Fraunhofer-Institut für Optronik, Systemtechnik und Bildauswertung IOSB

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Sensitivity enhanced roll-angle sensor by means of a quarter-waveplate

2021 , Chen, Chia-Wei , Hartrumpf, Matthias , Längle, Thomas , Beyerer, Jürgen

Attitude metrology (roll, pitch, and yaw) playsan important role in many different fields. Roll angle is con-sidered the most difficult measurement quantity in angulardisplacements compared to pitch and yaw angles becausethe rotation axis of the roll angle is parallel to the probebeam. In this work, a sensitivity enhanced roll-angle sensor is presented. The principle is based on the polarizationchange of a sensing unit (quarter-waveplate). The polarization model is analyzed by Mueller matrix formalism. TheStokes parameters are detected by a Stokes polarimeter.The novel coaxial design improves the sensitivity and reduce the complexity of optical system alignment by meansof a fixed quarter-waveplate. The proposed sensor providesa simple setup to measure roll angles with a high sensitivity of 0.006∘ and a long unambiguous measurement range of 180∘.

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Ellipsometric inline inspection of dielectric substrates with nonplanar surfaces

2020 , Hartrumpf, Matthias , Chen, Chia-Wei , Längle, Thomas , Beyerer, Jürgen

An analytical solution for the determination of either angle of incidence (AOI) and the refractive index from combined ellipsometric and reflectometric measurements at dielectric substrates is presented. The solution is of special importance for retroreflex ellipsometry (but not limited to this application). Overcoming the geometric restrictions of conventional ellipsometers, the patented retroreflex ellipsometry can detect changes of intensity and the state of polarization in or at test objects even with curved surfaces. In contrast to conventional ellipsometers where the AOI is set by the adjustment procedure, the AOI is usually unknown in retroreflex ellipsometry. For quantitative analysis, the knowledge of the AOI is nevertheless essential. The proposed combination of retroreflex-reflectometry and retroreflex-ellipsometry opens the path to precise measurements of either surface geometry and index of refraction of nonplanar dielectric substrates (e. g. surfaces of freeform optics).

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Direct-imaging DOEs for high-NA multi-spot confocal microscopy

2020 , Li, Zheng , Taphanel, Miro , Längle, Thomas , Beyerer, Jürgen

Diffractive lens arrays with overlapping apertures can produce spots with high numerical apertures (NAs). Such diffractive optical elements (DOEs) can replace high-NA objectives and measure a large area with high resolution in transmission microscopes. However, in reflection microscopes for surface measurements, the axial resolution is still limited by the objectives. Direct-imaging DOEs are proposed to solve the problem. They can perform high-NA multi-spot measurement in reflection configurations in both lateral and axial directions. Experiments demonstrate a lateral non-vanishing contrast up to 1448 lp/mm and an axial response on a plane mirror with a full width at half maximum (FWHM) of 2.24 mm.

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Sensor-realistic simulation of images in diffraction-limited imaging systems

2018 , Mohammadikaji, Mahsa , Bergmann, S. , Burke, Jan , Beyerer, Jürgen , Dachsbacher, C.

Die optische Abbildung ist der Baustein verschiedener Messsysteme, deren Abbildungsqualität unter anderem durch die Wahl der Abbildungsparameter, der Optik und des Bildsensors beeinflusst wird. Realistische Bildsimulationen können daher für die automatische Optimierung solcher Freiheitsgrade sehr vorteilhaft sein. In diesem Beitrag stellen wir ein physikalisch basiertes Modell für die Bilderzeugung vor, welches aus drei Teilen besteht: Geometrische Lichttransportsimulation mit verfügbaren Computergrafikverfahren, Einführung von Linsenabbildungseffekten mit Fourier-Optik und Integration der EMVA 1288 Sensorspezifikationen. Das vorgeschlagene Modell wurde für die Simulation der Bilder mit Speckle-Interferenz-Muster verwendet und verifiziert, indem eine gemessene Metalloberfläche von einer Laserlinie beleuchtet wird. Die Übereinstimmung der Simulationen mit realen Bildern zeigt, dass das Modell insbesondere bei beugungsbegrenzten Systemen genaue Annäherungen an die realen Bilder erzeugen kann. Zudem wird in diesem Paper der Simulationsprozess für kohärente und inkohärente Abbildungsfälle beschrieben, um Abbildungssysteme mit einer oder beiden Arten von Beleuchtungen simulieren zu können.

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Light field illumination. Problem-specific lighting adjustment

2021 , Kludt, Christian , 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, transparent objects, which change the directional 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. Thereby, 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 examples of customized illuminations.

