<figure><img src="image_1.png" /></figure>

# High-Precision Inspection of Micro-Components Using the Microferoscope:
Exploring the Potential of Digital Speckle-Pattern Interferometry

Jessica PLASSMANN<sup>1,2</sup>, Tim EIS<sup>2</sup>, Ann-Kathrin MOLITOR<sup>2</sup>, Valentin BASTGEN<sup>1,2</sup>, Georg von FREYMANN<sup>1,3</sup>, Michael SCHUTH<sup>2</sup>,

## Abstract

Reliable inspection of micro-scale components is essential in fields such as electronics and medical device manufacturing. To address this, a method combining digital speckle holography with microscopy has been developed. The resulting "Microferoscope" enables precise detection of structural defects in areas of just a few square millimetres. The method identifies minute deformations induced by controlled external stimuli, such as thermal or mechanical loading. Characteristic deformation patterns reveal structural anomalies. A dedicated module with orthogonal illumination allows for high-resolution measurement of deformations perpendicular to the component surface. Additionally, a three-dimensional module using laser diodes of different wavelengths enables the detection of both in-plane and out-of-plane deformations. Although high sensitivity in out-of-plane detection enhances defect recognition, it also increases susceptibility to environmental disturbances. This study compares both modules on micro-components to evaluate their sensitivity, robustness, and practical applicability.

**Keywords:** Electronic Speckle-Pattern Interferometry, ESPI, Micro-component inspection, Microscopy-based metrology, Structural defect detection

## 1. Introduction

Quality assurance in industrial serial testing relies on inspection methods capable of reliably and efficiently detecting even the smallest defects. Established techniques such as thermography, ultrasound, and X-ray inspection have proven effective within their specific application areas. Thermography utilizes the thermal properties of materials to visualize defects, contingent on the feasibility of adequate thermal excitation. Ultrasonic testing detects internal structural changes with high sensitivity but is generally limited to point-wise inspections, making large-area assessments more time-consuming. X-ray inspection offers detailed radiographic analysis, yet its use is constrained by stringent safety requirements associated with ionizing radiation. While these methods perform well for many inspection tasks, certain applications remain challenging and necessitate complementary or alternative techniques.

Shearography represents an alternative, albeit still relatively infrequently applied, interferometric measurement technique that quantifies deformation changes between two distinct loading states. Its inherent robustness against environmental perturbations suggests potential for broader implementation in serial testing contexts. Nevertheless, attaining high measurement accuracy necessitates a sufficiently large shear distance, which inherently constrains its effectiveness in detecting minute defects. Specifically, when the shear distance, defined as the angular displacement between successive image captures, exceeds an optimal range, the resultant difference images may exhibit non-overlapping defect representations, rendering such defects undetectable. This limitation is particularly critical in the inspection of safety-relevant micro-components, such as those employed in medical technology and microelectronics, where undetected defects could precipitate significant material,

<footnote n="1"><sup>1</sup> Department of Physics and research center OPTIMAS, RPTU University of Kaiserslautern-Landau
67663 Kaiserslautern, Germany</footnote>

<footnote n="2"><sup>2</sup> Department of Engineering technology center OGKB, Trier University of Applied Sciences, 54296 Trier,
Germany, plassmaj@hochschule-trier.de</footnote>

<footnote n="3"><sup>3</sup> Fraunhofer Institute for Industrial Mathematics ITWM, 67663 Kaiserslautern, Germany</footnote>
environmental, or personal harm. In these contexts, inspection methods with enhanced precision are indispensable [1][2].

A promising alternative is digital speckle pattern interferometry (ESPI), which differs from shearography by directly measuring deformations rather than deformation gradients. This characteristic enables the detection of even minute structural irregularities. Nevertheless, the heightened sensitivity inherent to holographic techniques also increases vulnerability to environmental disturbances, posing challenges for their application in industrial production settings [2].

