Fraunhofer Institute for Industrial Mathematics ITWM
Fraunhofer Institute for Industrial Mathematics ITWM
Langlebige Elektronik durch OCT
© Fraunhofer ITWM
Langlebige Elektronik durch OCT: Schutzlacke werden bei hochempfindlichen Baugruppen eingesetzt, um sie vor Umwelteinflüssen zu schützen, die Lebensdauer zu verlängern oder die Funktionalität zu gewährleisten. OCT prüft hierbei die für die Funktionalität wichtige Schichtdicke und Blasenfreiheit.

Long-lasting Electronics Thanks to OCT

Protective coatings are used on highly sensitive assemblies to protect them from environmental influences, extend their service life or guarantee their functionality. OCT checks the coating thickness and absence of bubbles, which are important for functionality.

© Fraunhofer ITWM

Mit OCT kann man die Schweißnaht von Kunststoffbeutel überprüfen, wichtig für die Dichtheit der Beutel.
© Fraunhofer ITWM
Messaufnahme: Mit OCT kann man die Schweißnaht von Kunststoffbeutel überprüfen, wichtig für die Dichtheit der Beutel. Auf dem Bild erkennt man die zwei Folien, die sich näherkommen und in Stoß verschwindet der Übergang, das ist ein Zeichen für eine gute Schweißung.

OCT for Weld Seam Inspection of Plastics

The enormous advantages of optical coherence tomography can be used for a wide range of applications in industrial production. For example, in the inspection of weld seams on plastic bags such as those used for medical products or foodstuffs.

© Fraunhofer ITWM

Non-destructive Material Testing with Optical Coherence Tomography (OCT)

High-Resolution Material Testing with OCT – Non-contact and Precise

Optical Coherence Tomography (OCT) is a high-resolution imaging method that has proven to be extremely useful not only in medical diagnostics, but also in non-destructive testing (NDT).

It makes internal structures of materials and complex components visible – without contact and without damaging them. Because no samples need to be removed, the object remains completely intact. This is particularly important when it comes to expensive materials or materials that are difficult to replace. OCT is becoming increasingly important in industrial applications – for example in manufacturing, the automotive industry, aerospace and materials research. It helps to make inspection processes more efficient, ensure the quality of products and materials and reduce costs.

Principle of Optical Coherence Tomography

In OCT technology, the light from a broadband light source of low coherence length is split into a reference arm and a sample arm (see illustration). In the sample arm, we focus the light on the object to be examined. It is reflected at the interfaces according to the Fresnel equations and then superimposed with the light from the reference arm. This superposition takes place in an optical coupler.

A diffraction grating directs the resulting short-coherent light onto a line detector. There, a spectral interference pattern is created by Fourier transformation, which we evaluate as a function of the wavelength. The resulting interference fringes are directly related to the depth of the reflective layers. As interference only occurs within the coherence length, this also determines the achievable depth resolution.

This punctual measurement technique can be extended to line and area scans (B-scan and C-scan) using galvo mirrors. In this context, single-point measurement is also referred to as an A-scan.

Schematic Representation of a Frequency Domain OCT System
© Fraunhofer ITWM
Schematic representation of a frequency-domain OCT system as used in optical coherence tomography (OCT) for high-resolution imaging.

Resolution and Measuring Performance

Typical OCT systems work with a central wavelength of around 840 nm and a bandwidth of 80 nm. This results in a coherence length of around 9 μm – the theoretical depth resolution is therefore around 4.5 μm. In practice, we achieve a reliable resolution from around 10 μm with real samples and optical components.

Thanks to high measurement speeds – up to 150,000 individual measurements (A-scans) per second and 150 B-scans per second – and line widths in the millimeter range, OCT is ideal for use in automated, inline-capable inspection systems.

Applications of OCT in Non-destructive Testing

Optical Coherence Tomography offers decisive advantages for non-destructive testing. Its high resolution in the micrometer range makes it possible to make the finest details and surface structures visible – even where other methods reach their limits.

OCT really comes into its own with multi-layered materials such as plastic films or painted components. It detects individual layers and uncovers potential delamination or other defects without damaging the material. Even with rough surfaces or highly scattering samples, OCT delivers reliable results in situations where many conventional inspection methods fail.

The following three illustrations are examples of how versatile and powerful OCT is in practice.

Non-destructive Material Testing with Optical Coherence Tomography (OCT): Painted Wood Sample
© Fraunhofer ITWM
The illustration shows a varnished wood sample. The homogeneous clear lacquer surface is just as visible as the textured wood grain underneath.
Non-destructive Material Testing with Optical Coherence Tomography (Oct): Tube
The illustration shows a multi-layer pipe wall. The outer layers are clearly different: while the first layer shows a smooth, clear structure, the second layer scatters strongly due to the addition of additives – an effect that OCT maps precisely.
Non-destructive Material Testing with Optical Coherence Tomography (Oct): Roll of Adhesive Tape with Many Individual Layers
© Fraunhofer ITWM
The illustration shows a roll of adhesive tape with many individual layers. OCT makes it clear: foil and adhesive film can be clearly distinguished from each other – as a sequence of thicker and thinner layers.

Limitations of OCT

Despite its strengths, OCT also has its limitations. A key challenge lies in the limited penetration depth. Although the method provides high-resolution images of surfaces and layers close to the surface, deeper structures in dense or highly scattering materials can only be depicted to a limited extent.

In such cases, the use of longer wavelength light sources can help to increase the penetration depth. However, this is usually at the expense of resolution, so a sensible compromise must always be found between depth and detail accuracy.

Project Examples

Measuring the Enamel Thickness of Winding Wire

Optical Coherence Tomography (OCT) can be used to measure the thickness and quality of thin layers of enamel on copper wires precisely and non-destructively.

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Coating Thickness Measurement

Using optical measurement techniques such as OCT and terahertz, the thickness of multilayer coatings on metal and plastic can be checked precisely and non-destructively to ensure quality and resource conservation.

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