lasers can be applied for all known temporal excitation
techniques and can, in addition to that, be used up to very high modulation frequencies as well as for
arbitrary temporal excitation functions.
5.2.5 Laser thermography with different spatial excitations
Depending on the specific laser used, different spatial excitation modes can be implemented. A focused
laser spot can be used e.g. for the flying spot technique, where the test object is continuously or randomly
scanned with a laser spot. In this case 3D heat transfer shall be considered. A focused laser line can be
used for techniques where the test object is continuously or randomly scanned with a laser line. In this
case only 2D heat transfer may be taken into account. A widened laser beam can be used to illuminate a
larger area homogeneously. Depending on the size of this area, only 1D heat transfer in depth direction
may be considered.
Illumination patterns like spots or lines cannot only be used to locate and quantify discontinuities, but
can also be exploited to determine the directional dependence on thermal material properties, e.g. the
thermal diffusivity.
5.3 Laser and laser optics requirements
5.3.1 Laser irradiance and wavelength
The laser system used for inspection shall meet the specific requirements of the thermographic testing
task. For a specified IR camera the main parameters determining the minimum detectable temperature
increase on the test object surface by the IR camera are the irradiance of the laser radiation and the optical
and thermal material properties of the test object. Concerning the irradiance of the laser radiation, the
8

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SIST EN 17501:2022
EN 17501:2022 (E)
relationship with the temperature increase is linear. This means that the choices of laser output power
and focusing optics directly influence the achievable temperature increase. In addition, the spectral
absorptance of the test object at the laser wavelength determines how much of the incident radiant
energy is converted into heat. Therefore, if the absorptance is small, as it is for instance for polished
metals or partially transparent polymers, the laser irradiance shall be accordingly high. In the best case,
the laser wavelength is chosen to be at maximum of the spectral absorptance of the test object.
Additionally, the laser wavelength shall be outside the spectral sensitivity range of the IR camera to avoid
damaging the detector and to avoid that the partially reflected laser radiation from the test object can be
confused with the thermal radiation from the test object. Besides the irradiance and test object
absorptance, the thermal material properties of the test object (i.e. thermal conductivity k, specific heat
c , and density ρ) determine the gained temperature increase due to laser heating. For the two limiting
p
-1/2
cases of pulse and lock-in thermography, the temperature rise ΔT is proportional to (k·c ·ρ) . This
p
means that for highly thermally conducting materials only a small temperature rise is generated.
Accordingly, the laser irradiance shall be chosen high enough in order to compensate for a sufficient
temperature rise during testing.
For maximum laser power it should be considered that the test object is not destructed by e.g. melting,
oxidation, illumination induced colour changes or ageing of polymers. These effects are influenced not
only by the laser irradiance, but also by pulse length, thermal and optical material properties, chemical
composition etc.
5.3.2 Spatial illumination shapes
5.3.2.1 General
The illumination pattern shall be chosen accordingly to the specific spatial and temporal excitation mode
used for testing. This can be performed by optical beam shaping devices.
Generally, fibre, solid state and gas lasers have a higher beam quality than diode lasers and can be focused
to a smaller width. For focused laser spots, the spot size is only given within the depth of field of the laser
optics.
If the laser radiation is brought to the system by an optical fibre, this fibre and any additional optics shall
be set so that the laser beam has a shape compatible with the inspection.
5.3.2.2 Spot
A round-shaped focused or collimated laser spot can be generated by an optical lens or mirror system. In
dependence on the type of laser and the optics used, different spot sizes can be obtained.
5.3.2.3 Straight line
A straight line-shaped focused laser spot can be generated by a cylinder lens or lens system. In
dependence on the type of laser and optics used, different line lengths, widths and shapes can be obtained.
In this case, the sensitivity of the detection is better for discontinuities whose orientation is parallel to
the laser line.
5.3.2.4 Area
A homogeneous illumination of a larger area is obtained by lens systems. If the laser source has to be used
for pulse or lock-in thermography, the dimension of the illuminated area shall be considerably larger than
the thermal diffusion length. Otherwise this configuration refers to the photothermal or flying spot
technique. A top-hat beam profile shall be chosen in favour of a Gaussian beam profile to avoid lateral
heat flows.
9

