Characterization of coatings with surface waves

Elastic waves can be used to determine the thickness and elastic properties of layered structures.
Even if the thickness of a layer is too small for pulse-echo measurements, the thickness or elastic properties of a layer can be calculated from the propagation behavior of surface waves. Depending on the frequency, surface waves penetrate the layer and substrate material below the surface to different depths. Low frequencies correspond to large penetration depths, high frequencies correspond to small penetration depths (see graph).
The different properties of the layer and substrate result in a frequency-dependent phase velocity of the surface wave. This can be measured and numerically modeled, and thus the thickness and elastic properties can be determined.
For more information, see our publication:

Process monitoring during heat treatment of metals

Heat treatment of metals is a standard process to bring controlled changes in the microstructure and thereby set the desired mechanical properties in the material.
However, currently used methods for microstructure analysis (micrographs, dilatometry, tensile tests, ...) have the disadvantage that they either cannot be used directly in the process or require special sample geometries. By measuring plate resonances on sheet metal specimens using laser ultrasound, Poisson's ratio and, if the thickness is known, also the longitudinal and transverse sound velocity can be determined without contact and with high time resolution during heat treatment (e.g., in a thermal simulator). The temperature variation of these parameters correlates with changes in the microstructure, allowing phase transitions to be monitored using this method.

Further information can be found in our publication:

Material characterization in the high GHz-range with PLUS

Optical pump-probe spectroscopy on the picosecond timescale (picosecond laser ultrasound “PLUS”) enables the study of elastic phenomena in the GHz- and even up to the THz-range.
We apply different detection schemes to investigate a large variety of materials from metals, semiconductors and polymers to novel nano- and low dimensional materials.
This gives us access to material intrinsic parameters like acoustic damping, heat diffusion and other dissipation mechanisms, doping profiles, as well as to structural properties like layer thicknesses in the sub-nm to low um range, the adhesion quality of thin coatings and other interface related properties.
Further information about a possible application can be found in

Monitoring precipitation hardening after solution annealing

Many metals are homogenized by solution annealing as part of the manufacturing process.
After quenching, however, the material is in a supersaturated state, and dissolved alloying constituents deposit over time into precipitate particles, e. g. Guinier-Preston (GP) zones. These have a strong influence on the strength and formability of the material. Using laser ultrasound, the changes in elastic and shear modulus of plates can be measured over many hours with a precision <1%, allowing the progress of precipitation hardening to be monitored with high time resolution.
One possible application is the adjustment of springback compensation depending on the stage of precipitation hardening in which the bent workpiece is in.
For further information, please refer to our publication:

Analysis of plate structures: elastic properties and thickness simultaneously

The longitudinal and transverse sound velocities allow conclusions to be drawn about the microstructure, for example the degree of recrystallization of metals during thermal treatments.
The thickness of a plate or sheet is also often required. Previous acoustic methods for measuring the speed of sound and the thickness of a plate require knowledge of the other quantity or spatial scanning. The method we have developed can determine these properties in a single measurement.
A specific surface wave and several plate resonances are excited simultaneously in the plate sample by a laser pulse with a periodic line pattern and then analyzed. This allows (with known density) the complete elastic characterization and simultaneous thickness determination of isotropic plates. A patent has been filed in this regard.
Further information can be found in our publication:

Characterization of Micro-Acoustic Systems

Microelectromechanical systems (MEMS) are integral components of cell phones and telecommunication infrastructure, whose development, production and quality testing are metrologically very challenging due to the high frequencies and small dimensions.
5G and future 6G standards drive the relevant frequency range up to about 20 GHz. We have developed a laser-ultrasonic method (Frequency Domain Laser-UltraSound "FreDomLUS") for spatially resolved characterization of single and multilayer systems with respect to elastic properties, attenuation, thickness and defects.
Figure (A) shows the sample dimensions and the determined damping αL on different single-layer constituent materials (Al and W) used in bulk acoustic wave "BAW" filters, as well as the thermoelastic limit.
For the experiment shown in Figure (B), a sample with a height profile in a checkerboard pattern was fabricated and scanned with FreDomLUS. The thickness variations of 8 nm could be reconstructed successfully and were also comparable to AFM results (Atomic Force Microscopy).
For more details, please see:; ICPPP21 conference presentation.

