SIST EN 17501:2022

SLOVENSKI STANDARD
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
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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.

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United Kingdom.
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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.

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
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

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.
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/
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
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
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
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
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.
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 safety requirements regarding the exposure of workers to risks arising from physical
agents (artificial optical radiation) (19th individual Directive within the meaning of Article 16(1) of
Directive 89/391/EEC) and [14]
Eye protection:
— EN 207
— EN 208
— EN 12254
— EN 62471
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 shall 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 irradiance 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.
5.4.2 Test object position and orientation

Key
1 scanning system rT radial distance of the IR camera and the surface of the test object
2 test object ϕ azimuthal angle between the IR camera and the surface of the test object
T
3 discontinuity θ polar angle between the IR camera and the
T
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 shall 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 camera is 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.
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 shall
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 shall be able to detect all temperature variations of the
(K, digital unit) 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.
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 measure for
the thermal stability is the short-term stability (given in K/s), which
should be as small as possible (according to VDI/VDE 5585-Blatt 1).
Automatic calibration corrections shall be disabled during the
acquisition.
Feature Minimum requirement
5 Detector format The detector can be a linear or matrix array. Its number of array
elements shall be defined according to the spatial resolution required,
the optics used and the area to be inspected.
6 Lens Characteristics such as focal length and aperture shall be compatible
with the performance of the application regarding sensitivity of
detection and spatial resolution. Depth of field shall be compatible
with the geometry of the test objects. Working distance shall be
compatible with test surroundings.
7 Spatial resolution The spatial resolution shall be at least three times smaller than the
(mm) length of the shortest discontinuity to be detected. The term length of
the discontinuity refers to its largest dimension in the observation
plane. Typically, very thin cracks with an opening smaller than the
spatial resolution can be found, if its length is sufficient. A measure for
the spatial resolution is the minimum measurable field of view for
temperature measurement (MFOV) according to
VDI/VDE 5585 Blatt 1. As a typical minimum requirement, the
relation between MFOV and instantaneous field of view IFOV should
be used: MFOV ≥ 3 IFOV. This means, the MFOV shall be at least as
large as the length of the shortest discontinuity to be detected. It can
be adapted using interchangeable lenses and/or extension rings.
8 Temporal resolution The acquisition frame rate shall be high enough to allow a correct
(s) sampling of the thermal signal, according to the feature of the
inspection (thermal properties of the materials, geometry of the
discontinuites, heating power distribution, scanning speed,
processing etc.).
9 Temporal stability (s) For certain types of processing, high temporal stability of the frame
rate is required. Alternatively, accurate timestamps can be recorded
for each frame.
10 Integration mode For the integration mode, a snapshot mode is preferable. For certain
types of processing, a correction shall be applied when using a rolling
frame integration mode.
11 Synchronisation For certain types of processing, it is necessary to synchronise the
acquisition to the scanning. For microbolometers cameras, the
synchronisation signal shall be released from the IR camera. Quantum
detectors cameras can also be synchronized externally.
12 Documentation The IR camera shall supply recordings that include radiometric data.
13 IR camera protection The sensor of the IR camera shall be protected so that the laser beam
against laser light does not hit the sensor, directly or indirectly. This is also required if
the laser light wavelength lies outside the spectral range of the IR
camera.
5.6 Data processing and analysis techniques
5.6.1 General
Conventional data processing techniques can be applied to thermal images, in order to visually enhance
the defect and/or as a preliminary step before the data analysis.
Some of the data processing and analysis techniques to be applied to laser thermography are described
in other standards. In particular, if a large illumination spot size (i.e. considerably larger than the thermal
diffusion length) is used, other testing techniques using, e.g. halogen or flash lamps can be applied as well.