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SIST EN 17501:2022
EN 17501:2022 (E)
5.3.2.5 Pattern
Specific illumination patterns, deviating from the above mentioned shapes, can be generated by, e.g. laser
arrays, lens arrays or diffractive optical elements. The shape of the patterns can be optimized to certain
types of discontinuities.
5.3.3 Switchable laser for lock-in thermography and other temporal techniques
In all temporal excitation techniques, and especially in those which correlate the temporal excitation
function with the temporal temperature response of the test object, the irradiance of the laser shall be
controlled as exactly as possible. A common approach is to control the output power of the laser system
by an external function generator via an analogue voltage input, which can be triggered by the IR camera.
For best performance, the output power shall have a linear relationship with the controlling voltage such
that the signal processing algorithm can be applied directly. If there is no linear relationship, the
characteristic curve of the laser (i.e. output power vs. controlling voltage) shall be taken into account
properly by the user. Concerning the temporal behaviour, the laser switch on and switch off times shall
be taken into account for the maximum possible modulation frequency.
5.3.4 Safety
The most important European standards on laser safety, safety of optical radiation and protection
measures are listed in the following:
— EN 60825, e.g.:
— EN 60825-1
— EN 60825-4
— EN ISO 11553, e.g.:
— EN ISO 11553-1
— EN ISO 11553-2
— Directive 2006/25/EC of the European Parliament and of the Council of 5 April 2006 on the minimum
health and safet
...

SLOVENSKI STANDARD
oSIST prEN 17501:2020
01-julij-2020
Neporušitvene preiskave - Termografsko preskušanje - Aktivna termografija z
laserskim vzbujanjem
Non-destructive testing - Thermographic testing - Active thermography with laser
excitation
Zerstörungsfreie Prüfung - Thermografische Prüfung - Aktive Thermografie mit Laser-
Anregung
Essais non destructifs - Analyse thermographique - Thermographie active avec
excitation laser
Ta slovenski standard je istoveten z: prEN 17501
ICS:
19.100 Neporušitveno preskušanje Non-destructive testing
oSIST prEN 17501:2020 en,fr,de
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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oSIST prEN 17501:2020

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oSIST prEN 17501:2020


DRAFT
EUROPEAN STANDARD
prEN 17501
NORME EUROPÉENNE

EUROPÄISCHE NORM

May 2020
ICS 19.100
English Version

Non-destructive testing - Thermographic testing - Active
thermography with laser excitation
Essais non destructifs - Analyse thermographique - Zerstörungsfreie Prüfung - Thermografische Prüfung -
Thermographie active avec excitation laser Aktive Thermografie mit Laser-Anregung
This draft European Standard is submitted to CEN members for enquiry. It has been drawn up by the Technical Committee
CEN/TC 138.

If this draft becomes a European Standard, 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.

This draft European Standard was established by CEN 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, Turkey and
United Kingdom.

Recipients of this draft are invited to submit, with their comments, notification of any relevant patent rights of which they are
aware and to provide supporting documentation.

Warning : This document is not a European Standard. It is distributed for review and comments. It is subject to change without
notice and shall not be referred to as a European Standard.


EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2020 CEN All rights of exploitation in any form and by any means reserved Ref. No. prEN 17501:2020 E
worldwide for CEN national Members.

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Contents Page
European foreword . 3
1 Scope . 4
2 Normative references . 4
3 Terms and definitions . 5
4 Qualification and certification of personnel . 5
5 Principle of laser thermography and instrumental setup . 6
5.1 General . 6
5.2 Typical configurations of excitation . 7
5.3 Laser and laser optics requirements . 8
5.4 Scanning system requirement . 10
5.5 Specifications of the IR camera . 12
5.6 Data processing and analysis techniques . 13
5.7 Data processing for crack characterization. 16
5.8 Data processing and analysis techniques for the determination of lateral thermal
diffusivity . 19
5.9 Data processing and analysis techniques for emissivity correction . 19
5.10 Data processing and analysis techniques for coating thickness control . 19
6 Reference test specimens . 19
7 Calibration, validation and performance of testing . 19
8 Evaluation, classification and registration of thermographic indications . 20
9 Reporting . 20
Annex A (informative) List of influential parameters for the NDT qualification of laser
thermographic system . 22
Annex B (informative) Reference blocks . 26
Bibliography . 31

