Blind structured illumination as excitation for super-resolution photothermal radiometry

ABSTRACT

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 surface of the sample 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 nonlinear joint-sparsity reconstruction algorithm. The structured illumination is realised 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 homogeneous illuminated thermographic reconstruction.

KEYWORDS:

• Super-resolution imaging
• thermography
• blind structured illumination
• VCSEL array

Acknowledgments

This work has been supported by the project “multimodal and in-situ characterisation of inhomogeneous materials” (MiCi) by the federal government of Upper Austria and the European Regional Development Fund (EFRE) in the framework of the EU-programme IWB2020. We also acknowledge funding from the federal government of Upper Austria within the strategic-economic research program “Innovative Upper Austria 2020”.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported by the EFRE- IWB2020 [MiCi]; Upper Austrian government [Innovative Upper Austria 2020].

Notes on contributors

Peter Burgholzer

Peter Burgholzer (1964) received his diploma 1989 (statistical physics, non-equilibrium thermodynamics) and his Ph.D. degree in technical science (texture, microstructure, and plasticity of aluminum sheets) at Johannes Kepler University of Linz, Austria, in1993. After a Postdoctoral Scholarship at the Joint Research Center of the European Commission in Ispra (Italy), Institute for Advanced Materials, he worked back in Linz mainly on new optical and acoustic methods to image subsurface structures in biomedical or composite samples. He has set up a research institute for non-destructive evaluation (RECENDT, before 2009 department of the UAR) in Linz on the university campus, which started as a “one-man-show” and now employs more than 40 researchers in physical acoustics and optics mainly in cooperative projects with industry.

Thomas Berer

Thomas Berer received his Ph.D. degree in technical science from the Department of Semiconductor Physics of the Johannes Kepler University of Linz, Austria, in 2007. His thesis was on lateral quantum dots in strained silicon/silicon-germanium heterostructures. Since 2007, he has been working for the Sensor Department of the Upper Austrian Research GmbH, which eventually became the RECENDT GmbH in 2009. In 2010 he became the head of the photoacoustic imaging group. His research covered photoacoustic imaging, laser-ultrasound, and fluorescence imaging. Particular interests included non-linear photoacoustics, photoacoustic microscopy, detection of acoustic waves with interferometric means, multimodal photoacoustic imaging with optical coherence tomography and fluorescence microscopy, image formation, and excitation of ultrasonic waves using modulated laser-diodes. Thomas Berer is now with Qorvo Inc., where his reasearch focuses on bulk acoustic wave resonators.

Mathias Ziegler

Mathias Ziegler (1977) received his diploma (2004) and his Dr. rer. nat. (2009) in physics from the Humboldt University Berlin and the Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy. After working in the field of semiconductor physics, spectroscopy and photonics towards the development of short-wavelength quantum cascade lasers and high-power diode laser bars, he joined the Federal Institute for Materials Research and Testing (BAM) in 2010 where he is now a senior researcher. His current research at BAM focuses on the development of active thermographic testing techniques for microscopic and macroscopic applications, in particular using high-power lasers.

Erik Thiel

Erik Thiel (Dipl.-Ing.), born 1981, received his diploma degree in mechatronic engineering from the TU Dresden, Germany in 2009. Afterwards, he started to work at W. O. M. GmbH as part of the photonic research group, as a system engineer for the 2PM FlySCAN technology. He left the company in order to work on his PhD in the field of advanced thermographic testing at Bundesanstalt für Materialforschung und – prüfung (BAM) in Berlin.

Samim Ahmadi

Samim Ahmadi (1993) started his studies in electrical engineering at the Technical University of Berlin (2012) and received the master's degree in electrical engineering at the TU Berlin (2017). Subsequently, he began to work on his PhD (2017) at the Federal Institute for Materials Research and Testing (BAM) in Berlin in the group dealing with thermographic processes. The PhD degree will be obtained in cooperation with the TU Berlin (Department of Communication and Information Theory in the Faculty of Electrical Engineering and Information Technology). His current research interest is focused on data science, where he works on optimization algorithms for solving inverse problems within thermography using compressed sensing.

Jürgen Gruber

Jürgen Gruber, born in 1983, studied sensors and microsystems at the University of Applied Sciences Upper Austria, School of Engineering. Currently he is a research associate at the FH OÖ F&E GmbH with a ten-year long experience in non-destructive testing with focus on Active Thermography and Digital Shearography.

Günther Mayr

Günther Mayr, born 1981, studied sensors and microsystems at the University of Applied Sciences Upper Austria, School of Engineering. After graduating, he worked as a research assistant at FH OÖ F&E GmbH. In 2015 he completed his dissertation at the Johannes Kepler University in the field of polymer science. Since January 2018, he is the head of the Josef Ressel Center for Thermal Non-Destructive Evaluation of Composites in Wels.

Günther Hendorfer

Günther Hendorfer, born in 1956, studied physics at the University of Innsbruck and Linz, Austria. He did his habilitation 1997 in experimental physics. 1999–2018 he was Head of Study Automation Engineering and Dean at the University of Applied Sciences, Campus Wels from 2012–2018. Since 2018 he is Provost of the University of Applied Sciences Upper Austria.