This study introduces the Microferoscope (MFK), a measurement system [3] that integrates digital speckle-pattern interferometry with microscopy, designed specifically to enable high-precision inspection of micro-components. Recognizing the importance of advancing and optimizing measurement techniques to expand the range of applicable testing scenarios, the MFK addresses these challenges by combining holography with microscopy. The system comprises two modules: one optimized for highly sensitive out-of-plane deformation measurements, and a second 3D imaging module (3D-module) that employs multiple laser wavelengths to achieve more robust measurements both in-plane and out-of-plane. The primary objective of this initial investigation is to evaluate and compare the sensitivity of the two modules for out-of-plane measurements. Building upon these findings, future studies will focus on implementing combined in-plane and out-of-plane measurement capabilities.

## 2. Fundamentals

### 2.1 Digital Speckle Pattern Interferometry: Setup and Operating Principle

Optical interferometry offers highly precise measurement techniques for a wide range of technical domains. ESPI utilizes the principle of interference to detect minute deformations of an object with sub micrometre accuracy. These measurements are based on the superposition of a reference wavefront $E_r$ with an object wavefront $E_o$ reflected from the surface of the object. The resulting phase shift $\Delta\varphi$ provides information about irregular deformations or defects in the measured object.

The measurement setup ([4], p. 77) includes a coherent light source which is split into two beams by a beam splitter. One beam is directed to the camera and serves as the reference wavefront $E_r$. The second beam is directed onto the object under test, reflected from its surface, and reaches the camera as the object wavefront $E_o$. This wavefront interferes with the reference wavefront $E_r$ and produces interference patterns that represent the object's deformations. According to [5], the intensity of the interference patterns is determined by the superposition of the reference wavefront $E_r$ and the object wavefront $E_o$. The intensity $I$ is given by the squared magnitude of the sum of the two wavefronts:

$$I = |E_r + E_o|^2 = |E_r|^2 + |E_o|^2 + 2\Re(E_r E_o^*) \quad (1)$$

The interference term $2\Re(E_r E_o^*)$, which contains the complex conjugate of the object wavefront $E_o^*$, provides the phase information necessary to determine the deformation of the object. To extract this phase information, the technique of temporal phase shifting (TPS) is commonly employed. In this method, a series of measurement images is recorded sequentially, each with a controlled phase shift $\Delta\varphi_i$ ($i = 1, 2, ..., N$). introduced in the reference beam. These phase shifts are commonly generated by a piezoelectrically actuated mirror, which modulates the
optical path length of the reference wavefront by sub-wavelength increments. By analyzing the
intensity variations across these phase-shifted images, the phase distribution of the object's
deformation can be precisely calculated. It is important to note that temporal phase shifting
(TPS) requires multiple sequential captures and precise control of the reference beam phase.
Due to this sequential acquisition, TPS is generally limited to static loading conditions, as any
motion or dynamic changes during the capture of phase-shifted images can significantly distort
the results. Even minor environmental disturbances or vibrations during the measurement
sequence can strongly affect accuracy.

Figure 2.1 shows the schematic setup of the ESPI system for an out of plane measurement using TPS. In many holographically operating interferometers, the reference wavefront is coupled coaxially to the observation axis ([6], Chap. 21.3). This coupling is not explicitly depicted in the figure, as the illustration aims to clarify the general setup and working principle. However, the precise illumination angle plays a crucial role in determining the type of measurement conducted. Details regarding the influence of this angle will be addressed in the following sections, as it is fundamental to the present investigation. The figure therefore focuses on the essential components and the method by which interference patterns are utilized to calculate surface deformations.