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European foreword
This document (prEN 17501:2020) has been prepared by Technical Committee CEN/TC 138 “Non-
destructive testing”, the secretariat of which is held by AFNOR.
This document is currently submitted to the CEN Enquiry.
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1 Scope
This document determines the guidelines and the specifications for non-destructive testing using active
thermography with laser excitation.
Active thermography with laser excitation is mainly applicable, but not limited to different materials
(e.g. composites, metals, ceramics) and to:
— the detection of surface-breaking discontinuities, particularly cracks;
— the detection of discontinuities located just below the surface or below coatings with an efficiency
that diminishes rapidly with a few mm depth;
— the detection of disbonds and delamination parallel to the examined surface;
— the measurement of thermal material properties, like thermal diffusivity;
— the measurement of coating thickness.
The requirements for the equipment, for the verification of the system, for the surface condition of the
part to be tested, for the scanning conditions, for the recording, the processing and the interpretation of
the results are specified. Acceptance criteria are not defined.
Active thermography with laser excitation can be applied in industrial production as well as in
maintenance and repair (vehicle parts, engine parts, power plant, aerospace, etc.).
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.
EN 16714-1, Non-destructive testing — Thermographic testing — Part 1: General principles
EN 16714-2, Non-destructive testing — Thermographic testing — Part 2: Equipment
EN 16714-3, Non-destructive testing — Thermographic testing — Part 3: Terms and definitions
EN 17119, Non-destructive testing — Thermographic testing — Active thermography
EN ISO 9934-2, Non-destructive testing — Magnetic particle testing — Part 2: Detection media
(ISO 9934-2)
EN 12464-1, Light and lighting — Lighting of work places — Part 1: Indoor work places
CEN/TR 14748, Non-destructive testing — Methodology for qualification of non-destructive tests
DIN 54184, Non-destructive testing — Pulse thermography using optical excitation
VDI/VDE 5585-1, Technical temperature measurement — Temperature measurement with
thermographic cameras — Metrological characterization
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3 Terms and definitions
For the purposes of this document, the terms and definitions given in EN 16714-3 and EN 17119 and
the following apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https://www.iso.org/obp
— IEC Electropedia: available at https://www.electropedia.org/
3.1
laser
light amplification by stimulated emission of radiation used as light source for thermal excitation in
laser thermography
3.2
scanning system
system which provides relative movement of the shaped laser beam, the surface of the test object
and/or the IR camera
3.3
flying spot technique
scanning of the shaped laser beam on the surface of the test object
3.4
spot diameter
full width at half maximum of the irradiation profile of the laser beam at the target plane
3.5
scanning speed
relative velocity of the laser spot at the target plane
3.6
illumination pattern
spatial distribution of the laser irradiation at the target plane
4 Qualification and certification of personnel
The competence of the test personnel using this document shall be demonstrated according to
EN ISO 9712 or an equivalent formalized system and the following:
— the relevant standards, rules, specifications, test instructions and description of the methods;
— the type of equipment and its operation;
— the mounting, design, structure and operation of the objects under test;
The test personnel shall have sufficient knowledge about the test object and about the possible
diagnostic findings.
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5 Principle of laser thermography and instrumental setup
5.1 General
Laser thermography is a technique of active thermography. A very basic experimental setup is given in
Figure 1. The absorption of laser radiation at the surface generates the heat flux into the test object
(also called photothermal effect). The illumination pattern of the laser can be designed to deliver a
focused spot, a line, or an area (e.g. square or circle). The presence of a discontinuity inside or on the
surface of the test object alters the heat flux in a specific way. As a result, the shape and symmetry of the
induced heat flux pattern allows the testing to be optimized to different classes of discontinuities, like,
e.g. planar and volume defects or linear cracks or measurement of material properties that influence the
diffusion of heat. The resulting temporal evolution of the thermal radiation distribution on the surface
during or after laser illumination is acquired by an infrared (IR) camera, and converted to a signal that
can be analysed by different signal processing algorithms. One advantage of applying a laser as thermal
energy source is that a large spatial distance between source and test object surface can be achieved (of
typically some centimetres up to several metres). A further advantage when utilizing a laser is given by
its narrow emission spectrum, which can be clearly separated from the IR camera detection range. This
characteristic facilitates the reflection configuration without using additional filters.
Typically, high-power lasers with an output power of several watts to kilowatts are used. Due to
2
focusing to, e.g. a spot or a line, a high irradiance (MW to GW per m ) can be achieved. In order to test
the entire test object surface, the illumination needs to be scanned over the test object surface by a
relative movement between test object, laser, and/or IR camera.
NOTE As laser sources, continuous as well as pulsed laser systems can be used.
Since lasers typically allow very high modulation rates (kHz-range is easily achievable), different
