TY - CONF A1 - Ziegler, Mathias A1 - Maierhofer, Christiane A1 - Krankenhagen, Rainer A1 - Röllig, Mathias T1 - Characterization of defects in fibre reinforced composites (FRC) using passive and active thermography N2 - Impact damages and delaminations in fibre-reinforced composites (FRC) might not be visible at the surface, but could have an influence on the resistance and on the long-term behaviour of the component. Therefore, and especially for safety relevant structures, non-destructive methods are required for the assessment of such damages. Active thermography methods are suitable to characterize damages after loading using different kind of excitation techniques and various configurations of infrared (IR) camera and heating sources. Here, flash lamps, impulse excitation with infrared radiator and lock-in technique with halogen lamps or widened laser beams are suited. In addition, non-optical sources like sonotrodes (requiring direct contact to the structure) or induction generators (only suited for carbon fibre reinforced polymer (CFRP) structures) could be applied as well. For the investigation of the evolution of the damage during the impact, passive thermography can be applied in-situ. Elastic and plastic deformations alter the temperature of the structure and thus the temperature on the surface. In this contribution, at first the general principles of quantitative defect characterisation in FRC using active thermography with flash, impulse and lock-in excitation are described. Optical and thermal properties of the FRC material and its anisotropy are considered. Results of phase differences obtained at flat bottom holes with flash and lock-in thermography are compared for qualifying both methods for quantitative defect characterization. Secondly, the damage evolution of CFRP and GFRP structures under impact load and static tensile loading is described. The spatial and temporal evolution of the surface temperature enables us to distinguish matrix cracks or fibre-matrix separation from delaminations between the layers. Afterwards, all results for loading defects, obtained by passive and active thermography, are compared with each other. Fig. 1 and 2 show the difference of passive and flash thermography obtained at impact and tensile loaded CFRP plates, respectively. As one purpose of these investigations is the development of standards within national (DIN) and European (CEN) standardisation bodies, new draft and final standards are presented and further needs are discussed at the end of the presentation. T2 - INTERNATIONAL SCHOOL OF QUANTUM ELECTRONICS, 62nd Course, Progress in Photoacoustic & Photothermal Phenomena CY - Erice, Italy DA - 06.09.2018 KW - Thermography KW - Flash thermography KW - Lock-in thermography KW - CFRP KW - GFRP PY - 2018 AN - OPUS4-46283 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - CONF A1 - Schlichting, Joachim A1 - Ziegler, Mathias A1 - Maierhofer, Christiane A1 - Kreutzbruck, Marc T1 - Flying laser spot thermography for the fast detection of surface breaking cracks N2 - We report on recent developments in the detection of surface breaking cracks using flying laser spot thermography. Application of an infrared camera for mapping the thermal radiation after excitation with a diode laser equipped with an optical scanner allows us to examine a surface containing cracks in an entirely non-destructive, contactless and fast way, without even moving the camera. We developed an efficient and robust algorithm that can be applied directly to the recorded thermal sequences, and that derives a single image containing all crack signatures. For this crack detection technique, no specific synchronisation between laser and camera is required. Hence, our approach is suitable for an upgrade of existing thermographic systems. The feasibility of the proposed procedure is proven by testing an artificial test sample and a piece of rail that comprises roll contact fatigue cracks and by comparing the results with magnetic particle testing. T2 - 18th WCNDT - World conference on nondestructive testing CY - Durban, South Africa DA - 2012-04-16 KW - Thermography KW - Cracks KW - Laser KW - Flying spot PY - 2012 SN - 978-0-620-52872-6 SP - 1 EP - 7 (Paper 499) AN - OPUS4-26352 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Hempel, M. A1 - Ziegler, Mathias A1 - Tomm, J.W. A1 - Elsaesser, T. A1 - Michel, N. A1 - Krakowski, M. T1 - Time-resolved analysis of catastrophic optical damage in 975 nm emitting diode lasers N2 - Catastrophic optical damage (COD) is analyzed during single current pulse excitation of 975 nm emitting diode lasers. Power transients and thermal images are monitored during each pulse. The COD process is unambiguously related to the occurrence of a “thermal flash” of Planck’s radiation. We observe COD to ignite multiple times in subsequent pulses. Thermography allows for tracing a spatial motion of the COD site on the front facet of the devices. The time constant of power decay after the onset of COD has values from 400 to 2000 ns, i.e., an order of magnitude longer than observed for shorter-wavelength devices. KW - Semiconductor laser KW - Thermography KW - Catastrophic optical damage KW - High-power diode lasers PY - 2010 U6 - https://doi.org/10.1063/1.3456388 SN - 0003-6951 SN - 1077-3118 VL - 96 IS - 251105 SP - 1 EP - 3 PB - American Institute of Physics CY - Melville, NY AN - OPUS4-21672 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Hempel, M. A1 - Tomm, J.W. A1 - Ziegler, Mathias A1 - Elsaesser, T. A1 - Michel, N. A1 - Krakowski, M. T1 - Catastrophic optical damage at front and rear facets of diode lasers N2 - Single-pulse tests of the catastrophic optical damage (COD) are performed for three batches of diode lasers with different gain-regions. The tests involve in situ inspection of front, rear, and side of the devices by a thermocamera. Devices with an Al-containing gain-region show COD at the front facet, as expected for strong facet heating via surface recombination and reabsorption of laser light. In contrast, Al-free devices with low surface recombination rates tend to fail at the rear facet, pointing to a different heating scenario. The high carrier density at the rear facet favors heating and COD via Auger recombination processes. KW - Thermography KW - Catastrophic optical damage KW - High power diode lasers KW - Destructive testing KW - Non-destructive testing KW - Auger effect KW - Carrier density KW - Heating KW - Laser beam effects KW - Optical testing KW - Semiconductor lasers KW - Surface recombination PY - 2010 U6 - https://doi.org/10.1063/1.3524235 SN - 0003-6951 SN - 1077-3118 VL - 97 IS - 23 SP - 231101-1 - 231101-3 PB - American Institute of Physics CY - Melville, NY AN - OPUS4-22763 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Hempel, M. A1 - Ziegler, Mathias A1 - Schwirzke-Schaaf, S. A1 - Tomm, J.W. A1 - Jankowski, D. A1 - Schröder, D. T1 - Spectroscopic analysis of packaging concepts for high-power diode laser bars N2 - Double-side cooled high-power diode laser bars packaged by different techniques on different types of passive heat sinks are analyzed in terms of packaging-induced strain. Reference data from standard devices being single-side cooled only and packaged by conventional soft and hard soldering are also presented. Thermal profiling across the devices complements the results. The most suitable packaging architecture and technique for double-side cooled bars is identified. Measurements of the laser emission near field and electroluminescence pattern provide direct reference to the functionality of the devices. Furthermore, a type of cross calibration of the methods used for strain analysis is made, since all techniques are applied to the same set of bars. This involves micro photoluminescence, micro Raman, and degree-of-polarization electroluminescence spectroscopy. KW - Spectroscopy KW - Semiconductor laser KW - Thermography KW - High-power diode lasers KW - Non-destructive testing PY - 2012 U6 - https://doi.org/10.1007/s00339-012-6799-4 SN - 0947-8396 VL - 107 IS - 2 SP - 371 EP - 377 PB - Springer CY - Berlin AN - OPUS4-25789 LA - deu AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Ahmadi, Samim A1 - Kästner, L. A1 - Hauffen, Jan Christian A1 - Jung, P. A1 - Ziegler, Mathias T1 - Photothermal-SR-Net: A Customized Deep Unfolding Neural Network for Photothermal Super Resolution Imaging N2 - This article presents deep unfolding neural networks to handle inverse problems in photothermal radiometry enabling super-resolution (SR) imaging. The photothermal