<figure><img src="image_2.png" /><figcaption>Figure 2.1. Typical setup for out-of-plane digital holography using temporal phase shifting (TPS). Phase information is obtained by introducing defined phase steps via a piezo-actuated mirror in the reference beam path, allowing phase-shifted holograms to be recorded on the camera, after [7]. On the left, an exemplary measurement of a flat-bottom hole under central out-of-plane loading is depicted. Below it, a cross-sectional profile through the centre illustrates the characteristic sawtooth-like phase signal. The bottom image shows the demodulated displacement.</figcaption></figure>

In contrast, in-plane holography is employed to measure deformations occurring within the
plane of the object's surface. This typically involves a dual-beam illumination setup, where the
specimen is symmetrically illuminated by two coherent light beams ([2], Chap. 3.2.5). While
understanding this principle is important for the design rationale of the 3D module, the current
study focuses exclusively on out-of-plane measurements. This focus serves to establish a
foundational proof of concept. Although many defects initially exhibit out-of-plane
deformations, addressing in-plane measurements will be reserved for future studies to fully
utilize the capabilities of the three-dimensional system.
## 2.2 Mathematical Fundamentals: Sensitivity of Holography

Holography does not provide absolute height measurements of surface points but quantifies relative deformations by evaluating the phase difference between two loading states. For the general illumination geometry shown in Figure 2.1, where the illumination angle is denoted as $\Theta_{xz}$, the relationship between the measured phase shift and the deformation components of the examined object is given by [7], Chap. 2.4.a, as follows:

$$N = \frac{1}{\lambda} [D_u \sin(\Theta_{xz}) + D_w (1 + \cos(\Theta_{xz}))] \quad (2)$$

Here, $N$ represents the measured fringe order resulting from the interference of the reference and object wavefronts. The fringe order corresponds to the number of interference fringes visible on the detector. Each fringe corresponds to a phase jump that occurs when the deformation of the object's surface exceeds a length proportional to the wavelength of the laser. This relationship is illustrated in Figure 2.1 by the cross-sectional view of the resulting interference pattern and can be converted into an actual deformation measurement through demodulation. $D_u$ denotes the in-plane deformation in the x-direction, meaning the deformation that occurs parallel to the objects surface. For clarity, this paper adopts a simplified two-dimensional representation, considering only one in-plane direction while omitting the orthogonal in-plane component along the y-axis. This simplification facilitates the explanation of the fundamental relationships and aligns with the study's focus on out-of-plane deformations. In contrast, $D_w$ represents the out-of-plane deformation component normal to the object's surface (z-direction). The parameter $\lambda$ denotes the wavelength of the employed laser source and directly affects the measurement resolution. The equation indicates that the measured phase shift consists of two distinct components. The first is an in-plane term $D_u \sin(\Theta_{xz})$, which corresponds to deformation parallel to the surface in the x-direction, as represented within the two-dimensional simplification adopted in this analysis. The second component is an out-of-plane term $D_w (1 + \cos(\Theta_{xz}))$, which characterizes deformation normal to the surface. The factor $(1 + \cos(\Theta_{xz}))$ inherently increases the sensitivity of the out-of-plane component by the addition of the constant term 1. Consequently, irrespective of the illumination angle $\Theta_{xz}$, out-of-plane displacement contributions exert a comparatively greater influence on the measured phase shift than those arising from in-plane deformations. This intrinsic weighting results in an increased responsiveness of the measurement system to surface-normal displacements.

Furthermore, the measured phase shift is inversely proportional to the wavelength $\lambda$ and depends on the illumination angle $\Theta_{xz}$. Specifically, a shorter wavelength results in an increased phase response, thereby enhancing measurement sensitivity. Additionally, as the illumination angle decreases from 90° toward 0°, the contribution of out-of-plane deformation to the measured phase shift becomes more pronounced, further amplifying the system's sensitivity to surface-normal displacements.

# 3. Setup and Methodology

## 3.1 Operating Principle and Setup of the Microferoscope

The Microferoscope represents a unique integration of a conventional microscope and a specially developed add-on module. A fundamental advantage of the MFK lies in its modular design, which allows the measurement unit to be integrated directly into the microscope as a
plug-in component. This design enables rapid retrofitting of the system for both visual inspection and microscopic ESPI-based deformation analysis.