temporal excitation and testing modes can be implemented.
The quality of the thermographic signal depends on a number of experimental setup and test object
parameters. In the following a list of preferable prerequisites to improve signal quality (i.e. signal to
noise ratio and contrast to noise ratio of detected discontinuity) is given:
— high laser power and/or irradiance;
— high and spatially homogenous spectral absorptance of the test object at the laser wavelength;
— high and spatially homogenous spectral emissivity of the test object at the IR camera wavelength;
— high temporal and mechanical stability between the devices moving relatively, i.e. the laser, the
scanning system and the IR camera;
— high responsivity and low NETD of the IR camera.
If, for instance, the absorptance or emissivity of the test object surface is not spatially homogenous, this
leads to in homogeneously distributed thermal radiation which can be misinterpreted as a defect
indication. In this case an additional coating of the surface might be applied.
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Key
1 Scanning system 5 Laser beam
2 Test object 6 IR camera
3 Discontinuity 7 Thermal radiation
4 Laser
Figure 1 — Schematic of laser thermography with movement of the test object
5.2 Typical configurations of excitation
5.2.1 General
According to EN 17119, laser thermography can be performed in static as well as in dynamic
configuration and with different types of temporal and spatial excitation.
5.2.2 Laser thermography in static configuration (without relative movement)
Laser thermography can be realized without any relative movement between illumination, test object
and IR camera. In this case, one or many thermogram(s) of the whole scene are acquired at the time(s)
of interest. The thermogram or thermogram sequence acquired can then be processed to indicate if any
discontinuity is present in the test object. In particular, pulse and lock-in thermographic testing
techniques that do not rely on a relative movement and that apply either a very short optical impulse or
a periodic optical excitation can be implemented using a laser as optical energy source.
5.2.3 Laser thermography in dynamic configuration (with relative movement)
Laser thermography can be realized with relative movement between illumination, test object and IR
camera. In this case, the IR camera is set to view either the area where the illumination is present, or an
area where the illumination is no longer present but where its thermal impact is still visible (time-lag).
The thermogram or thermogram sequence acquired can then be processed to indicate if any
discontinuity is present in the test object.
5.2.4 Laser thermography with different temporal excitations
Depending on the specific laser used, different temporal excitation modes can be implemented. In
particular, if a very short laser impulse is used, this resembles the pulse thermographic testing
technique using optical sources. Another known temporal technique is lock-in thermographic testing
using an optical source whose irradiance is periodically modulated, e.g. by a sine, a triangle or a
rectangular function. Laser sources allow for continuous wave (cw) operation and can be modulated up
to very high modulation rates (>10 kHz-range). Hence lasers can be applied for all known temporal
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excitation techniques and can, in addition to that, be used up to very high modulation frequencies as
well as for arbitrary temporal excitation functions.
5.2.5 Laser thermography with different spatial excitations
Depending on the specific laser used, different spatial excitation modes can be implemented. A focused
laser spot can be used e.g. for the flying spot technique, where the test object is continuously or
randomly scanned with a laser spot. In this case 3D heat transfer shall be considered. A focused laser
line can be used for techniques where the test object is continuously or randomly scanned with a laser
line. In this case only 2D heat transfer shall be taken into account. A widened laser beam can be used to
illuminate a larger area homogeneously. Depending on the size of this area, only 1D heat transfer in
depth direction shall be considered.
Illumination patterns like spots or lines cannot only be used to locate and quantify discontinuities, but
can also be exploited to determine the directional dependence on thermal material properties, e.g. the
thermal diffusivity.
5.3 Laser and laser optics requirements
5.3.1 Laser irradiance and wavelength
The laser system used for inspection shall meet the specific requirements of the thermographic testing
task. For a specified IR camera the main parameters determining the minimum detectable temperature
increase on the test object surface by the IR camera are the irradiance of the laser radiation and the
optical and thermal material properties of the test object. Concerning the irradiance of the laser
radiation, the relationship with the temperature increase is linear. This means that the choices of laser
output power and focusing optics directly influence the achievable temperature increase. In addition,
the spectral absorptance of the test object at the laser wavelength determines how much of the incident
radiant energy is converted into heat. Therefore, if the absorptance is small, as it is for instance for
polished metals or partially transparent polymers, the laser irradiance shall be accordingly high. In the
best case, the laser wavelength is chosen to be at maximum of the spectral absorptance of the test
object. Additionally, the laser wavelength should be outside the spectral sensitivity range of the IR
camera to avoid damaging the detector and to avoid that the partially reflected laser radiation from the
test object can be confused with the thermal radiation from the test object. Besides the irradiance and
test object absorptance, the thermal material properties of the test object (i.e. thermal conductivity k,
