Fraunhofer-Institut für Optronik, Systemtechnik und Bildauswertung IOSB

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  • Publication
    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∘.
  • Publication
    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.
  • Publication
    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.
  • Publication
    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).
  • Publication
    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.
  • Publication
    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.
  • Publication
    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.