The initial MFK prototype was developed to measure both out-of-plane and in-plane deformations of test specimens. Its interferometric setup was originally based on a Mach–Zehnder interferometer, but it was subsequently adapted to accommodate the optical and spatial constraints inherent to microscopy. The microscope itself features a focal length adjustable in thirteen discrete increments ranging from 35.5 millimetres to 575 millimetres, permitting the examination of surface areas from 40 by 25 millimetres down to 2.5 by 2 millimetres with high precision. A key development aspect was the integration of laser light through an optical fibre, which is guided along the optical path of the measurement device. The system’s internal shading mechanisms enable switching between in-plane and out-of-plane measurements depending on their configuration.

The prototype underwent continuous refinement to address limitations and to expand its applicability. The current iteration, designated MFK 2, replaces the previously employed temporal phase shifting technique with spatial phase shifting (SPS). This method uses a specially designed aperture to generate a carrier frequency, which, combined with a fast Fourier transform, permits extraction of phase information from a single image per measurement state. This approach is particularly suited for capturing dynamic loads, as the sequential image acquisition required by temporal phase shifting is prone to errors in such scenarios. Although SPS reduces light efficiency due to the aperture, leading to a slight decrease in measurement accuracy compared to temporal phase shifting, it significantly improves suitability for dynamic measurements. The MFK 2 comprises two principal modules: the out-of-plane module (oop module) and the 3D module, each optimized for specific measurement tasks as illustrated in Figure 3.1.

<figure><img src="image_3.png" /><figcaption>Figure 3.1 The left image shows the 3D module mounted below a microscope. A camera is connected to the microscope via a video tube for digital image acquisition. A bimetallic strip is positioned as a test object beneath the setup and bends upwards toward the microscope when electrically activated. Illumination is provided by three laser diodes, each representing a different wavelength, identifiable by their elliptical beam profiles. The right image shows the oop module, which is inserted into the optical path of the microscope. Laser light is coupled laterally into the beam path using a fibre optic input. ([7], p. 147 ff)</figcaption></figure>

The oop module has been designed to be more compact and lightweight compared to its predecessor and is mounted directly to the lower part of the microscope, increasing the setup height by only thirty-five millimetres. A key advantage of this module is its zero-degree illumination angle, which enables precise measurement of out-of-plane deformations while
avoiding image shadowing. The laser beam is directed downward under the microscope, and
the zero-degree coupling angle minimizes stray reflections into the environment, thereby
enhancing operational safety.

The 3D module allows for full-field deformation measurement encompassing both out-of-plane and in-plane components. Compared to the original MFK, this module employs a geometric arrangement of three laser diodes emitting at different wavelengths (blue, green, red), enhancing measurement sensitivity through wavelength-dependent fringe order. As mathematically derived in Section 2.2, the fringe order is inversely proportional to the laser wavelength, meaning shorter wavelengths yield higher fringe orders, thereby improving sensitivity and resolution. Conversely, longer wavelengths result in lower fringe orders and reduced sensitivity to small deformations. While higher sensitivity achieved with shorter wavelengths improves resolution, it can also lead to challenges such as decorrelation effects when deformations exceed certain thresholds. This limitation can compromise measurement accuracy for large or complex displacements. The use of parallel measurements employing an additional wavelength provides a means to mitigate these issues by extending the measurable deformation range and reducing decorrelation risks. This configuration is particularly advantageous for capturing complex deformations involving multiple spatial components and facilitates targeted adjustment of measurement resolution and sensitivity according to the expected deformation characteristics of the test specimens. Additionally, the module supports full three-dimensional measurement by capturing out-of-plane deformation as well as in-plane displacements along both the x- and y-directions. The arm length has been reduced from 300 millimetres to 180 millimetres, which improves sensitivity in out-of-plane measurements and enhances precision. Moreover, the module's weight has been decreased to approximately 400 grams.