specific heat c , and density ρ) determine the gained temperature increase due to laser heating. For the
p
two limiting cases of pulse and lock-in thermography, the temperature rise ΔT is proportional to
-1/2
(k*c *ρ) . This means that for highly thermally conducting materials only a small temperature rise is
p
generated. Accordingly, the laser irradiance shall be chosen high enough in order to compensate for a
sufficient temperature rise during testing.
NOTE For maximum laser power it should be considered that the test object is not destructed by e.g. melting,
oxidation, illumination induced colour changes or ageing of polymers. These effects are influenced not only by the
laser irradiance, but also by pulse length, thermal and optical material properties, chemical composition etc.
5.3.2 Spatial illumination shapes
5.3.2.1 General
The illumination pattern shall be chosen accordingly to the specific spatial and temporal excitation
mode used for testing. This can be performed by optical beam shaping devices.
Generally, fibre, solid state and gas lasers have a higher beam quality than diode lasers and can be
focused to a smaller width. For focused laser spots, the spot size is only given within the depth of field of
the laser optics.
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If the laser radiation is brought to the system by an optical fibre, this fibre and any additional optics
should be set so that the laser beam has a shape compatible with the inspection.
5.3.2.2 Spot
A round-shaped focused or collimated laser spot can be generated by an optical lens or mirror system.
In dependence on the type of laser and the optics used, different spot sizes can be obtained.
5.3.2.3 Straight line
A straight line-shaped focused laser can be generated by a cylinder lens or lens system. In dependence
on the type of laser and optics used, different line lengths, widths and shapes can be obtained. In this
case, the sensitivity of the detection is better for discontinuities whose orientation is parallel to the
laser line.
5.3.2.4 Area
A homogeneous illumination of a larger area is obtained by lens systems. If the laser source has to be
used for pulse or lock-in thermography, the dimension of the illuminated area shall be considerably
larger than the thermal diffusion length. Otherwise this configuration refers to the photothermal or
flying spot technique. A top-hat beam profile shall be chosen in favour of a Gaussian beam profile to
avoid lateral heat flows.
5.3.2.5 Pattern
Specific illumination patterns, deviating from the above mentioned shapes, can be generated by, e.g.
laser arrays, lens arrays or diffractive optical elements. The shape of the patterns can be optimized to
certain types of discontinuities.
5.3.3 Switchable laser for lock-in thermography and other temporal techniques
In all temporal excitation techniques, and especially in those which correlate the temporal excitation
function with the temporal temperature response of the test object, the irradiance of the laser shall be
controlled as exactly as possible. A common approach is to control the output power of the laser system
by an external function generator via an analogue voltage input, which can be triggered by the IR
camera. For best performance, the output power should have a linear relationship with the controlling
voltage such that the signal processing algorithm can be applied directly. If there is no linear
relationship, the characteristic curve of the laser (i.e. output power vs. controlling voltage) shall be
taken into account properly by the user. Concerning the temporal behaviour, the laser switch on and
switch off times shall be taken into account for the maximum possible modulation frequency.
5.3.4 Safety
The most important European standards on laser safety, safety of optical radiation and protection
measures are listed in the following:
— EN 60825 Safety of laser products contains several relevant parts, e.g.:
— EN 60825-1 Safety of laser products — Part 1: Equipment classification and requirements
— EN 60825-4 Safety of laser products — Part 4: Laser guards
— EN ISO 11553 Safety of machinery — Laser processing machines contains several relevant parts,
e.g.:
— EN ISO 11553-1 Safety of machinery — Laser processing machines — Part 1: General safety
requirements
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— EN ISO 11553-2 Safety of machinery — Laser processing machines — Part 2: Safety
requirements for hand-held laser processing devices
Eye protection:
— EN 207 Personal eye-protection equipment — Filters and eye-protectors against laser radiation
(laser eye-protectors)
— EN 208 Personal eye-protection — Eye-protectors for adjustment work on lasers and laser systems
(laser adjustment eye-protectors)
— EN 12254 Screens for laser working places — Safety requirements and testing
— EN 62471 Photobiological safety of lamps and lamp systems
During testing, the workplace shall be illuminated adequately and appropriately according to
EN 12464-1. If necessary, protective measures related to the working safety regulations and regulations
for artificial optical radiation are expected to be considered.
During testing, it shall be ensured that there are no flammable materials in the vicinity of the equipment
and the investigated object.
Further on, there is a risk of burning on heated parts of the radiation sources, of the test object and of
further objects within the beam path.
5.4 Scanning system requirement
5.4.1 General
The scanning system should allow the laser to reach the whole area to be inspected from one or
multiple positions of the laser, the IR camera and/or the test object while keeping the laser beam and
the inspection IR camera in focus. The maximum scanning speed is linked to the laser power density
and IR camera frame rate.
According to the application, the movement of any part of the scanning system will be step-by-step or
continuous.
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5.4.2 Test object position and orientation