Fiber directions in composite materials

Do you want to gain a deeper insight and understanding of interior properties of your fiber-reinforced plastics? Are you interested in the anisotropies of your components or do you need to determine the orientation of the fibers inside a CFRP-injection moulded component?
We can help with our Terahertz-Technology (THz), OCT technology, and Laser-Ultrasound Technology.

Carbon fiber reinforced composites - CFRP

In the mainly manual production process of CFRP components delamination, inclusions and inner cracks represent critical problems for the mechanical strength. These errors are not visible from the outside because of the black coloring of the material. Using Laser-Ultrasound components can be scanned non-destructively and imperfections can be detected.

Glass fiber reinforced plastics (GFRP)

Do you want to learn more about your fiberglass components?
Would you like to take a deeper look into your components before they are delivered to customers?
Our OCT technology makes it possible by enabling you to detect defects, cracks, local fiber orientation etc.

Tribological coatings

Metallic components are often coated dielectrically in order to optimize the operating characteristics (such as tribology).
With Terahertz technology (THz) such coatings can be measured efficiently inline.

3D printing / generative manufacturing / additive manufacturing

In recent years the rapid development and quality improvements in the field of 3D printing for plastic and metal components has enabled many applications, including the serial production of critical components. Using the OCT inspection technique (for plastics, ceramics) and Laser-Ultrasonics (for metals, hard plastics) allow the detection of certain defects offline or even inline and thus enable to monitor the process closely in order to set corrective actions if needed.

Polymer coatings

The OCT (Optical Coherence Tomography) provides insight into nearly all common polymer materials used in the industrial environment. As a tomographic measurement technique, OCT delivers information about the internal structure in order to detect and characterise cracks, defects, inclusions, pores, etc. OCT not only controls the quality and functionality of the plastic products, but also delivers relevant information to help understand and optimize the manufacturing process.

Hidden (adhesive-) layers

With the Terahertz technology (THz) it is possible to look through relatively thick plastic layers and test hidden layers non-destructively (e.g. adhesive layers). The terahertz waves used are not harmful for health (no ionizing radiation), but still make it possible to look inside many optically non-transparent materials.

Testing of hidden adhesive layers

The homogeneity of adhesive layers, which is not visible or accessible after the joining process of the components, should often be tested. The modern technique of Photoacoustic Imaging provides a solution in such cases. For example you can see here (b) errors in the adhesive layer between (a) a non-transparent plastic component and a metal component – and (c) the adhesive surface in a PAI-Scan.

Energy, resources, and raw material efficiency

Using process-integrated techniques from the NDT (Non-Destructive Testing) technologies portfolio means that errors can be identified at an early stage – saving time, energy, and cost.
With direct control of your processes, you can always operate them optimally without compromise. This results in a reduction of energy input, materials consumption, and process time. Through 100% quality control, only certified zero-defect products are delivered to the customers.
Dare to go to the limits within product and process design – through integrated process analytics and 100% quality control you save time and money, and it’s hassle-free. We can support you in these matters!

Inline quality assurance in welding

Non-destructive testing of welds is possible with Laser-Ultrasound. Thanks to the good automation capacity also inline measurements at high speed can be carried out, depending on the required resolution. With a repetition rate of 10 Hz, 100 Hz or even higher, flaws, inclusions, hot cracks and pore concentrations can be found. 

Spot welding

By means of Laser-Ultrasound also spot welds can be scanned in a short time and zones of actual material connection (weld nugget) can be imaged and evaluated.

