Optical Coherence Tomography

Optical Coherence Tomography

Measurement and imaging method

The non-destructive sensing and imaging of the inner structure of materials is essential in many industrial processes for quality control and the development of new materials.

Standard non-destructive measurement methods, like ultrasound, exhibit a resolution of approximately 100 µm and often require direct contact between the probe and sample (e.g. with the aid of contact gels).

Optical coherence tomography (OCT) is an emerging optical imaging method, which allows the acquisition of cross-sections in a non-destructive and contactless manner. Today, OCT systems provide axial resolutions ranging from 1 – 10 µm and allow real-time in situ and in vivo imaging.
Optical coherence tomography is based on the phenomenon of white light interferometry and is very sensitive to small changes in the refractive index of the sample. Therefore it delivers complementary information to other imaging techniques like X-ray computed tomography (CT), and magnetic resonance imaging (MRI). The principle behind OCT is depicted in figure 1. Near infrared light waves are partially reflected at different depths within the sample, and their arrival times at a detector are interferometrically compared with a reference wave. The detected signal contains information on the position of the scatterers within the sample, such as their reflectivity, velocity, and polarization properties. Cross-sectional images can be reconstructed by collecting depth scans at different adjacent positions.

Schematic OCT structure with interferometric detection principle of the reflected light waves

Applications

So far, the main research in the field of OCT has been directed towards biomedical applications, like ophthalmology, dermatology, dentistry, and development biology. However, in recent years, the number of applications outside the field of biomedicine has increased substantially.

At RECENDT we are interested in the application of OCT for the non-destructive testing of materials. This includes the use of OCT as a tool for quality control in industrial processes and in food, and to provide complementary information for the development of new materials. Examples include the detection of cracks and faults in glass fiber composites and injection moulded polymers; the evaluation of photo-resist layers and micro-fabricated devices (fig. 2); the determination of particle distribution in protective coatings and pore size in polymer foams (fig. 3, movie); and the evaluation of multilayer foils (fig. 4), as used, for example, in the packaging industry.

Figure 2: OCT cross-section (a) and horizontal en-face image (b), acquired parallel to the interface of photo-resist moulds for miniature gear-wheels (in cooperation with microresist GmbH; design of the wheels by micromotion GmbH).
Figure 3: 3D-reconstruction of a polymer foam sample and corresponding en-face scan, acquired at a depth of ~100 µm.

Figure 4: OCT cross-section of a welded multilayer foil exhibiting a defect (arrow).

Figure 5: Projection of a 3D-OCT volume of a mourning butterfly.

Polarization-sensitive optical coherence tomography

At RECENDT we also specialize in the development of polarization sensitive OCT (PS-OCT). This method allows us to analyse the internal stress states of materials and products, e.g. of molded parts (Fig. 6).

Figure 6: (a) The conventional OCT image shows the structure of a photoresist mold for micro-gears, (b) the PS-OCT image shows internal stresses, and (c) the orientation of the internal stresses measured by PS-OCT.

Industrial Applications

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.

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) or Laser-Ultrasonics Technology (LUS, for metals) allows to monitor the quality of the products directly during the printing process. This gives the possibility to intervene immediately to correct occurring faults inline.

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.

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.

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.

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.

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!

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.

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.

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.

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.

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!

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 Ultrasonics Technology.

Polymer-extrusion

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.

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.

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 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.  

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

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.