## 3.2 Specimen Characterization and Strategy

The test specimen consists of a cylindrical aluminium body with a central flat-bottom hole of 5 mm diameter and a residual wall thickness of 0.5 mm at the surface. The outer diameter of the cylinder was chosen to ensure sufficient structural stiffness, thereby minimizing deformations outside the weakened zone. This design allows for isolating the deformation in the vicinity of the flat-bottom hole without distortion of the measurement results by global deformation effects. Figure 3.2, right, shows a cross-section of the test specimen. During the measurements, the component was fixed below the MFK to ensure reproducibility of the results. The load was applied by rear-side, concentric pressurization directly into the hole at pressure levels of 1 bar and 3 bar.

In the following, two distinct investigations were conducted on the test specimen. The first study employed both the oop and the 3D module to evaluate the sensitivity of out-of-plane deformation measurements. To ensure consistency and comparability, the 3D module was temporarily equipped with the same expanded neodymium-doped yttrium aluminum garnet laser source (Nd:YAG laser) used in the oop module, thereby standardizing the laser wavelength and enabling an isolated assessment of the influence of the measurement geometry. For our measurements, we use the second harmonic of the Nd:YAG laser, corresponding to a wavelength of 532 nm. The second investigation focused on the impact of different laser wavelengths and sources on the measurement results. Here, the conventional laser diodes typically integrated in the 3D module were compared with the Nd:YAG laser under identical illumination conditions.
<figure><img src="image_4.png" /><figcaption>Figure 3.2. Left: Fixed test object, right: CAD representation of the test object in cross section through the introduced defect.</figcaption></figure>

# 4. Results and Discussion

## 4.1 Influence of Illumination Angle on Out-of-Plane Measurement Sensitivity

Quantitative deformation measurements were conducted using both the oop and the 3D modules, each equipped with an expanded Nd:YAG laser operating at its second harmonic ($\lambda$ = 532 nm). The comparison focused on the number of detected interference fringes and the overall quality of the fringe patterns, with a reference image captured in the unloaded state (0 bar). The results are summarized in Figure 4.1.

<figure><img src="image_5.png" /><figcaption>Figure 4.1. Comparison of the unfiltered measurement results of the test specimen under pressure loads of 1 bar and 3 bar respectively, using an illumination wavelength of 532 nm. Larger illumination angles reduce the number of phase fringes, thereby decreasing the measurement sensitivity.</figcaption></figure>

At an overpressure of 1 bar, both modules exhibited a single interference fringe. However, the fringe patterns recorded with the oop module demonstrated significantly higher clarity and reduced noise levels. To substantiate this observation, identical kernel filters were applied to both datasets, clearly revealing the elevated noise present in the 3D module measurements. This increased noise level can contribute to greater uncertainty during quantitative evaluation. At
3 bar, the specimen experienced more pronounced deformation, reflected by the presence of two interference fringes in both measurement systems.

The observed fringe patterns can be interpreted quantitatively based on the geometric illumination configuration. Under normal incidence ($\Theta_{xz} = 0^{\circ}$), the displacement field in the oop module is directly related to the fringe order. This relationship can be derived from Equation (2), which defines the phase shift as a function of the optical path difference, leading to the expression:

$$N = \frac{\lambda}{2D_w} \qquad (3)$$

While this expression allows a direct estimation of displacement from the number of observed fringes, it is important to emphasize that the primary objective of the measurement is not the precise quantification of absolute deformation. Rather, the method is intended to sensitively detect local anomalies in the deformation field. In this context, the exact displacement value at a given fringe is of secondary relevance, particularly since rigid-body motions or global structural shifts can contribute to the overall optical path difference and thus to additional phase shifts that are not linked to localized mechanical deformation.