Breast cancer – early detection

Photoacoustic imaging is an effective tool for diagnostic imaging for the early detection of breast cancer. This technology uses optical as well as acoustic effects. Thus, this method provides a good combination of diagnostic accuracy and spatial resolution in imaging. Therefore, photoacoustic imaging is perfectly suitable as an alternative for mammography – and it completely avoids harmful radiation exposure for the patients.


Operation accompanying imaging without X-ray exposure

In some situations it is helpful for the operating surgeon to use the support of a continuous imaging system (e.g. during a brain operation). In order to keep the X-ray radiation exposure as low as possible for the patients and the medical staff, Photoacoustic imaging photoacoustic imaging (another research area of RECENDT) can be applied.

Point-of-Care – Diagnostics for blood and urine

Spectroscopic methods provide highly efficient analysis of blood and urine samples. These methods allow a quick determination of various clinical parameters under Point-of-Care (POC) conditions. With such modern technologies time-consuming laboratory analysis are not necessary, and in addition, minimal amounts of liquid can be reliably measured. Thus, such approaches can be applied to intensive care and paediatrics / neonatology, or on-site use for emergency doctors.

Point-of-Care – Diagnostic of malign changes of skin (in-vivo)

Modern technologies are able to provide an immediate assessment of suspicious changes in skin – ideally by combining several methods: for example, contactless Infrared or Raman Spectroscopy can create spectral/chemical fingerprints of skin changes, and the Optical Coherence Tomography (OCT) is able to show microstructural skin changes.

PAC – The research network

Process Analytical Technologies (e.g. Infrared Spectroscopic Methods) are able to monitor chemical processes inline and control them precisely, which enables perfect optimization.
We have conducted intensive research in this field since 2010 and can act as your contact point to the entire research network PAC -

Process analytics & process optimization

In food production (bio) chemical process steps are frequently performed, for example, in beer production.
With our expertise we can assist in the application of Process Analytical Methods, e.g. varieties of Spectroscopy, to safeguard and optimize your processes!

Spectroscopic imaging

The spectroscopic control of chemical compoundings (e.g. of pills) can be achieved by implementing different technologies (NIR, MIR, Raman, THz and also Quantum Sensing). Spatially resolved spectroscopy is also possible – moreover you can get a picture, for instance, of the API (Active Pharmaceutical Ingredient) distribution in the product!

Process optimization for batch-processes or continuous-processes

Through exact knowledge of the current conditions in a batch-reactor or in a continuous process, the chemical processes can be regulated precisely. Therefore it is often possible to reduce process temperature and time reserves. Energy saving significantly raises profits, and leads to a quick amortization of such process optimization. Our Spectroscopy experts can support you!

Fruits & Vegetables

The structure of fruits (or generally speaking biological samples) can be analysed by OCT (Optical Coherence Tomography) – the image shows apples as an example. With such analysis, we can draw conclusions about possible shelf life and determine the quality criterion that influences the acceptance by customers.

Sealing or bonding processes

Do you operate sealing processes e.g. for food packaging? With our Laser-Ultrasonics or (depending on the material) OCT technology we are able to measure the sealing seams inline in the process. We provide similar testing methods for bonding processes, e.g., hot melt adhesives for packaging.

Beverages in bottles and cups

Bottles (glass or plastics) and plastic bowls can be tested on the exact adherence to thickness specifications or on possible defects by OCT (Optical Coherence Tomography). Thus, we can ensure highly automated packaging processes.

Bakery products

In the field of bakery production our Spectroscopy experts deal with:

  • Incoming goods and monitoring of raw materials
  • Monitoring of mixtures and ingredients
  • Monitoring of the homogeneity of dough in different process steps
  • Characterization and process validation in coating process steps
  • Multimodal studies for the crystallization of chocolate
  • Online process optimization based on incoming goods inspection (measurement of the Raw-Material-Variability), and process modelling
  • Detection of foreign objects: THz scans are able to detect non-metallic foreign bodies
  • Detection of defective goods (e.g. cracked biscuits in a closed package, ready for sale)

Blow-extrusion films / cast-sheet

OCT technology provides real-time high-resolution images of the inner structure of plastics, e.g., the layer construction of a multilayer film. You can determine the thickness of the inner layers right inline during the production process with an accuracy of less than 1 micrometer and adjust your processes (blow-extrusion film or cast-sheet) for an optimal production!