SR approach is a well-known technique to overcome the spatial resolution limitation in photothermal imaging by extracting high-frequency spatial components based on the deconvolution with the thermal point spread function (PSF). However, stable deconvolution can only be achieved by using the sparse structure of defect patterns, which often requires tedious, handcrafted tuning of hyperparameters and results in computationally intensive algorithms. On this account, this article proposes Photothermal-SR-Net, which performs deconvolution by deep unfolding considering the underlying physics. Since defects appear sparsely in materials, our approach includes trained block-sparsity thresholding in each convolutional layer. This enables to super-resolve 2-D thermal images for nondestructive testing (NDT) with a substantially improved convergence rate compared to classic approaches. The performance of the proposed approach is evaluated on various deep unfolding and thresholding approaches. Furthermore, we explored how to increase the reconstruction quality and the computational performance. Thereby, it was found that the computing time for creating high-resolution images could be significantly reduced without decreasing the reconstruction quality by using pixel binning as a preprocessing step. KW - Deep unfolding KW - Defect reconstruction KW - Elastic net KW - Inverse problems KW - Iterative shrinkage thresholding KW - Neural network KW - Nondestructive testing (NDT) KW - Photothermal imaging KW - Super resolution (SR) KW - Thermography PY - 2022 U6 - https://doi.org/10.1109/tim.2022.3154803 SN - 1557-9662 VL - 71 SP - 1 EP - 9 PB - IEEE AN - OPUS4-54678 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Hauffen, J. C. A1 - Kästner, L. A1 - Ahmadi, Samim A1 - Jung, P. A1 - Caire, G. A1 - Ziegler, Mathias T1 - Learned block iterative shrinkage thresholding algorithm for photothermal super resolution imaging N2 - Block-sparse regularization is already well known in active thermal imaging and is used for multiple-measurement-based inverse problems. The main bottleneck of this method is the choice of regularization parameters which differs for each experiment. We show the benefits of using a learned block iterative shrinkage thresholding algorithm (LBISTA) that is able to learn the choice of regularization parameters, without the need to manually select them. In addition, LBISTA enables the determination of a suitable weight matrix to solve the underlying inverse problem. Therefore, in this paper we present LBISTA and compare it with state-of-the-art block iterative shrinkage thresholding using synthetically generated and experimental test data from active thermography for defect reconstruction. Our results show that the use of the learned block-sparse optimization approach provides smaller normalized mean square errors for a small fixed number of iterations. Thus, this allows us to improve the convergence speed and only needs a few iterations to generate accurate defect reconstruction in photothermal super-resolution imaging. KW - Thermography KW - Laser KW - Machine learning KW - Optimization KW - Non-destructive testing KW - NDT KW - Neural network KW - Defect reconstruction KW - Block-sparsity KW - Active thermal imaging KW - Regularization KW - Laser thermography PY - 2022 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:b43-554598 SN - 1424-8220 VL - 22 IS - 15 SP - 1 EP - 15 PB - MDPI CY - Basel AN - OPUS4-55459 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - CONF A1 - Myrach, Philipp A1 - Ziegler, Mathias A1 - Maierhofer, Christiane A1 - Kreutzbruck, Marc ED - Chimenti, D.E. ED - Bond, L.J. ED - Thompson, D.O. T1 - Influence of the acquisition parameters on the performance of laser-thermography for crack detection in metallic components N2 - We present a systematic study on the performance of laser-thermography for the detection of surface cracks in metallic components. Scanning a metallic surface with laser causes local heating that is mapped simultaneously by an IR-camera and allows identifying cracks with sub-µm openings. The detectability, however, depends on a number of acquisition parameters (e.g. scanning speed, laser power, IR-camera resolution) that typically relate on each other. Most importantly, the detection-sensitivity of surface breaking cracks is given by a particular combination for the acquisition parameter values. As a result, this sensitivity is adaptable within wide ranges allowing the detection of cracks with openings ranging from 200 to 0.1 µm at testing speeds of 100 to 0.05 cm²/s. By examining artificial as well as fatigue cracks, we demonstrate that the method can be even applied to shiny surfaces with no need of pretreatments, which makes it an entirely contactless, remote and automatable NDT technique. A comparison with magnetic particle testing shows that laser-thermography has the potential to become a strong competitor to conventional surface inspection methods in the future. T2 - 40th Annual review of progress in quantitative nondestructive evaluation CY - Baltimore, Maryland, USA DA - 21.07.2013 KW - Thermography KW - Laser-thermography KW - Crack detection PY - 2014 SN - 978-0-7354-1212-5 SN - 978-0-7354-1211-8 U6 - https://doi.org/10.1063/1.4865018 SN - 0094-243X N1 - Serientitel: AIP conference proceedings – Series title: AIP conference proceedings VL - 1581 SP - 1624 EP - 1630 PB - AIP Publishing AN - OPUS4-31260 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Tomm, J.W. A1 - Ziegler, Mathias A1 - Hempel, M. A1 - Elsaesser, T. T1 - Mechanisms and fast kinetics of the catastrophic optical damage (COD) in GaAs-based diode lasers N2 - COD diagram determined for a batch of broad-area AlGaAs diode lasers. The time to COD within a single current pulse is plotted versus the actual average optical power in the moment when the COD takes place. Full circles stand for clearly identified COD events (right ordinate), whereas open circles (left ordinate) represent the pulse duration in experiments, where no COD has been detected. A borderline (gray) exists between two regions, i. e., parameter sets, of presence (orange) and absence of COD (blue). This borderline is somewhat blurred because of the randomness in filamentation of the laser nearfield and scatter in properties of the involved individual devices. KW - Semiconductor laser KW - Thermography KW - Catastrophic optical damage KW - High-power diode lasers KW - Destructive testing KW - Non-destructive testing PY - 2011 U6 - https://doi.org/10.1002/lpor.201000023 SN - 1863-8880 VL - 5 IS - 3 SP - 422 EP - 441 PB - Wiley-VCH CY - Weinheim AN - OPUS4-23624 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Ziegler, Mathias A1 - Hempel, M. A1 - Larsen, H. E. A1 - Tomm, J. W. A1 - Andersen, P. E. A1 - Clausen, S. A1 - Elliott, S. N. A1 - Elsaesser, T. T1 - Physical limits of semiconductor laser operation: A time-resolved analysis of catastrophic optical damage N2 - The early stages of catastrophic optical damage (COD) in 808 nm emitting diode lasers are mapped by simultaneously monitoring the optical emission with a 1 ns time resolution and deriving the device temperature from thermal images. COD occurs in highly localized damage regions on a 30 to 400 ns time scale which is determined by the accumulation of excess energy absorbed from the optical output. We identify regimes in which COD is avoided by the proper choice of operation parameters. KW - Semiconductor laser KW - Thermography KW - Catastrophic optical damage KW - High-power diode lasers PY - 2010 U6 - https://doi.org/10.1063/1.3463039 SN - 0003-6951 SN - 1077-3118 VL - 97 IS - 021110 SP - 1 EP - 3 PB - American Institute of Physics CY - Melville, NY AN - OPUS4-21741 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Ziegler, Mathias A1 - Myrach, Philipp A1 - Neding, Benjamin ED - J. Grosch, ED - E.J. Mittemeijer, T1 - Wärmebehandlung und zerstörungsfreie Prüfung: Oberflächenrisse mit der Laser-Thermografie finden N2 - Die Thermografie als zerstörungsfreies Prüfverfahren erlaubt das Erkennen einer Vielzahl von Defekten in unterschiedlichsten Werkstoffen. Hierzu ist zum einen eine Energiequelle erforderlich, welche einen Wärmestrom im Werkstück erzeugt, und zum anderen eine Infrarot-Kamera, welche den orts- und zeitabhängigen Temperaturverlauf der Werkstückoberfläche analysiert. Neue Ansätze unter Verwendung von Lasern ermöglichen nun auch die Prüfung auf winzige Oberflächenrisse. Die aktuelle Grenze der Nachweisempfindlichkeit für mittels klassischer Thermografie nur schwer prüfbare hochreflektierende