In practical application, only the discrete fringe positions are evaluated, and the displacement is assumed to lie within the range defined by successive fringe orders. To support this estimation, numerical simulations using ANSYS Workbench 2024 R2 were conducted. For the selected specimen geometry and material configuration, the simulations predict an out-of-plane deformation of approximately 320 nm at 1 bar and 630 nm at 3 bar. Substituting these values into Equation (3) yields theoretical fringe orders of $N_{\text{oop, 1 bar}} \approx 1.2$ and $N_{\text{oop, 3 bar}} \approx 2.37$. These values correlate well with the experimentally observed fringe patterns, in which the oop module consistently displays between one and two fringes at 1 bar and between two and three fringes at 3 bar. This consistency reinforces the validity of the simulated deformation fields and illustrates the method's capability to capture small-scale deformation changes with high sensitivity. The strong correspondence between theoretical and measured fringe orders further underscores the utility of the oop module as a reliable tool for anomaly detection in precision metrology applications.

To enable a direct comparison with the oop module, the expected fringe orders for the 3D module were estimated using Equation (2), which accounts for the oblique illumination geometry. Assuming negligible in-plane displacement ($D_u \approx 0$), the fringe order simplifies to a function of out-of-plane deformation $D_w$ and the illumination angle $\Theta_{xz} = 45^{\circ}$. For a wavelength of 532 nm and the deformation amplitudes obtained from numerical simulation the resulting fringe orders are approximately $N_{3Dg, 1 bar} \approx 1.03$ and $N_{3Dg, 3 bar} \approx 2.02$. These values are consistent with the measured interference patterns, which show one to two fringes at 1 bar and two to three fringes at 3 bar. Despite the lower sensitivity due to the oblique illumination, the 3D module still captures the expected deformation-induced phase shifts, albeit with reduced fringe clarity compared to the oop module.

## 4.2 Influence of Laser Wavelength and Source on Measurement Accuracy

Figure 4.2 focuses specifically on the 3D module by comparing measurements obtained at different illumination wavelengths. For this purpose, three different laser sources were used: the green wavelength (532 nm), generated by the Nd:YAG laser source described above; and
red (657 nm) and blue (448 nm), both emitted from conventional laser diodes. At an overpressure of 1 bar, interference fringes could already be observed with blue and green laser light, whereas no discernible fringe formation occurred under red illumination. This observation correlates well with the calculated fringe orders based on the simulation-derived displacement of approximately 320 nm at this pressure level. For the blue diode ($\lambda$ = 448 nm), the corresponding fringe order amounts to $N_{3Db, 1bar} \approx 1.22$, and for the red diode ($\lambda$ = 657 nm), $N_{3Dr, 1bar} \approx 0.83$, explaining the visibility threshold reached only in the blue and green measurements.

When the pressure is increased to 3 bar, producing an estimated out-of-plane deformation of 630 nm, two interference fringes are detected for both green and blue illumination, while the red laser still produces only a single visible fringe. These results again align with the calculated fringe orders: for 448 nm, $N_{3Db, 3bar} \approx 2.4$, and for 657 nm, $N_{3Dr, 1bar} \approx 1.64$. The red wavelength thus remains below the threshold required for clear separation of a second fringe, consistent with the experimental image data.

<figure><img src="image_6.png" /><figcaption>Figure 4.2. Comparison of the unfiltered measurement results of the test specimen obtained with the 3D module using different illumination wavelengths under pressure loads of 1 bar and 3 bar respectively. Shorter wavelengths yield higher spatial resolution, and the purity of the laser source affects the resulting phase contrast.</figcaption></figure>