Meat production / meat and sausage manufacturing

The meat production / meat and sausage manufacturing is another research and application field for our Spectroscopy group.

You may be interested in the following questions:

  • Is the work area absolutely clean or are biofilms forming on the surfaces?
  • How can the quality of a piece of meat be valuated?
  • In which production step is it decided whether the meat processed is “steak” or “minced”?
  • How can the origin of the product and a possible “bio” status be controlled?
  • Does the packaging guarantee that the quality produced will finally be delivered to the customer?
  • Is it possible to control the quality status (and the current BBD) of the sellable finished and packed product?  

Cereals and coatings

In product development non-destructive testing and analysis methods can help to monitor product features in real-time and to learn more about the product. The example shows a surface structure of breakfast cereals with different sugar coatings, measured by OCT (Optical Coherence Tomography). Such recordings are also available as real-time videos, which show the behaviour during hydration.

Spectroscopy in the product development, production and QA

For the production monitoring / quality control or the product development, spectroscopic technologies (NIR, MIR, Raman, THz) can play a significant role. These methods make it possible to obtain very precise measurements of mixing ratios, compositions, ingredients, reaction progresses, and similar chemical information without sample preparation and costly laboratory analyses.
Such real-time measured data provides an ideal basis for your process optimization!

Raw Material Variability

What does "Raw Material Variability" mean to us? You are required to achieve the same high quality standard for each of your products – irrespective of changing properties of different raw material supplies – in order to meet the expectation of your customers. Our portfolio of process analytics can support you by:

  • Monitoring the relevant parameters of your raw materials at the incoming goods inspection
  • Understanding, modeling, and monitoring your processes
  • Providing a guarantee of constant product quality even with changing raw materials!

We can also be your contact for the whole team of experts within the research network PAC.


If you are committed to providing high quality seeds, we can help you in the following ways:

  • Contactless determination of residual moisture
  • Quantitative and qualitative determination of seed ingredients (e.g. proteins, carbohydrates (sugar, starch) etc.)
  • Analysis of incrustation and pelleting
  • Evaluation of the vitality and germination capacity
  • Soil analysis (phosphates, nitrates…)
  • Outgassing of the seeds (e.g. during storage) may deliver information about the status and possible deterioration (e.g. fungal infection)

Ask our Spectroscopy experts!

Microstructure of metals in-situ

By offering the possibility to investigate glowing samples, Laser-Ultrasound also enables the determination of in-situ metallurgical information during thermo-mechanical cycles of new steel grades. Therefor, samples can be inductively heated and deformed in our modified quenching and deformation dilatometer (by Linseis Messgeraete). From the measured parameters, like speed of sound and acoustic damping, it is possible to conclude on grain growth, phase transitions and texture changes with appropriate calibration.

Hardness penetration depth in steel

The determination of the hardness penetration depth in thermally hardened components is essential for quality control. State-of-the-art is to cut samples and do etching and hardness measurements. Laser-Ultrasonics opens a non-destructive alternative. Thereby, zones of different microstructures at arbitrary positions can be imaged up to a tomographic representation of the hardness penetration depth.

Multilayer systems and welding seams in polymer films

Multilayer-films carry great importance especially in the food industry. For example, the thickness and homogeneity of barrier layers have a great influence on the shelf-life of food. With the OCT technology you can measure the thickness of the individual layers with high-resolution inline during the process. Moreover, you can see and detect delaminations and welding seams in real-time and possibly regulate and optimize the production process based on this measurement data.