Metalloberflächen liegt mit Rissbreiten und -tiefen bis hinunter in den Sub-Mikrometerbereich und damit im Bereich der fluoreszierenden Magnetpulver- und Eindringprüfung. Dabei sind jedoch weder Verbrauchsmittel, noch eine Oberflächenpräparation notwendig und eine Prüfung kann berührungslos und automatisiert über Entfernungen bis in den Meter-Bereich erfolgen. Bei der Wärmebehandlung und insbesondere beim Randschichthärten mittels Laserstrahlung stehen ebenfalls hochleistungsfähige Energiequellen zur Erzeugung intensiver dynamischer Temperaturgradienten zur Verfügung. Damit eröffnet sich unmittelbar die Möglichkeit der Integration der thermografischen Prüfung in die Fertigung. Der Beitrag stellt die neuesten Entwicklungen der vielversprechenden Laser-Thermografie-Technik vor und zeigt, warum deren Einsatz potentiell die Standardprüfverfahren Magnetpulver- bzw. Eindringprüfung für eine Reihe von Prüfproblemen beerben könnte. KW - Zerstörungsfreie Prüfung KW - Thermografie KW - Infrarot-Sensor KW - Rissprüfung KW - Lasertechnik KW - Nondestructive testing KW - Thermography KW - Thermal infrared sensor KW - Crack inspection KW - Laser techniques PY - 2015 U6 - https://doi.org/10.3139/105.110264 SN - 0341-101x SN - 1867-2493 VL - 70 IS - 4 SP - 190 EP - 195 PB - Hanser CY - München AN - OPUS4-34189 LA - deu AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - CONF A1 - Puthiyaveettil, N. A1 - Krishna, S. A1 - Kidangan, R. A1 - Unnikrishnakurup, Sreedhar A1 - Krishnamurthy, C. V. A1 - Ziegler, Mathias A1 - Myrach, Philipp A1 - Balasubramaniam, Krishnan T1 - In-line laser thermography for crack detection at elevated temperature: A Numerical modeling study N2 - The detection and characterization of cracks prior to damage is a technologically and economically highly significant task and is of very importance when it comes to safety-relevant structures. The evaluation of a components life is closely related to the presence of cracks in it. Laser thermography has already high capability for the detection of surface cracks and for the characterization of the geometry of artificial surface flaws in metallic samples. Crack detection in metallic samples at high temperature is highly significant in present manufacturing scenario. During the casting process of billets, surface cracks form, due to the suboptimal cooling rates. These cracks reduce value of the billet and must be removed using machining process after cooling. This secondary process increases cost of manufacturing. In this work we developed a heat transfer model for laser thermography to study the thermal contrast variation with increase in surface temperature using finite element method (FEM). Here we are mainly concentrating the capability of the scanning laser thermography in crack detection which are in elevated temperature and numerical modeling study of thermal contrast variation of crack with respect increase in metal surface temperature. This study is important to prove the capability of laser thermography for crack detection in elevated temperature. Since we are using High power CW Laser to local heating of the metal surface which can give relatively high thermal contrast even at elevated temperature compare to other heating source. Here we are modeled and simulated 2D laser scanning across a surface breaking crack and developed an algorithm to produce the vicinity of crack. The algorithm we developed applied for various surface temperature data. And validated the credibility of the algorithm with experimental data. T2 - 13th Quantitative Infrared Thermography Conference 2016 CY - Gdansk, Poland DA - 04.07.2016 KW - Thermal contrast KW - Laser thermography KW - Thermography KW - Surface cracks KW - Elevated temperatures KW - FEM PY - 2016 UR - http://www.ndt.net/article/qirt2016/papers/092.pdf U6 - https://doi.org/10.21611/qirt.2016.092 VL - 2016 SP - 588 EP - 596 PB - QIRT 2016 Proceedings AN - OPUS4-39105 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Ziegler, Mathias A1 - Thiel, Erik A1 - Studemund, T. T1 - Thermography using a 1D laser array – From planar to structured heating N2 - In the field of optically excited thermography, flash lamps (impulse shaped planar heating) and halogen lamps (modulated planar heating) have become established for the specific regimes of impulse and lock-in thermography. Flying-spot laser thermography is implemented by means of a rasterized focused laser, e. g. for crack detection (continuous wave operation) and photothermal material characterization (high-frequency modulated). The availability of novel technologies, i. e. fast and high-resolution IR cameras, brilliant innovative light sources and high-performance data acquisition and processing technology will enable a paradigm shift from stand-alone photothermal and thermographic techniques to uniform quantitative measurement and testing technology that is faster and more precise. Similar to an LED array, but with irradiance two orders of magnitude higher, a new type of brilliant laser source, i. e. the VCSEL array (vertical-cavity surface-emitting laser), is now available. This novel optical energy source eliminates the strong limitation to the temporal dynamics of established light sources and at the same time is spectrally clearly separated from the detection wavelength. It combines the fast temporal behavior of a diode laser with the high optical irradiance and the wide illumination area of flash lamps. In addition, heating can also be carried out in a structured manner, because individual areas of the VCSEL array can be controlled independently of each other. This new degree of freedom enables the development of completely new thermographic NDT methods. KW - Thermography KW - Laser thermography KW - Laser KW - Lock-in KW - VCSEL KW - Thermal wave KW - Photothermal PY - 2018 UR - https://www.hanser-elibrary.com/doi/abs/10.3139/120.111209 U6 - https://doi.org/10.3139/120.111209 SN - 0025-5300 VL - 60 IS - 7-8 SP - 749 EP - 757 PB - Carl Hanser Verlag CY - München AN - OPUS4-45482 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - CONF A1 - Ziegler, Mathias A1 - Thiel, Erik A1 - Ahmadi, Samim T1 - Lock-in Thermography using High-Power Laser Sources N2 - Optical lock-in thermography is a completely contactless and very sensitive NDT technique. As an optical source of energy, incandescent lamps are most commonly used because they are relatively inexpensive and offer high irradiances at the test specimen. However, they are strongly restricted by their low modulation bandwidth with a maximum modulation frequency of only about 1 Hz. The use of high-power kilowatt-class laser sources, e.g. diode laser arrays, pushes this constraint beyond 100 Hz. This allows for the exploration of the near-surface region of metals and layer systems with better and more accurate penetration depth and depth resolution. Moreover, these lasers are virtually free of any additional thermal radiation that could interfere with the “true” thermal response emitted from the heated sample. In turn, they can be easily used in a one-sided test configuration. We present current activities with kilowatt-class high-power laser sources for advanced lock-in thermography and focus on the application of laser arrays that offer a very high irradiation strength over a large sample area beyond the mentioned advantages. T2 - 12th European Conference on Non-destructive Testing CY - Gothenburg, Sweden DA - 11.06.2018 KW - Thermography KW - Laser Thermography KW - Lock-in Thermography PY - 2018 AN - OPUS4-45445 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - CONF A1 - Ziegler, Mathias A1 - Thiel, Erik A1 - Ahmadi, Samim T1 - Lock-in Thermography using High-Power Laser Sources N2 - Optical lock-in thermography is a completely contactless and very sensitive NDT technique. As an optical source of energy, incandescent lamps are most commonly used because they are relatively inexpensive and offer high irradiances at the test specimen. However, they are strongly restricted by their low modulation bandwidth with a maximum modulation frequency of only about 1 Hz. The use of high-power kilowattclass laser sources, e.g. diode laser arrays, pushes this constraint beyond 100 Hz. This allows for the exploration of the near-surface region of metals and layer systems with better and more accurate penetration depth and depth resolution. Moreover, these lasers are virtually free of any additional thermal radiation that could interfere with the “true” thermal response emitted from the heated sample. In turn, they can be easily used in a one-sided test