The measurement results also show that the phase contrast in the measurement with green laser light is significantly more pronounced than with the red and blue laser diodes. The interference fringes are more sharply defined and less affected by noise, enabling more accurate evaluation. This is likely due to the respective laser light sources used: while a Nd:YAG laser with high spectral purity was employed for the green wavelength, the red and blue wavelengths were generated using laser diodes. On the one hand, laser diodes exhibit a shorter coherence length, which influences the measurement result and is further reduced by imaging optics, glass elements, or lenses used for beam shaping ([7], see p. 14 ff). On the other hand, laser diodes have a broader emission spectrum due to their emission characteristics. This broader spectrum represents an additional source of error particularly in spatial phase shifting. This method relies on the controlled refraction of laser light at an aperture to generate a carrier frequency from which the phase information is extracted. Since the phase shift directly depends on the wavelength, a broad emission spectrum leads to superposition effects of different spectral components, which distort the interference pattern and reduce measurement accuracy. The
results thus illustrate that, in addition to the wavelength, the spectral purity and coherence of
the light source have a significant influence on the quality of phase measurement.

**4. Conclusions**

The conducted measurements demonstrate that the oop module, owing to its higher sensitivity under normal incidence and optimized measurement conditions, provides accurate phase images for deformation detection. In contrast, the 3D module exhibits a reduced sensitivity due to its oblique illumination angle of 45°, resulting in a lower fringe order for the same deformation magnitude. This reduced sensitivity can complicate the interpretation of phase data, as in-plane displacement components may also influence the measurements. Furthermore, the use of laser diodes with broader spectral bandwidth and shorter coherence length in the 3D module contributes to increased noise levels and less well-defined interference fringes, especially under red illumination. Conversely, the oop module benefits from the use of a spectrally pure laser source, enabling sharper fringe contrast and more reliable quantitative evaluation.

These findings highlight that each measurement module possesses specific advantages and limitations depending on the measurement geometry and laser source characteristics. Currently, a research initiative funded by the Central Innovation Programme for SMEs (ZIM) is focused on integrating the strengths of both modules into a unified system. The aim is to develop a more robust and flexible measurement setup that meets industrial requirements for accuracy and efficiency in serial inspection of miniature components. One promising direction involves exploring steeper illumination angles to enhance fringe contrast and phase image clarity. While the present study focused on out-of-plane deformation measurements, the long-term objective is to establish an illumination strategy that enables high-quality results for both out-of-plane and in-plane measurements. This integrative approach has the potential to improve system versatility and broaden its application scope.

**Acknowledgements**

The authors would like to thank the Ministry of Science and Health Rhineland-Palatinate and Trier University of Applied Sciences for supporting the publication of this research through the Young Researchers Fund. We also acknowledge the BMWK's ZIM program, which made the employment of researchers such as Ms. Plassmann possible. We also thank our colleagues for their valuable feedback and support during this research.

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## Appendix

J. Plassmann, C. Petry, A.-K. Bömkes and M. Schuth, „Anwendung der shearografischen Dehnungsmessung mit transienter Wärmeanregung zur zerstörungsfreien Prüfung für die Detektion von Rissen in faserverstärktem Kunststoff im Vergleich zur Thermografie“, DGZfP Jahrestagung, June 2021.

A. Simeth, J. Plassmann and P. Plapper, “Detection of Fluid Level in Bores for Batch Size One Assembly Automation Using Convolutional Neural Network”, IFIP Advances in Information and Communication Technology, 10.1007/978-3-030-85906-0_10, August 2021.

J. Plassmann, A.-K. Bömkes, C. Petry and M. Schuth, „Anwendung der Shearografie für die schnelle Prüfung von Bauteilen aus der Massenproduktion“, DGZfP Jahrestagung, Kassel, May 2022.

J. Plassmann, N. Schuler, M. Schuth and G. v. Freymann, „Entwicklung eines ganzheitlichen Automationsansatzes für die Speckle-interferometrischen Verfahren der Digital-Holografie und -Shearografie“, HAW-Forschungstag Rheinland-Pfalz 2024, Zweibrücken, June 2024

J. Plassmann, T. Eis, A.-K. Molitor, V. Bastgen and M. Schuth, „Mikroferoskop im Fokus: Anwendungspotenziale der digitalen Speckle-Holografie in der Serienprüfung von Kleinstbauteilen“, DGZfP Jahrestagung, Berlin, May 2025