Key
1 Scanning system r Radial distance of the IR camera and the surface of the test object
T
2 Test object φ Azimuthal angle between the IR camera and the surface of the test object
T
3 Discontinuity θT Polar angle between the IR camera and the
surface of the test object
4 Laser φ Azimuthal angle between the laser beam and the surface of the test object
L
5 IR camera θ Polar angle between the laser beam and the surface of the test object
L
rL Radial distance of the laser beam output and the surface of the test object
Figure 2 — Example of a scanning configuration for laser thermography
The depth of field of the IR camera and, in case of a focused laser, of the laser restrict the test object
orientation relative to the IR camera and the laser. Furthermore, both the absorptance and the
emissivity of the test object in the spectral range of the laser and the IR camera, respectively, depend on
the incidence angle. If those quantities are changed between or during measurements, the heating of the
test object and the thermal radiation detected by the IR camera will change as well. Therefore, the
relative position and orientation of test object, laser, and IR camera need to be defined and documented
well. One possible configuration given in spherical coordinates is illustrated in Figure 2.
5.4.3 Movement of the test object
In this case, the measurement system comprising the laser and IR camera is fixed and the test object is
moved in front of it during the acquisition (see Figure 2).
5.4.4 Movement of the whole measurement system
In this case, the test object is fixed. The measurement system comprising the laser and IR cameras
moved in front of the test object during the acquisition.
5.4.5 Movement of the laser beam through optics
In this case, the test object and the IR camera are both fixed during the acquisition, and an optical setup,
such as mirrors, goniometers or laser scanning heads, is used to deflect the laser beam.
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5.4.6 Movement of the laser beam and IR camera through optics
In this case, the test object and the IR camera are both fixed during the acquisition, and an optical setup,
such as mirrors or goniometers, is used to deflect both the thermal radiation and the laser beam.
5.4.7 Setup stability
When an acquisition requires the analysis of a thermogram sequence, it is primordial that the pixel
correspondence between the different images in the sequence is realized properly. Hence, the setup
should have a stability against vibrations and thermal drift such that the uncertainty of the movement
of any part of the setup is smaller than the spatial resolution of the IR camera.
5.5 Specifications of the IR camera
Table 1 provides the minimum requirements of the features of the IR camera.
Table 1 — Minimum requirements of the features of the IR camera
Feature Minimum requirement
1 Spectral range (µm) According to EN 16714-2. In addition, the spectral range of the IR
camera shall not overlap with the wavelength of the laser.
2 Temperature range The IR camera should be able to detect all temperature variations of
(K, digital unit) the test object. A corresponding temperature range shall be set for
this purpose. For IR cameras without temperature calibration, a
corresponding display range is set, e.g. by selecting a suitable
integration time.
3 Thermal resolution The thermal resolution can be described by the noise equivalent
(K, digital unit) temperature difference NETD (according to EN 16714-2) or an
equivalent parameter. This parameter shall be sufficiently smaller
than the apparent temperature difference induced by the heat
source. Typically it should be at least 2 to 3 times less and in some
specific cases it could be even lower (e.g. lock-in thermography).
4 Thermal short-term The temperature drift during the acquisition shall be smaller than the
stability (K/s) NETD so that it does not impact the thermal resolution. A mea
...

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