configuration. We present current activities with kilowatt-class highpower laser sources for advanced lock-in thermography and focus on the application of laser arrays that offer a very high irradiation strength over a large sample area beyond the mentioned advantages. T2 - 12th European Conference on Non-destructive Testing CY - Gothenburg, Sweden DA - 11.06.2018 KW - Thermography KW - Laser Thermography KW - Lock-in Thermography PY - 2018 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:b43-454466 UR - http://cdn.ecndt2018.com/wp-content/uploads/2018/05/ecndt-0139-2018.pdf SN - 978-91-639-6217-2 SP - ECNDT-0139-2018,1 EP - 7 AN - OPUS4-45446 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - CONF A1 - Ziegler, Mathias A1 - Thiel, Erik A1 - Ahmadi, Samim T1 - Lock-in Thermography using High-Power Laser Sources N2 - Optical lock-in thermography is a completely contactless and very sensitive NDT technique. As an optical source of energy, incandescent (i.e. halogen) lamps are most commonly used because they are relatively inexpensive and offer high irradiances at the test site. However, they are strongly restricted by their low modulation bandwidth with a maximum modulation frequency of only about 1 Hz. The use of high-power kilowatt-class laser sources, e.g. diode laser arrays, pushes this constraint beyond 100 Hz, see Fig.1. This allows for the exploration of the near-surface region of metals and layer systems with better and more accurate penetration depth and depth resolution. Moreover, these lasers are virtually free of any additional thermal radiation that could interfere with the “true” thermal response emitted from the heated sample. In turn, they can be easily used in a one-sided test configuration. Using the one-dimensional solution to the thermal heat diffusion equation together with the absorptance of the material which is illuminated with a harmonically modulated light source, we can calculate the temperature oscillation at the surface of a solid. As a second step, we calculate the corresponding oscillation of the total thermal emission using Stefan-Boltzmann law as a first order approximation and taking into account the emissivity of the material. Within this framework we can calculate the minimal irradiance of a light source necessary to provoke a measurable signal within a thermographic camera at a noise equivalent temperature difference (NETD) of 30 mK. In Fig. 2 this relationship is displayed for a wide spectrum of modulation frequencies and for a number of different light sources scaled to the same electrical input power and illumination area. Using this figure, it is now easily possible to analyze the range of materials to be tested using lock-in thermography, since only the materials (dotted lines) below the irradiance-vs-frequency curves (solid lines) are heated in excess of the camera’s NETD. This figure clearly shows that laser sources considerably increase the application range of lock-in thermography, since especially for metals with a high reflectance and high thermal diffusivity a high irradiance is vitally important to allow for lock-in texting. We present current activities with kilowatt-class high-power laser sources for advanced lock-in thermography and focus on the application of laser arrays that offer a very high irradiation strength over a large sample area beyond the mentioned advantages. T2 - Conference QIRT 2018 CY - Berlin, Germany DA - 25.06.2018 KW - Thermography KW - Laser Thermography KW - Lock-in Thermography PY - 2018 AN - OPUS4-45447 LA - deu AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - CONF A1 - Ziegler, Mathias A1 - Thiel, Erik A1 - Ahmadi, Samim T1 - Lock-in Thermography using High-Power Laser Sources N2 - Optical lock-in thermography is a completely contactless and very sensitive NDT technique. As an optical source of energy, incandescent (i.e. halogen) lamps are most commonly used because they are relatively inexpensive and offer high irradiances at the test site. However, they are strongly restricted by their low modulation bandwidth with a maximum modulation frequency of only about 1 Hz. The use of high-power kilowatt-class laser sources, e.g. diode laser arrays, pushes this constraint beyond 100 Hz, see Fig.1. This allows for the exploration of the near-surface region of metals and layer systems with better and more accurate penetration depth and depth resolution. Moreover, these lasers are virtually free of any additional thermal radiation that could interfere with the “true” thermal response emitted from the heated sample. In turn, they can be easily used in a one-sided test configuration. Using the one-dimensional solution to the thermal heat diffusion equation together with the absorptance of the material which is illuminated with a harmonically modulated light source, we can calculate the temperature oscillation at the surface of a solid. As a second step, we calculate the corresponding oscillation of the total thermal emission using Stefan-Boltzmann law as a first order approximation and taking into account the emissivity of the material. Within this framework we can calculate the minimal irradiance of a light source necessary to provoke a measurable signal within a thermographic camera at a noise equivalent temperature difference (NETD) of 30 mK. In Fig. 2 this relationship is displayed for a wide spectrum of modulation frequencies and for a number of different light sources scaled to the same electrical input power and illumination area. Using this figure, it is now easily possible to analyze the range of materials to be tested using lock-in thermography, since only the materials (dotted lines) below the irradiance-vs-frequency curves (solid lines) are heated in excess of the camera’s NETD. This figure clearly shows that laser sources considerably increase the application range of lock-in thermography, since especially for metals with a high reflectance and high thermal diffusivity a high irradiance is vitally important to allow for lock-in texting. We present current activities with kilowatt-class high-power laser sources for advanced lock-in thermography and focus on the application of laser arrays that offer a very high irradiation strength over a large sample area beyond the mentioned advantages. T2 - Conference QIRT 2018 CY - Berlin, Germany DA - 25.06.2018 KW - Thermography KW - Laser Thermography KW - Lock-in Thermography PY - 2018 AN - OPUS4-45449 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - CONF A1 - Burgholzer, P. A1 - Berer, T. A1 - Ziegler, Mathias A1 - Thiel, Erik A1 - Ahmadi, Samim A1 - Gruber, J. A1 - Mayr, G. A1 - Hendorfer, G. T1 - Blind structured illumination as excitation for super-resolution photothermal radiometry N2 - Using an infrared camera for radiometric imaging allows the contactless temperature measurement of multiple surface pixels simultaneously. From the measured surface data, a sub-surface structure, embedded inside a sample or tissue, can be reconstructed and imaged when heated by an excitation light pulse. The main drawback in radiometric imaging is the degradation of the spatial resolution with increasing depth, which results in blurred images for deeper lying structures. We circumvent this degradation with blind structured illumination, combined with a non-linear joint sparsity reconstruction algorithm. The ground-breaking concept of super-resolution can be transferred from optics to thermographic imaging. T2 - Conference QIRT 2018 CY - Berlin, Germany DA - 25.06.2018 KW - Thermography KW - Laser thermography KW - Super resolution PY - 2018 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:b43-454506 SN - 978-3-940283-94-8 SP - We.3.A.2, 1 EP - 7 AN - OPUS4-45450 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - CONF A1 - Thiel, Erik A1 - Kreutzbruck, M. A1 - Studemund, T. A1 - Ziegler, Mathias T1 - Thermal wave interference with high-power VCSEL arrays for locating vertically oriented subsurface defects N2 - Among the photothermal methods, full-field thermal imaging is used to characterize materials, to determine thicknesses of layers, or to find inhomogeneities such as voids or cracks. The use of classical light sources such as flash lamps (impulse heating) or halogen lamps (modulated heating) led to a variety of nondestructive testing methods, in particular, lock-in and flash-thermography. In vertical-cavity surface-emitting lasers (VCSELs), laser light is emitted perpendicularly to the surface with a symmetrical beam profile. Due to the vertical structure, they can be arranged in large arrays of many thousands of individual