If a very short laser pulse is used, this resembles the pulse thermography testing technique using optical
sources, see DIN 54184. In case of a periodically modulated (e.g. by a sine or a rectangular function) laser
beam, this resembles the lock-in technique which is described in EN 17119.
Some data processing and analysis techniques are based on the possibility of using the laser to realize
different illumination patterns or a relative movement between the heating location and the discontinuity
inside the test object. Due to the resulting large amount of degrees of freedom, it is not possible to cover
all imaginable data processing and analysis techniques. Some examples for the most important data
processing and analysis techniques are described below.
5.6.2 Spot with relative movement
5.6.2.1 General data evaluation
As described in 5.2.5, one implementation of laser thermography for surface breaking crack detection is
the flying laser spot technique. In this technique, the test object is scanned with a focused laser spot due
to a relative movement between test object, laser and/or IR camera. In order to receive a single image
containing all indications from a thermogram sequence, specialized image and signal processing
algorithms shall be applied. The presence of a discontinuity can be found in the spatial, in the temporal,
or in the frequency domain. Hence, different approaches exist. Some use a local spatial analysis of the
dynamic temperature field, e.g. by applying a differential edge detection filter, before reassembling the
resulting image, whereas some first reassemble the thermogram sequence into a new sequence, e.g. by
sorting or by tracking the laser position, and detect the crack induced perturbations afterwards. In all
approaches, special attention shall be paid to the correct image registration of all used data. Only then
the localization of the crack is accurate. In the following, selected suitable data analysis procedures are
described in more detail, but also other techniques may be applied.
5.6.2.2 Data analysis by spatial derivative and temporal data sorting
This procedure for data analysis can be applied, if the laser spot is raster scanned along the surface of the
test object continuously, stepped or randomly. There is no relative movement between test object and IR
camera. It contains the following steps [15], [16], see Figure 3:
a) Subtraction of one thermogram or an average of several thermograms from the whole thermogram
sequence, which have been recorded from the scene before heating. After subtraction, mainly only
temperature rises due to laser heating are appearing in the thermograms.
b) For each thermogram of the sequence T(x,y,t), the first spatial derivatives along the two main image
axes (i.e. in x- and y-direction) are calculated and T’ (x,y,t) and T’ (x,y,t) are obtained. This can be done
x y
e.g. by applying a Prewitt or Sobel filter. If cracks are only expected to be orientated e.g. in x-direction,
only the first spatial derivative in y-direction needs to be calculated.
Alternatively, the second spatial derivatives are calculated or a discrete Laplace filter is applied
(weighted sum of both second spatial derivatives). Note that each derivation of noisy data leads to
an increased noise in the resulting data set. Therefore, in order to apply a derivative filter, a good
signal-to-noise ratio is important.
c) The two data sets of the derivatives in x- and y-direction are joined and sorted according to their
values in ascending order into a resulting thermogram sequence:
′ ′′ ′ ′
T xy,, n ∈ T xy,,t, T xy,,t and T xy,, n ≤+T xy,, n 1 (1)
( ) ( ) ( ) ( ) ( )
{ }
sort xy sort sort
n is the index of the images in the sequence, and N is the total number of images in one sequence.
After applying Formula (1), for n = 1 the resulting data set contains the common minimum, for n =
2N, it contains the common maximum. An appropriate visualization of the crack indications might be
obtained for n equal or close to these extreme values (e.g. n = 4 or n = 2N − 4).
d) Alternatively to steps b) and c), the temperature values for each pixel are averaged along the time
scale of the whole sequence. An image of these mean values shows indications that point towards the
position of the cracks, but usually with less spatial resolution and less signal to noise ratio than with
steps b) and c).
NOTE Instead of using thermogram sequences consisting of temperature values (i.e. by using a temperature
calibrated IR camera) it is also possible to use thermogram sequences consisting of camera readings that are
proportional to the radiance of the heated test object.
Key
1 raw data 4 sort
2 laser scanning direction 5 laser excitation
3 single pixel 6 resulting image with crack indications
Figure 3 — Example of data processing after data recording with a scanning laser spot
Figure 3 shows an example of data processing after data recording with a scanning laser spot, which was
scanned in y-direction. In this figure the following details are shown:
— Figure 3 a) Images after applying a spatial derivative in y-direction to the thermograms
— Figure 3 b) Thermograms of the raw data set after subtraction of a thermogram before heating
— Figure 3 c) Images after applying a spatial derivative in x-direction to the thermograms
— Figure 3 d) Example of the temporal evolution of one pixel after the application of the spatial
derivative in y-direction
— Figure 3 e) Example of the temporal evolution of one pixel after the application of the spatial
derivative in x-direction
— Figure 3
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