Fraunhofer-Institut für Optronik, Systemtechnik und Bildauswertung IOSB

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  • Publication
    Robustness estimation of simple lens systems by machine learning
    ( 2021)
    Chen, Chia-Wei
    ;
    Zhou, Bowen
    ;
    Längle, Thomas
    ;
    Beyerer, Jürgen
    Tolerance analysis and tolerance sensitivity optimization (desensitization) are important and necessary for manufacturability. However, compared to the optimization of optical performance, tolerance analysis is still time-consuming. A machine learning approach for the fast robustness estimation of lens systems is proposed. The results of the machine learning estimation and the other four different methods are compared with the results of the Monte Carlo analysis. The proposed model is added to the merit function in commercial software for optimization to reduce the sensitivity.
  • Publication
    Resolution enhancement of low-NA objectives in confocal fluorescence microscopy by diffractive lens arrays
    ( 2021)
    Li, Zheng
    ;
    Taphanel, Miro
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    Längle, Thomas
    ;
    Beyerer, Jürgen
    In traditional confocal microscopes, high resolution and large fields of view are difficult to achieve simultaneously by microscope objectives, which limit the space-bandwidth products of the systems. With the proposed diffractive lens arrays, an objective with a low numerical aperture can have much higher resolution while maintaining its large field of view. Application of the diffractive lens arrays in fluorescence microscopy is presented in this work. The resolution enhancement is demonstrated by experiments with the fluorescent beads.
  • Publication
    Light Field Illumination. A Universal Lighting Approach for Visual Inspection
    ( 2020)
    Kludt, Christian
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    Dippon, Lukas
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    Längle, Thomas
    ;
    Beyerer, Jürgen
    Choosing a proper lighting approach is a crucial task in designing visual inspection systems. It becomes especially challenging for complex-shaped objects, which change the direction and distribution of incoming light in various ways. We overcome this challenge by constructing a light field display and deploy it as a highly tunable lighting device. By programmatically controlling the spatial position of the individual light sources while simultaneously controlling their angular direction of emission, an object-specific light field can be generated, which highlights the features of the object under test with maximum contrast. We explain the calibration procedure, the rendering pipeline and present first results of the device's performance.
  • Publication
    Automatic visual inspection based on trajectory data
    ( 2019)
    Maier, Georg
    ;
    Mürdter, N.
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    Gruna, Robin
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    Längle, Thomas
    ;
    Beyerer, Jürgen
    Automatic inspection tasks have successfully been implemented in several industrial fields and are of growing importance. Visual inspection using optical sensors is wide spread due to the vast variety of different sensors, observable features and comparatively low prices. It seems obvious that corresponding systems are blind towards mechanical features and inspection of those typically requires highly specialized, inflexible and costly systems. Recently, we have shown in the context of sensor-based sorting that tracking objects over a time period allows deriving motion-based features which potentially enable discrimination of optically identical objects, although an optical sensor is used. In this paper, we take one step back from the specific application and study the classification of test objects based on their trajectories. The objects are observed while receiving a certain impulse. We further refrain from manually designing features but use raw coordinates as extracted from a series of images. The success of the method is demonstrated by discriminating spheres made of similar plastic types while bouncing off a plane.
  • Publication
    Application of DOE in confocal microscopy for surface measurement
    ( 2019)
    Li, Zheng
    ;
    Taphanel, Miro
    ;
    Längle, Thomas
    ;
    Beyerer, Jürgen
    In traditional confocal microscopy, there is a trade-off between spatial resolution and field of view due to the limitations of objectives. To solve this problem, diffractive optical elements (DOEs) with overlapping apertures are used to generate high-NA illumination spots in a large area. However, currently such DOEs can only be used as illuminators which are not suitable for 3D surface measurements. In this work, the idea of superposition is utilized to expand the scope of application of the DOEs. These DOEs are designed by simulation and tested in the experiments. The results show that the proposed DOEs can be used in 3D surface measurements and have the potential to solve the problem of high-NA objectives.
  • Publication
    Simplified Stokes polarimeter based on division-of-amplitude
    ( 2019)
    Negara, Christian
    ;
    Li, Zheng
    ;
    Längle, Thomas
    ;
    Beyerer, Jürgen
    A polarization state detector (PSD) measures the state of polarization of the detected light. The state of polarization is fully described by the Stokes vector containing four Stokes parameters. A division-of-amplitude photopolarimeter (DOAP) measures the four Stokes parameters by simultaneously acquiring four intensities using photodetectors. A key component of the DOAP is the first beam splitter, which splits up the incoming beam into two beams. The effect of the beam splitter on the state of polarization of the reflected (r) and transmitted (t) beam is determined by six parameters: R, T, PSr, PSt, Dr, and Dt. R and T are the reflectance and transmittance, and (PSr, Dr) and (PSt, Dt) are the ellipsometric parameters of the beam splitter in reflection and transmission, respectively. To measure the Stokes vector with high accuracy, the six optical parameters must be chosen appropriately. In previous work, the optimal parameters of the beam splitter have been determined as R = T = 1/2, cos2 2PSr = 1/3, PSt = p/2 - PSr, and Dr-Dt modulo p=p/2 by calculating the maximum of the absolute value of determinant of the instrument matrix. Using additional quarter-wave plates eliminates the constraint on the retardance and hence simplifies the manufacturing process of the beam splitter, especially when broadband application is intended. To compensate a suboptimal value of Dr-Dt, the azimuthal angles of the principal axes of the retarders must be adjusted, for which we provide analytic formulas. Hence, a DOAP with retarders is also optimal in the sense that the same values for the determinant and condition number of the instrument matrix are obtained. When using two additional retarders, it is necessary to install both on the same light path in order to obtain an optimal DOAP. We will show that is also possible to get an optimal DOAP with only one additional quarter-wave plate instead of two, if one of the Wollaston prisms is rotated.