lasers, which allows power scaling into the kilowatt range. Recently, a high-power yet very compact version of such a VCSEL-array became available that offers both the fast timing behavior of a laser as well as the large illumination area of a lamp. Moreover, it allows a spatial and temporal control of the heating because individual parts of the array can be controlled arbitrarily in frequency, amplitude, and phase. In conjunction with a fast infrared camera, such structured heating opens up a field of novel thermal imaging and testing methods. As a first demonstration of this approach, we chose a testing problem very challenging to conventional thermal infrared testing: The detection of very thin subsurface defects perpendicularly oriented to the surface of metallic samples. First, we generate destructively interfering thermal wave fields, which are then affected by the presence of defects within their reach. It turned out that this technique allows highly sensitive detection of subsurface defects down to depths in excess of the usual thermographic rule of thumb, with no need for a reference or surface preparation. T2 - 44TH ANNUAL REVIEW OF PROGRESS IN QUANTITATIVE NONDESTRUCTIVE EVALUATION CY - Provo, Utah DA - 16.07.2017 KW - Laser applications KW - Thermography KW - VCSEL KW - Subsurface defects PY - 2018 SN - 978-0-7354-1644-4 U6 - https://doi.org/10.1063/1.5031547 SN - 0094-243X VL - 1949 SP - UNSP 060001, 1 EP - 8 AN - OPUS4-45171 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Burgholzer, P. A1 - Berer, T. A1 - Ziegler, Mathias A1 - Thiel, Erik A1 - Ahmadi, Samim A1 - Gruber, J. A1 - Mayr, G. A1 - Hendorfer, G. T1 - Blind structured illumination as excitation for super-resolution photothermal radiometry N2 - Photothermal radiometry with an infrared camera allows the contactless temperature measurement of multiple surface pixels simultaneously. A short light pulse heats the sample. The heat propagates through the sample by diffusion and the corresponding temperature evolution is measured at the sample’s surface by an infrared camera. The main drawback in radiometric imaging is the loss of the spatial resolution with increasing depth due to heat diffusion, which results in blurred images for deeper lying structures. We circumvent this information loss due to the diffusion process by using blind structured illumination, combined with a non-linear joint sparsity reconstruction algorithm. The structured illumination is realized by parallel laser lines from a vertical-cavity surface-emitting laser (VCSEL) array controlled by a random binary pattern generator. By using 150 different patterns of structured illumination and our iterative joint sparsity algorithm, it was possible to resolve 1 mm thick lines at a distance down to 0.5 mm, which results in a resolution enhancement of approximately a factor of four compared to the resolution of 5.9 mm for homogenous illuminated thermographic reconstruction. KW - Super-resolution imaging KW - Thermography KW - Blind structured illumination KW - VCSEL array PY - 2019 U6 - https://doi.org/10.1080/17686733.2019.1655247 SN - 1768-6733 VL - 17 IS - 4 SP - 268 EP - 278 PB - Taylor & Francis Group CY - Milton, UK AN - OPUS4-49122 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Puthiyaveettil, N. A1 - Thomas K, R. A1 - Unnikrishnakurup, S. A1 - Myrach, Philipp A1 - Ziegler, Mathias A1 - Balasubramaniam, K. T1 - Laser line scanning thermography for surface breaking crack detection modeling and experimental study N2 - Crack detection in metallic samples at high surface temperature, hostile and hazardous environments, etc. is challenging situation in any manufacturing industries. Most of the present NDE methods are suitable only for lower surface temperatures, especially room temperature. In this situation, we need a fast and non-contact NDT method which can be applied even in high sample surface temperature. Laser thermography is one of the techniques having a high potential in non-contact inspection. As a preliminary investigation, in this article, we have studied the potentiality of laser line thermography in crack detection at room temperature. In laser line thermography, a continuous wave (CW) laser is used to generate a laser line, which in turn is used to scan the metal surface. The heat distribution over the sample surface is recorded by an infrared thermal (IR) camera. Two different approaches are reported in this work. Firstly, a stationary laser line source and its interaction with cracks; secondly, moving laser line source scanning over a surface with crack. When the distance between crack centre to laser line centre increases, crack detectability will decrease; and when laser power increases, crack detectability will increase. A dedicated image processing algorithm was developed to improve the detectability of the cracks. To understand the heat transfer phenomenon, a simplified 3D model for laser thermography was developed for the heat distribution during laser heating and was validated with experimental results. Defects were incorporated as a thermally thin resistive layer (TTRL) in numerical modeling, and the effect of TTRL in heat conduction is compared with experimental results. KW - Thermography KW - Laser Thermography KW - Cracks KW - FEM KW - NDT PY - 2019 U6 - https://doi.org/10.1016/j.infrared.2019.103141 VL - 104 SP - 103141 PB - Elsevier B.V. AN - OPUS4-49941 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Myrach, Philipp A1 - Jonietz, Florian A1 - Meinel, Dietmar A1 - Suwala, Hubert A1 - Ziegler, Mathias T1 - Calibration of thermographic spot weld testing with X-ray computed tomography N2 - The paper presents an attempt for the calibration of an active thermography method that is suitable for the non-destructive evaluation of spot welds. Nowadays, the quality of spot welds is commonly characterised by the application of random chisel tests, which are time consuming, expensive and destructive. Recently a non-destructive testing method by means of active thermography was proposed that relies on the fact that the mechanical connection formed by the spot weld also serves as a thermal bridge between the two steel sheets joined in the welding process. It is shown in this paper that this thermal bridge can be thermographically characterised by extracting a measure for the spot weld diameter and hence the quality of the spot weld. The determination of the absolute value of the diameter hereby relies on a calibration of the testing system, which is performed by means of X-ray computed tomography in this study. The experiments were carried out using different experimental approaches, namely transmission as well as reflection geometry wSetup in reflectionith laser illumination. A comprehensive evaluation of samples produced using different welding currents, hence different quality, was carried out in order to validate the thermographic results. KW - Thermography KW - Spot welds KW - Spot welding KW - Computed thomography KW - Non-destructive testing PY - 2017 U6 - https://doi.org/10.1080/17686733.2017.1281554 SN - 1768-6733 SN - 2116-7176 VL - 14 IS - 1 SP - 122 EP - 131 PB - Taylor & Francis CY - London AN - OPUS4-40180 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Ziegler, Mathias A1 - Thiel, Erik T1 - Wenn die Wärme Wellen schlägt N2 - Mit Laserlicht kann man eine Materialoberfläche berührungslos und schnell moduliert aufheizen. Dabei entsteht eine stark gedämpfte Wärmewelle, die tief ins Material eindringen kann. Erzeugt und überlagert man solche thermischen Wellen auf kohärente Weise, dann kann man damit versteckte Materialfehler zerstörungsfrei und sehr präzise aufspüren. Sogar eine bildgebende Tomografie ist denkbar. KW - Thermography KW - Laser thermography KW - Lock-in thermography KW - NDT KW - Thermal waves PY - 2018 U6 - https://doi.org/10.1002/piuz.201801512 SN - 0031-9252 VL - 49 IS - 6 SP - 296 EP - 303 PB - Wiley-VCH Verlag GmbH & Co. KGaA CY - Weinheim AN - OPUS4-46630 LA - deu AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Thiel, Erik A1 - Ziegler, Mathias A1 - Studemund, Taarna T1 - Localization of subsurface defects in uncoated aluminum with structured heating using high-power VCSEL laser arrays N2 - We report on photothermal detection of subsurface defects by coherent superposition of thermal wave fields. This is made possible by structured heating using high-power VCSEL laser arrays whose individual emitter groups can be arbitrarily controlled. In order to locate the defects, we have developed a scanning method based on the continuous wavelet