Visualization of multilayer structures

As illustrated in this example of an organic photovoltaic cell, multilayer structures can be measured and analysed with OCT (Optical Coherence Tomography). Organic PV-cells consist of organic semiconducting materials, transparent electrodes, and a protective coating. As a result of this, a homogeneous layer structure without defects, inclusions etc. is relevant for the quality, functionality, and long-lasting performance of the product.

Multilayer pipe extrusion (Co-extrusion)

With THz technology, individual layer thicknesses can be measured inline in the multilayer pipe extrusion process (co-extrusion). This allows a precise regulation and optimization, with savings in energy and raw material, which does not hinder quality assurance.

Plastic parts

The OCT technology can be used for the online monitoring of dynamic processes, as the OCT images are generated in real-time. If you visualize the local distribution of internal stress in plastic parts you can avoid problems with your components!


Almost every plastic product runs through an extrusion process at least once during production. The flow behaviour of plastics, or the shearing forces in the extruders and injectors, appropriately determines the mechanic properties of the final product. With Doppler-OCT (image to the right) and time-resolved OCT (“particle image velocimetry”, image to the left) it is possible to measure the flow conditions in the manufacturing process in order to optimize the simulation models and tools with this data.

Casting process

For the casting of metals it may be interesting to monitor the depth of melting, but also to find hot cracks (aka solidification crack) in the freshly cast and still hot bars. Both is possible with Laser-Ultrasonics and was already investigated at RECENDT. In the figure a reconstruction and the corresponding photo of the cross section through a circular ingot of aluminum can be seen, which has an obvious center crack.

Testing of surfaces and topographies

As illustrated in this image of a coin, the topography or roughness (parameters cf. ISO 25178) of a sample can easily be determined by Optical Coherence Tomography (OCT). The advantage of using OCT, compared to other methods, is the additional possibility of measuring steep edges. The accuracy in the measurement of height profiles is below 1 micron.

Bondings and Solder Joints

The Laser-Ultrasonics technology is also applicable for the testing of adhesion and bonding layers, e.g., solder joints. As the high-frequency ultrasonic waves are influenced at the interfaces, an image of this critical inner zone can be generated.

Coating technology

In multilayer coatings the individual layers often have individual functions, e.g., protection from mechanical impacts and contamination, corrosion protection, adhesive layers, etc. Therefore, a homogeneous layer structure without defects, inclusions, etc. is essential for the quality and functionality of the product. With OCT (Optical Coherence Tomography) it is possible to measure the thickness of coatings with micrometre precision (even for most multilayer coatings).

Bionic surfaces / functionalized surfaces

In a research cooperation with the Institute of Biomedical Mechatronics (Johannes Kepler University, Linz), the skin structure of a Texan horned lizard was artificially recreated. With its unique skin structure, this animal can move condensed water from the whole body surface directly to the mouth. The successful application of OCT played a major role in the success of this project. As an instrument for imaging and characterization of the surface, OCT contributed to the understanding of functionality of the surface. Applications for similar “functional surfaces” are “Lab-on-chip” applications, wound compresses, textile fibers, and the distribution of lubricant on friction bearings.

Measuring laser structured surfaces

With laser-based methods surfaces can be structured for different purposes. The OCT technology (Optical Coherence Tomography) is a perfect tool for imaging and measuring such structures in the micrometre range (e.g. microscopic laser drilled holes shown in the picture) and can also be measured online and in real-time during the process.

Tablet coating

We provide our patented OCT technology in the form of an inline-integrated measurement system for the coating of tablets, which allows the exact regulation, monitoring and optimization for the coating process!

GMP - Good Manufacturing Practice

In order to ensure the health and safety of customers of pharmaceutical products, the regulators have introduced a strict set of rules in recent years. In addition to Good Manufacturing Practice (GMP) and Quality by Design (QbD), Process Analytical Technology (PAT) also plays an important role.