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Sensor-Realistic Simulations for Evaluation and Planning of Optical Measurement Systems with an Application to Laser Triangulation

2020 , Mohammadikaji, Mahsa , Bergmann, Stephan , Beyerer, Jürgen , Burke, Jan , Dachsbacher, Carsten , Prautzsch, Hartmut

Image acquisition plays a central role in optical measurement systems. To configure machine vision setups, engineers often go through some empirical best practices to evaluate the measurement in different geometrical and optical configurations. This approach turns out to be expensive, tedious, and sometimes even unsuitable for nontrivial tasks. To automate this process, realistic synthetic images allow us to evaluate and optimize imaging setups without employing physical parts and sensors. Available computer graphics rendering techniques can be very beneficial for realistic simulation of images in machine vision systems. Simulating optical measurement systems, however, requires taking extra simulation components into account, including the wave optics effects and realistic spectral response and stochastic noise of the digital sensor. In this article, we focus on physical simulation of optical measurement systems and further propose and verify a sensor-realistic simulation framework for this purpose. We demonstrate the results in different steps for a laser triangulation measurement system, and verify them against experimental images, both qualitatively and quantitatively. The measurement with a laser light source results in wave optics effects which cannot be modeled by conventional ray-tracing methods. The concepts of this article are, however, general to many optical systems with both coherent and/or incoherent light sources. The agreement of simulations with reality is a promising proof of concept for machine vision experts, to move towards automating the setup planning process by means of sensor-realistic simulations.

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Kalibrierfreie 3D-Rekonstruktion von gekrümmten Oberflächen mittels orthographischer Projektion unter Verwendung von Reflexionsmarkern

2019 , Negara, Christian , Längle, Thomas , Beyerer, Jürgen

Die optische Vermessung gekrümmter Oberflächen erfolgt meist über problemspezifische Messverfahren und Algorithmen zur 3D-Rekonstruktion, die an die optischen Eigenschaften der zu untersuchenden Oberfläche angepasst sind. In diesem Artikel wird ein Verfahren zur 3D-Rekonstruktion von Freiformflächen vorgestellt, das das Oberflächenrelief in einem telezentrischen Strahlengang mit einer telezentrischen Abbildungsoptik und einer koaxialen telezentrischen Beleuchtung erfasst. Die untersuchte Oberfläche des Prüflings kann dabei texturiert oder untexturiert und transparent, spiegelnd oder diffus sein. Die Oberfläche wird dabei nicht direkt vermessen, sondern mittels darauf aufgebrachter retroreflektierender Marker rekonstruiert. Da zwischen den Markern keine Messwerte vorliegen, können nur Oberflächen rekonstruiert werden, die eine örtliche Glattheit aufweisen. Mit einem Verfahren für monokulares Stereo werden aus einer Bildserie simultan sowohl die Kameraposen als auch die 3D-Koordinaten der Reflexionsmarker berechnet. Die sonst übliche Kalibrierung der extrinsischen Kameraparameter entfällt. Anschließend wird eine parametrische B-Spline-Oberfläche gefittet, aus der sowohl das Oberflächenrelief als auch die Oberflächennormalen berechnet werden können.

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Direct-imaging DOEs for high-NA multi-spot confocal surface measurement

2021 , Li, Zheng , Taphanel, Miro , Längle, Thomas , Beyerer, Jürgen

Diffractive lens arrays with overlapping apertures can produce spot arrays with high numerical apertures (NAs). Combined with low-NA objectives, they can measure a large area with high lateral resolution. However, for surface measurements, the axial resolution of such setups is still fundamentally limited by the objectives. In this work, we propose a new design of diffractive optical elements (DOEs) to overcome this problem. The proposed Direct-imaging DOEs can perform 3D high-NA multi-spot surface measurements. Laterally, a non-vanishing contrast up to 1448 lp/mm is measured with a USAF resolution target. Axially, an average height of 917.5 nm with a standard deviation of 49.9 nm is measured with a calibrated step height target of 925.5 nm.

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Modern Non-Destructive Testing

2020 , Beyerer, Jürgen , Hanke, Randolf

Modern non-destructive testing (NDT) philosophy's focus is on acquiring information and linking further with process knowledge for all processes in the entire material and product life cycle. This information about materials and products plays an increasingly central role in the value-added chain of life cycle processes, from their creation (new materials, production) and use in products (maintenance, wear) to their disposal (recycling, re-use, up-cycling). Each phase in which the matter, the material or the resulting product is considered provides different insights and information that are highly relevant for its continuation in the value creation chain.

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Measurement of ellipsometric data and surface orientations by modulated circular polarized light

2019 , Chen, Chia-Wei , Hartrumpf, Matthias , Längle, Thomas , Beyerer, Jürgen

Ellipsometry is a widely used optical method for the characterization of materials and thin films. However, only flat or nearly flat surfaces can be measured since small deviations of the angle of incidence might lead to significant experimental errors. In order to overcome the geometrical limitations of ellipsometry, the retroreflex ellipsometry developed at Fraunhofer IOSB is used. Based on this configuration, a new illumination method is proposed. When the sample is illuminated by modulated circular polarized light (LCP and RCP), the ellipsometric parameters (PS, D) and the tilted angle (F) can be determined directly. Combined with reflectance measurement or prior knowledge of optical properties of samples, the surface orientation can be obtained.

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