transformation with complex Morlet wavelet using the destructive interference effect of thermal waves. This approach can also be used for thermally very fast and highly reflective materials such as uncoated aluminum. We show that subsurface defects at an aspect ratio of defect width to defect depth down to 1/3 are still detectable in this material. KW - Thermography KW - Heat diffusion KW - Laser thermography KW - Structured heating KW - NDT KW - Subsurface defects KW - Thermal wave KW - VCSEL KW - Wavelet transformation PY - 2019 U6 - https://doi.org/10.1007/s10765-018-2478-9 SN - 1572-9567 SN - 0195-928X VL - 40 IS - 2 SP - 17, 1 EP - 13 PB - Springer Science+Business Media, LLC, part of Springer Nature 2019 AN - OPUS4-47208 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - CONF A1 - Ziegler, Mathias A1 - Thiel, Erik A1 - Studemund, Taarna T1 - Locating subsurface defects with structured heating using high-power VCSEL-laser arrays N2 - Among the photothermal methods, full-field thermal imaging is used to characterize materials, to determine thicknesses of layers, or to find inhomogeneities such as voids or cracks. The use of classical light sources such as flash lamps (impulse heating) or halogen lamps (modulated heating) led to a variety of nondestructive testing methods, in particular lock-in and flash-thermography. In vertical cavity surface emitting lasers (VCSELs), laser light is emitted perpendicular to the surface with a symmetrical beam profile. Due to the vertical structure, they can be arranged in large arrays of many thousands of individual lasers, which allows power scaling into the kilowatt range. Recently, a high-power yet very compact version of such a VCSEL-array became available that offers both the fast timing behavior of a laser as well as the large illumination area of a lamp. Moreover, it allows a spatial and temporal control of the heating because individual parts of the array can be controlled arbitrarily in frequency, amplitude, and phase. In conjunction with a fast infrared camera, such structured heating opens up a field of novel thermal imaging and testing methods. As a first demonstration of this ansatz, we chose a testing problem very challenging to conventional thermal infrared testing: The detection of very thin subsurface defects perpendicularly oriented to the surface of metallic samples. First, we generate destructively interfering thermal wave fields which are then affected by the presence of defects within their reach. It turned out that this technique allows highly sensitive detection of subsurface defects down to depths in excess of the usual thermographic rule of thumb, with no need for a reference or surface preparation. T2 - 19th International Conference on Photoacoustic and Photothermal Phenomena CY - Bilbao, Spain DA - 16.07.2017 KW - Thermografie KW - Laser KW - Zerstörungsfreie Prüfung KW - Thermography KW - Non-destructive testing PY - 2017 AN - OPUS4-41829 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - CONF A1 - Ziegler, Mathias A1 - Ahmadi, Samim A1 - Thiel, Erik A1 - Krankenhagen, Rainer A1 - Maierhofer, Christiane T1 - Benefits & peculiarities of using highpower lasers for lock-in thermography N2 - Optical lock-in thermography is a completely contactless and very sensitive NDE technique. As an optical source of energy, incandescent (i.e. halogen) lamps are most commonly used because they are relatively inexpensive, do not need any work safety measures and offer high irradiances at the test site. However, they are strongly restricted by their low modulation bandwidth with a maximum modulation frequency of only about 1 Hz. The use of high-power kilowatt-class laser sources, e.g. diode laser arrays, pushes this constraint beyond 100 Hz. This allows for the exploration of the near-surface region of metals and layer systems with better and more accurate penetration depth and depth resolution. Moreover, these lasers are virtually free of any additional thermal radiation that could interfere with the “true” thermal response emitted from the heated sample. In turn, they can be easily used in a one-sided test configuration. Altogether using lasers considerably increases the application range of lock-in thermography, since especially for metals with a high reflectance and high thermal diffusivity a high irradiance is vitally important to allow for lock-in testing [1, 2]. We report on the mentioned benefits of using such high-power lasers and analyze the range of materials to be tested using lock-in thermography in dependence on the laser irradiance, the modulation frequency, the infrared camera as well as the optical and thermal material parameters. In this context, we also address a number of systematic errors caused by the use of ideal and non-ideal heat sources. For example, the measured phase angle in lock-in thermography depends on the irradiance and the modulation bandwidth of the source. This in turn has a decisive influence on the uncertainty in the quantification of, e.g. layer thicknesses. T2 - INTERNATIONAL SCHOOL OF QUANTUM ELECTRONICS, 62nd Course, Progress in Photoacoustic & Photothermal Phenomena CY - Erice, Italy DA - 06.09.2018 KW - Thermography KW - Laser thermography KW - Lock-in thermography KW - NDT PY - 2018 AN - OPUS4-46282 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Ahmadi, Samim A1 - Lecompagnon, Julien A1 - Hirsch, Philipp Daniel A1 - Burgholzer, P. A1 - Jung, P. A1 - Caire, G. A1 - Ziegler, Mathias T1 - Laser excited super resolution thermal imaging for nondestructive inspection of internal defects N2 - A photothermal super resolution technique is proposed for an improved inspection of internal defects. To evaluate the potential of the laser-based thermographic technique, an additively manufactured stainless steel specimen with closely spaced internal cavities is used. Four different experimental configurations in transmission, reflection, stepwise and continuous scanning are investigated. The applied image post-processing method is based on compressed sensing and makes use of the block sparsity from multiple measurement events. This concerted approach of experimental measurement strategy and numerical optimization enables the resolution of internal defects and outperforms conventional thermographic inspection techniques. KW - Super Resolution KW - Laser Thermography KW - Non Destructive Testing KW - Comressed Sensing KW - Inverse Problem KW - Thermography PY - 2020 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:b43-519016 VL - 10 IS - 1 SP - 22357 PB - Springer Nature AN - OPUS4-51901 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Lecompagnon, Julien A1 - Ahmadi, Samim A1 - Hirsch, Philipp Daniel A1 - Rupprecht, C. A1 - Ziegler, Mathias T1 - Thermographic detection of internal defects using 2D photothermal super resolution reconstruction with sequential laser heating N2 - Thermographic photothermal super resolution reconstruction enables the resolution of internal defects/inhomogeneities below the classical limit, which is governed by the diffusion properties of thermal wave propagation. Based on a combination of the application of special sampling strategies and a subsequent numerical optimization step in post-processing, thermographic super resolution has already proven to be superior to standard thermographic methods in the detection of one-dimensional defect/inhomogeneity structures. In our work, we report an extension of the capabilities of the method for efficient detection and resolution of defect cross sections with fully two-dimensional structured laser-based heating. The reconstruction is carried out using one of two different algorithms that are proposed within this work. Both algorithms utilize the combination of several coherent measurements using convex optimization and exploit the sparse nature of defects/inhomogeneities as is typical for most nondestructive testing scenarios. Finally, the performance of each algorithm is rated on reconstruction quality and algorithmic complexity. The presented experimental approach is based on repeated spatially structured heating by a high power laser. As a result, a two-dimensional sparse defect/inhomogeneity map can be obtained. In addition, the obtained results are compared with those of conventional thermographic inspection methods that make use of homogeneous illumination. Due to the sparse nature of the reconstructed defect/inhomogeneity map, this comparison is performed qualitatively. KW - Thermography KW - Super resolution KW - NDT KW - Inspection KW - Internal defects PY - 2022 