  • Publication
    SNR-optimized image fusion for transparent object inspection
    ( 2018)
    Meyer, J.
    ;
    Melchert, Wolfgang
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    Hartrumpf, Matthias
    ;
    Längle, Thomas
    ;
    Beyerer, Jürgen
    Automated visual inspection of transparent objects is important for many industrial fields. Especially the detection of scattering impurities inside complexly shaped transparent objects is a demanding task. Usually, so-called dark field approaches are employed in this case. However, these methods often fail due to direct reflections of the light sources, e.g., at the test object's surface which cannot be distinguished from signals of real material defects. This paper introduces an inspection approach which captures images at different illumination modalities and fuses them while optimizing the signal-to-noise ratio. Two fusion strategies are presented, which employ prior knowledge in order to obtain optimized inspection images. The signal component of the observed images is defined as the signal corresponding to visualized defects. Conversely, all light reaching the sensor due to scattering or reflections caused by the test object's geometry is regarded as noise. The signal values and noise values depend on both the pixel position and the respective illumination source. Prior knowledge about the signal and noise components allows to estimate the spatially resolved SNR for every illumination channel. The images resulting from the fusion step show scattering material defects with high contrast whereas surface reflections are nearly completely mitigated by the SNR-optimized fusion strategies. Several experiments state the performance of the presented approaches.
  • Publication
    Kalibrierfreie 3D-Rekonstruktion von spiegelnden Oberflächen mittels Orthogonalprojektionen unter Verwendung von Reflexionsmarkern
    ( 2018)
    Negara, Christian
    ;
    Längle, Thomas
    In diesem Artikel wird ein Verfahren zur 3D-Rekonstruktion von Freiformflachen vorgestellt, das sich besonders in Laborumgebungen zur einfachen Erfassung der 3DForm mit einer telezentrischen Optik und telezentrischem, koaxialen Beleuchtung eignet. Die untersuchte Oberfläche des Prüflings kann dabei eben oder gekrümmt und transparent oder spiegelnd sein und wird durch auf der Oberfläche aufgebrachten retroreflektierenden Markern erfasst.
  • Publication
    Application of area-scan sensors in sensor-based sorting
    ( 2018)
    Maier, Georg
    ;
    Pfaff, F.
    ;
    Pieper, C.
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    Gruna, Robin
    ;
    Noack, B.
    ;
    Kruggel-Emden, H.
    ;
    Längle, Thomas
    ;
    Hanebeck, U.D.
    ;
    Wirtz, S.
    ;
    Scherer, V.
    ;
    Beyerer, Jürgen
    In the field of machine vision, sensor-based sorting is an important real-time application that enables the separation of a material feed into different classes. While state-of-the-art systems utilize scanning sensors such as line-scan cameras, advances in sensor technology have made application of area scanning sensors feasible. Provided a sufficiently high frame rate, objects can be observed at multiple points in time. By applying multiobject tracking, information about the objects contained in the material stream can be fused over time. Based on this information, our approach further allows predicting the position of each object for future points in time. While conventional systems typically apply a global, rather simple motion model, our approach includes an individual motion model for each object, which in turn allows estimating the point in time as well as the position when reaching the separation stage. In this contribution, we present results from our collaborative research project and summarize the present advances by discussing the potential of the application of area-scan sensors for sensor-based sorting. Among others, we introduce our simulation-driven approach and present results for physical separation efficiency for simulation-generated data, demonstrate the potential of using motion-based features for material classification and discuss real-time related challenges.
  • Publication
    Optical realization and calibration of a light field generator
    ( 2018)
    Meyer, J.
    ;
    Längle, Thomas
    ;
    Beyerer, Jürgen
    In this contribution, we introduce a novel light source allowing to generate multiple collimated light beams of which both the spatial location of the origin and the direction can be controlled. The realization of such a light field generator is based on a high-resolution display on top of which a micro lens array is mounted in the distance of its focal length. Consequently, the micro lenses emit collimated light beams if one of the underlying pixels is turned on. In order to be able to turn on a light ray originating from a certain macro pixel with a certain direction, a mapping from a two-dimensional spatial macro pixel coordinate and a two-dimensional angular coordinate to the two-dimensional coordinate of the respective pixel of the underlying display is needed. In our paper, we solve this calibration task with a suitable optical setup and appropriate processing methods. As an example application, we demonstrate how our prototype can successfully be used for the visual inspection of transparent objects by means of the inverse light field illumination method.