With OCT technology RECENDT (in collaboration with the Research Center Pharmaceutical Engineering, RCPE, Graz) offers, e.g., a worldwide and unique tool that can be used inline, at-line or offline. With this tool the layer thickness of tablet coatings can be measured during the coating process and the process can be controlled and optimized.  

Explosives or drugs identification

Spectroscopic methods allow the easy identification of substances. Using such methods makes it possible to identify explosives and to distinguish between non-hazardous materials and hazardous or illegal substances.

Moreover, with Terahertz Spectroscopy, such analysis can also be performed for substances inside closed and non-transparent containers / objects (e.g. letters and cardboard boxes).

Security checks

Terahertz imaging received significant media attention in connection with the so called “body scanners” for airport security checks. Indeed, it is possible to detect THz radiation, which is emitted from every body, through clothing using the THz technique. These images can help to detect shadows, such as hidden weapons.

Photoacoustic spectroscopy

Contactless Photoacoustic Spectroscopy may also be used for the Spectroscopic measurements of solids and liquids. This technique offers very high sensitivity in certain applications as well as offering the possibility of spatially resolved measurements. Talk to our experts about possible solutions for your applications.

Adhesive bonding processes

An adhesive bond can only be as good as the production process in which it was produced. Besides the optimum choice and quality of the adhesive agent, the preparation of the surfaces and the perfect process control also play important roles in ensuring utmost performance of the bond in terms of load capacity and fatigue strength.
By utilizing spectroscopic technologies we can help you to monitor and control all chemical parameters (e.g., in mixtures, reaction monitoring, cleanliness of surfaces, hardening) directly inline during the process.

EEffG - The federal energy-efficiency-law

The energy-efficiency-law (EEffG) was created in 2014 with an aim to improve the energy efficiency of industrial enterprises by 20%. We can provide the support to help achieve these goals. By improving energy efficiency your company can also save a lot of money. Additionally, the EEffG develops a market for efficiency-enhancement-activities.
With our range of technologies in each of our research areas, we are able to support you.   

Analytics for paper & the paper industry

Paper is an extremely diverse material and its characterization requirements are relevant for production, processing, and application. We can support you in many ways with our testing methods (IR & Raman Spectroscopy, THz Spectroscopy, OCT, and Laser-Ultrasound), for example, in chemical analytics (directly in the production process) and the characterization of surfaces and fibers for both absorbency and anisotropy.

Embedded particles in lacquer coatings

Embedded particles in lacquer coatings

In lacquer coating layers (or similar coatings), embedded particles (of different materials) are very important for the functionalization of the coating or for e.g. a perfect automotive-metallic-coating.
With the low-cost OCT method we are able to monitor and characterize the partial embedding, which helps to secure and optimize the production process.

Spatially resolved spectroscopy

Do you want to know the exact local distribution (in micrometer range) of your chemical components? With Mid-Infrared-Microscopy we can chemically characterize and measure materials and cross-sections (e.g. residues or inclusions) with a spatial resolution as small as 5 µm. Quantum Sensing offers interesting opportunities in that area too.

Process-Up scaling

When developing new processes, and especially when bringing a process from lab-scale to production-scale, spectroscopic technologies play a crucial role in guaranteeing optimum performance. We can assist with selection of the best-suited measurement technique. Combine your process expertise with our analytical and monitoring experience to optimize your processes and maximize your output.

Nano-Medicine & Pharmaceutical Development

We are unable to develop the API - this might be your expertise! But when it comes to carrier development and process upscaling, you can rely on our experience of the application of process for analytical technologies (e.g. spectroscopic techniques) to serve as a support unit for precision medicine / nano medicine / personalized medicine and pharmaceutical development in general.

Inner structures in high-scattering materials

You need to inspect inner structures in, e.g., industrial ceramics, filled polymers or paints, composite materials? Often, this inspection task is hindered by the intense scattering effects. By applying mid-infrared OCT (Optical Coherence Tomography) or our latest Quantum Sensing technologies, we can overcome those problems and provide an increased probing depth and still 8 µm axial resolution.