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:b43-548351 SN - 1089-7550 VL - 131 IS - 18 SP - 1 EP - 12 PB - AIP Publishing AN - OPUS4-54835 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - CONF A1 - Lecompagnon, Julien A1 - Hirsch, Philipp Daniel A1 - Rupprecht, C. A1 - Ziegler, Mathias ED - Maldague, X. T1 - Detection of internal defects applying photothermal super resolution reconstruction utilizing two-dimensional high-power random pixel patterns N2 - In this work, we report on our progress for investigating a new experimental approach for thermographic detection of internal defects by performing 2D photothermal super resolution reconstruction. We use modern high-power laser projector technology to repeatedly excite the sample surface photothermally with varying spatially structured 2D pixel patterns. In the subsequent (blind) numerical reconstruction, multiple measurements are combined by exploiting the joint-sparse nature of the defects within the specimen using nonlinear convex optimization methods. As a result, a 2D-sparse defect/inhomogeneity map can be obtained. Using such spatially structured heating combined with compressed sensing and computational imaging methods allows to significantly reduce the experimental complexity and to study larger test surfaces as compared to the one-dimensional approach reported earlier. T2 - Quantitative Infrared Thermography 2022 CY - Paris, France DA - 04.07.2022 KW - Thermography KW - Super resolution KW - NDT KW - inspection KW - Internal defects KW - DMD KW - DLP PY - 2022 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:b43-577795 SN - 2371-4085 SP - 1 EP - 7 PB - QIRT Council AN - OPUS4-57779 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - CONF A1 - Ziegler, Mathias A1 - Thiel, Erik A1 - Ahmadi, Samim A1 - Hirsch, Philipp Daniel A1 - Burgholzer, P. A1 - Mayr, G. T1 - Thermography using High-Power Laser Arrays N2 - Due to their high irradiance and wide modulation bandwidth, high-power lasers open up a wide field of application. For example, the classical methods of pulse and lock-in thermography can be realized in high quality. In addition, structured heating is also possible by using arrays of such lasers. This makes it possible to implement new thermographic methods, such as interference-based detection of cracks or super resolution. T2 - Fifth NDTonAIR Training Event: Thermography Workshop CY - Linz, Austria DA - 13.02.2019 KW - Thermography KW - Laser Thermography KW - Super Resolution KW - Thermal Waves KW - NDT PY - 2019 AN - OPUS4-47491 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Thummerer, G. A1 - Mayr, G. A1 - Hirsch, Philipp Daniel A1 - Ziegler, Mathias A1 - Burgholzer, P. T1 - Photothermal Image Reconstruction in Opaque Media with Virtual Wave Backpropagation N2 - Thermographic reconstruction of defects that lie in the bulk of a sample is a difficult task because entropy production during heat diffusion leads to information loss. To reconstruct defects one has to solve an inverse heat conduction problem. The quality of the reconstruction is closely related to the information content of the observed data set that is reflected by the decreasing ability to spatially resolve a defect with growing defect depth. In this work we show a 2D reconstruction of rectangular slots with different width-to-depth ratios in a metallic sample. For this purpose, we apply the virtual wave concept and incorporate positivity and sparsity as prior information to overcome the diffusion-based information loss partially. The reconstruction is based on simulated and experimental pulse thermography data. In the first reconstruction step, we compute a virtual wave field from the surface temperature data. This allows us, in the second step, to use ultrasonic backpropagation methods for image reconstruction. KW - Virtual wave concept KW - Thermography KW - Photothermal Technique KW - Image reconstruction PY - 2020 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:b43-506166 VL - 112 SP - 102239 PB - Elsevier Ltd. CY - Netherlands AN - OPUS4-50616 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - CONF A1 - Lecompagnon, Julien A1 - Ahmadi, Samim A1 - Hirsch, Philipp Daniel A1 - Ziegler, Mathias ED - Zalameda, J. N. ED - Mendioroz, A. T1 - Full-frame thermographic super-resolution with 2D-structured laser heating N2 - Thermographic super-resolution techniques allow the resolution of defects/inhomogeneities beyond the classical limit, which is governed by the diffusion properties of thermal wave propagation. Photothermal super-resolution is based on a combination of an experimental scanning strategy and a numerical optimization which has been proven to be superior to standard thermographic methods in the case of 1D linear defects. In this contribution, we report on the extension of this approach towards a full frame 2D photothermal super-resolution technique. The experimental approach is based on a repeated spatially structured heating using high power lasers. In a second post-processing step, several measurements are coherently combined using mathematical optimization and taking advantage of the (joint) sparsity of the defects in the sample. In our work we extend the possibilities of the method to efficiently detect and resolve defect cross sections with a fully 2D-structured blind illumination. T2 - Thermosense: Thermal Infrared Applications XLIII CY - Online meeting DA - 12.04.2021 KW - Thermography KW - super-resolution KW - NDT KW - Inspection KW - Image resolution PY - 2021 U6 - https://doi.org/10.1117/12.2586093 VL - 11743 SP - 11743-26, 10 PB - SPIE AN - OPUS4-52524 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - CONF A1 - Ahmadi, Samim A1 - Thiel, Erik A1 - Karagianni, Christina A1 - Hirsch, Philipp Daniel A1 - Burgholzer, P. A1 - Mayr, G. A1 - Jung, P. A1 - Caire, G. A1 - Ziegler, Mathias T1 - Photothermal super resolution image reconstruction using structured 1D laser illumination N2 - The separation of two closely spaced defects in fields of Thermographic NDE is very challenging. The diffusive nature of thermal waves leads to a fundamental limitation in spatial resolution. Therefore, super resolution image reconstruction can be used. A new concerted ansatz based on spatially structured heating and joint sparsity of the signal ensemble allows for an improved reconstruction of closely spaced defects. This new technique has been studied using a 1D laser array with randomly chosen illumination pattern. This paper presents the results after applying super resolution algorithms, such as the iterative joint sparsity (IJOSP) algorithm, to our processed measurement data. Different data processing techniques before applying the IJOSP algorithm as well as the influence of regularization parameters in the data processing techniques are discussed. Moreover, the degradation of super resolution reconstruction goodness by the choice of experimental parameters such as laser line width or number of measurements is shown. The application of the super resolution results in a spatial resolution enhancement of approximately a factor of four which leads to a better separation of two closely spaced defects. T2 - Quantitative Nondestructive Evaluation Conference 2019 CY - Portland, OR, USA DA - 14.07.2019 KW - Super resolution KW - Photothermal KW - Thermography KW - Laser PY - 2019 SP - Paper 8593, 1 PB - ASME AN - OPUS4-50924 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - CONF A1 - Ziegler, Mathias A1 - Ahmadi, Samim A1 - Hirsch, Philipp Daniel A1 - Lecompagnon, Julien A1 - Hassenstein, Christian A1 - Thiel, Erik A1 - Pech May, Nelson Wilbur T1 - Using spatial and temporal shaping of laser-induced diffuse thermal wave fields in thermography N2 - The diffuse nature of thermal waves is a fun-damental limitation in thermographic nonde-structive testing. In our studies we investigated different approaches by shaping the thermal wave fields which result from heating. We have used high-power laser sources to heat metallic samples. Using these spatial and temporal shaping techniques leads to a higher detection sensitivity in our measurements with the infra-red camera. In this contribution we show our implementation of shaping laser-induced diffuse thermal wave fields and the effect on the defect reconstruction quality. T2 - SMSI 2020 Conference CY - Online meeting DA - 22.06.2020 KW - Thermal wave KW - Diffusion KW - High-power laser KW - Thermography KW - Spatiotemporal shaping PY - 2020 U6 - https://doi.org/10.5162/SMSI2020/C5.1 SP - 179 EP - 180 AN - OPUS4-50897 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - CONF A1 - Lecompagnon, Julien A1 - Ahmadi, Samim A1 - Hirsch, Philipp Daniel A1 - Rupprecht, C. A1 - Ziegler, Mathias ED - Kimata, M. ED - Shaw, J. A. ED - Valenta, C. R. T1 - Investigations on photothermal super resolution reconstruction using 2D-structured illumination patterns N2 - Active thermography as a nondestructive testing modality suffers greatly from the limitations imposed by the diffusive nature of heat conduction in solids. As a rule of thumb, the detection and resolution of internal defects/inhomogeneities is limited to a defect depth to defect size ratio greater than or equal to one. Earlier, we demonstrated that this classical limit can be overcome for 1D and 2D defect geometries by using photothermal laser-scanning super resolution. In this work we report a new experimental approach using 2D spatially structured illumination patterns in conjunction with compressed sensing and computational imaging methods to significantly decrease the experimental complexity and make the method viable for investigating larger regions of interest. T2 - Future Sensing Technologies Conference 2021 CY - Online meeting DA - 15.11.2021 KW - Thermography KW - Super resolution KW - NDT KW - Inspection KW - Image resolution PY - 2021 U6 - https://doi.org/10.1117/12.2603838 VL - 11914 SP - 124 EP - 131 PB - International Society for Optics and Photonics. SPIE AN - OPUS4-53745 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - CONF A1 - Lecompagnon, Julien A1 - Ahmadi, Samim A1 - Hirsch, Philipp Daniel A1 - Ziegler, Mathias T1 - 2D-Photothermal Super Resolution with Sparse Matrix Stacking N2 - Thermographic super resolution techniques allow the spatial resolution of defects/inhomogeneities below the classical limit, which is governed by the diffusion properties of thermal wave propagation. In this work, we report on the extension of this approach towards a full frame 2D super resolution technique. The approach is based on a repeated spatially structured heating using high power lasers. In a second post-processing step, several measurements are coherently combined using mathematical optimization and taking advantage of the (joint) sparsity of the defects in the sample T2 - Sensor and Measurement Science International Conference SMSI 2021 CY - Online meeting DA - 03.05.2021 KW - Thermography KW - Super-resolution KW - NDT KW - Inspection KW - Image resolution PY - 2021 SN - 978-3-9819376-4-0 U6 - https://doi.org/10.5162/SMSI2021/C2.2 VL - SMSI 2021 - Sensors and Instrumentation SP - 183 EP - 184 AN - OPUS4-52589 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - CONF A1 - Ziegler, Mathias A1 - Lecompagnon, Julien A1 - Hirsch, Philipp Daniel A1 - Rupprecht, C. T1 - Thermographic super resolution reconstruction using 2D pseudo-random pattern illumination N2 - Thermographic non-destructive testing is based on the interaction of thermal waves with inhomogeneities. The propagation of thermal waves from the heat source to the inhomogeneity and to the detection surface according to the thermal diffusion equation leads to the fact that two closely spaced defects can be incorrectly detected as one defect in the measured thermogram. In order to break this spatial resolution limit (super resolution), the combination of spatially structured heating and numerical methods of compressed sensing can be used. The improvement of the spatial resolution for defect detection then depends in the classical sense directly on the number of measurements. Current practical implementations of this super resolution detection still suffer from long measurement times, since not only the achievable resolution depends on performing multiple measurements, but due to the use of single spot laser sources or laser arrays with low pixel count, also the scanning process itself is quite slow. With the application of most recent high-power digital micromirror device (DMD) based laser projector technology this issue can now be overcome. T2 - ICPPP21: International Conference on Photoacoustic and Photothermal Phenomena CY - Bled, Slovenia DA - 19.06.2022 KW - Thermography KW - Super resolution KW - NDT KW - Inspection KW - Internal defects KW - DMD KW - DLP PY - 2022 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:b43-551224 UR - https://indico.ung.si/event/5/contributions/237/ AN - OPUS4-55122 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Lecompagnon, Julien A1 - Hirsch, Philipp Daniel A1 - Rupprecht, C. A1 - Ziegler, Mathias T1 - Nondestructive thermographic detection of internal defects using pixel-pattern based laser excitation and photothermal super resolution reconstruction N2 - In this work, we present a novel approach to photothermal super resolution based thermographic resolution of internal defects using two-dimensional pixel pattern-based active photothermal laser heating in conjunction with subsequent numerical reconstruction to achieve a high-resolution reconstruction of internal defect structures. With the proposed adoption of pixelated patterns generated using laser coupled high-power DLP projector technology the complexity for achieving true two-dimensional super resolution can be dramatically reduced taking a crucial step forward towards widespread practical viability. Furthermore, based on the latest developments in high-power DLP projectors, we present their first application for structured pulsed thermographic inspection of macroscopic metal samples. In addition, a forward solution to the underlying inverse problem is proposed along with an appropriate heuristic to find the regularization parameters necessary for the numerical inversion in a laboratory setting. This allows the generation of synthetic measurement data, opening the door for the application of machine learning based methods for future improvements towards full automation of the method. Finally, the proposed method is experimentally validated and shown to outperform several established conventional thermographic testing techniques while conservatively improving the required measurement times by a factor of 8 compared to currently available photothermal super resolution techniques. KW - Thermography KW - Super resolution KW - NDT KW - Inspection KW - Internal defects KW - DMD KW - DLP PY - 2023 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:b43-570551 SN - 2045-2322 VL - 13 SP - 1 EP - 13 PB - Nature Research AN - OPUS4-57055 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - CONF A1 - Lecompagnon, Julien A1 - Hirsch, Philipp Daniel A1 - Rupprecht, C. A1 - Ziegler, Mathias ED - Mendioroz, A. ED - Avdelidis, N. P. T1 - Thermographic testing using 2D pseudo-random illumination and photothermal super resolution reconstruction N2 - Due to the diffusive nature of heat propagation in solids, the detection and resolution of internal defects with active thermography based non-destructive testing is commonly limited to a defect-depth-to-defect-size ratio greater than or equal to one. In the more recent past, we have already demonstrated that this limitation can be overcome by using a spatially modulated illumination source and photothermal super resolution-based reconstruction. Furthermore, by relying on compressed sensing and computational imaging methods we were able to significantly reduce the experimental complexity to make the method viable for investigating larger regions of interest. In this work we share our progress on improving the defect/inhomogeneity characterization using fully 2D spatially structured illumination patterns instead of scanning with a single laser spot. The experimental approach is based on the repeated blind pseudo-random illumination using modern projector technology and a high-power laser. In the subsequent post-processing, several measurements are then combined by taking advantage of the joint sparsity of the defects within the sample applying 2D-photothermal super resolution reconstruction. Here, enhanced nonlinear convex optimization techniques are utilized for solving the underlying ill-determined inverse problem for typical simple defect geometries. As a result, a higher resolution defect/inhomogeneity map can be obtained at a fraction of the measurement time previously needed. T2 - Thermosense: Thermal Infrared Applications XLIV CY - Orlando, Florida, USA DA - 05.04.2022 KW - Thermography KW - Super resolution KW - NDT KW - Material testing KW - Internal defects KW - DMD KW - DLP PY - 2022 U6 - https://doi.org/10.1117/12.2618562 SN - 0277-786X VL - 12109 SP - 1 EP - 10 PB - SPIE AN - OPUS4-54909 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER -