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Item-typ:Veröffentlichung, Fast form measurements using a digital micro-mirror device in imaging with partially coherent illuminationWe present a new technique for fast form measurement based on imaging with partially coherent illumination. It consists of a 4 f-imaging system with a digital micro-mirror device (DMD) located in the Fourier plane of its two lenses. The setup benefits from spatially partially coherent illumination that allows for depth discrimination and a DMD that enables a fast depth scan. Evaluating the intensity contrast, the 3D form of an object is reconstructed. We show that the technique additionally offers extended depth of focus imaging in microscopy and short measurement times of less than a second.Wissenschaftlicher ArtikelBand:45Heft:2243 30 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Improved 3D form profiler based on extending illumination aperture(Society of Photo‑Optical Instrumentation Engineers (SPIE), 2023-08-15); ; We recently demonstrated that the 3D shape of micro-parts can be measured using LED illumination based on contrast evaluation. The technique is based on imaging the object under test using partially coherent illumination. The limited spatial coherence of LED illumination was utilized to discriminate depth. For a fast depth scan without mechanically moving parts, an electrically tunable lens (ETL) in a 4f optical configuration is used. This approach is efficient, takes less than a second to capture all required images, is eye-safe, and offers a depth of focus of a few millimeters. However, the main limitation of the proposed approach arises from the underlying assumption that a small angle of variation of illumination is required. Such a small illumination aperture affects the axial scan resolution that dominates the measurement uncertainty. In the present paper we propose a method to overcome the main limitation of the above-mentioned profiling technique. The approach combines measurements with multiple illumination directions achieved by illuminating the object simultaneously with several independent light sources. In this way, the full-width half maximum of the contrast envelope is reduced. A tilted metallic plate is used for proof of concept.Konferenzbeitrag32 78 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, The coherence function and its information content for optical metrology(De Gruyter Oldenbourg, 2022-05-24); ; ; ; The coherence function offers new possibilities for optical metrology that are not available with conventional wave field sensing. Its measurement involves a spatio-temporal sampling of the wave fields modulated by the object under investigation. Temporal sampling is well known e. g. by means of White Light Interferometry (WLI) and spatial sampling can e. g. performed by Computational Shear Interferometry (CoSI). The present paper describes an approach that combines both temporal and spatial sampling using a robust common-path setup. While the evaluation of the coherence function is more elaborate than approaches that either sample the temporal or the spatial domain, an information theoretical treatment shows that it also delivers more information about the object under investigation. Our approach is based on the mutual information that represents the reduction of uncertainty about the object as a consequence of the measurements performed. Using a simplified measurement case, we calculate the mutual information for different measurement situations and demonstrate that spatio-temporal sampling of the coherence function results in a higher mutual information as compared to classical approaches. Based on the proposed approach, we identify further open research tasks for an efficient information extraction from the coherence function to surpass current limitations of optical metrology.Wissenschaftlicher ArtikelBand:89Heft:6101 96 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Approaching optical metrology with multiple light sources and compressive sensing(Society of Photo‑Optical Instrumentation Engineers (SPIE), 2022-05-20); ; Complex optical surfaces such as aspheres and freeforms are used in optical systems to reduce aberrations or to achieve high optical performance with a compact design and less optical surfaces. Due to limited acceptance angles of conventional interferometric techniques, there is still no satisfactory solution for their form measurement that is at the same time precise, flexible, and fast. Often these surfaces are surveyed by pointwise measurement, or the aperture problem is overcome by elaborately compensating wave front deviations either through compensator lenses or computer-generated holograms. Alternatively, several subapertures are used to capture the whole surface. These, however, have to be recorded in sequence since the superposition of multiple independent wave fields cannot be assigned a time-independent wave front. Instead, we present a compressive sensing approach for Multiple Aperture Shear-Interferometry (MArS) which captures multiple overlapping subapertures simultaneously and allows a flexible measurement of aspheres with multi-spot illumination. MArS uses the mutual intensity as the primary measurand which is still well defined for superposed mutually incoherent wave fields. The mutual intensity is sparse in phase space for there are only a finite number of distinct wave fields at every surface point. Utilizing this sparsity, the presented compressive sensing approach avoids superflously large space-bandwidth products and significantly reduces the number of necessary measurements.Konferenzbeitrag32 26 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, The coherence function for optical metrology: a new paradigm and the role of information theory and compressed sensing(Society of Photo‑Optical Instrumentation Engineers (SPIE), 2023-08-10); ; ; ; The coherence function offers new possibilities for optical metrology not available with conventional wave field sensing. Its measurement involves spatio-temporal sampling of wave fields modulated by the object under investigation. While the evaluation of the coherence function is more elaborate than conventional approaches, an information theoretical treatment shows that it also delivers more information about the object under investigation. In order to achieve efficient information extraction from the coherence function, advanced approaches involving compressed sensing are required to obtain optical metrology techniques that are simultaneously precise, robust and fast as well as suited for complex measurement situations.Konferenzbeitrag35 36 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, High resolution lensless microscopy based on Fresnel propagation(Society of Photo‑Optical Instrumentation Engineers (SPIE), 2024-07-12); ; Digital holography allows for the recording and reconstruction of three-dimensional images using interference and diffraction principles. The propagation of light from the hologram plane to the reconstruction plane is a crucial step, often achieved through Fresnel propagation, a method that inherently transforms the reconstructed pixel pitch to provide diffraction-limited imaging. However, the accuracy of this method is limited by the Fresnel approximation, especially in applications such as digital holographic microscopy. We present a simple method that significantly improves the accuracy of the Fresnel approximation by incorporating higher orders of the binomial approximation. We validate the effectiveness of our approach through high numerical aperture simulations and experimental results, demonstrating superior sub-micron resolution and reduced distortions compared with standard Fresnel propagation.Wissenschaftlicher ArtikelBand:63Heft:11201 160 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Exploring the coherence function for optical metrology and beyondI will report on several strategies to exploit the structure of the coherence function for optical metrology and wave field analysis in general. For the case of optical metrology, I will show that properties like the surface topology or refractive index distributions can be determined by solely investigating the coherence function of light that has previously interacted with the specimen under test. Since the coherence function can be measured by a simple shear interferometer, this suggests that a large variety of metrology tasks can be accomplished by a common-path architecture. A prime advantage of this approach is the combination of interferometric precision with inherent mechanical robustness. Furthermore, because a shear interferometer is essentially an imaging system that provides a twin-image, methods based on sensing the coherence function are very flexible and largely independent from the distance between sensor and object.Konferenzbeitrag40 21 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Characterization of terahertz wavefront aberrations using computational shear-interferometry(Society of Photo‑Optical Instrumentation Engineers (SPIE), 2022-11-10); ; ; ; We propose a new solution for sensing a terahertz (THz) wavefront based on a THz reference-less shear interferometer. The key component of the experimental configuration of the proposed interferometer is a THz Ronchi phase grating (RPG). The RPG is custom designed and fabricated for a 0.28 THz source using mechanical milling on a block of high-density polyethylene with a computer numerical control machine. It acts as a shearing element that generates two diffraction orders, thereby, creating two laterally shifted copies of the investigated wavefront in the sensor plane where a THz camera is placed. The direction of the shear can be varied by rotating the grating. Since the grating is a phase grating, the diffraction efficiency is very high. The approach is verified experimentally by demonstrating interferograms of a spherical wave and wavefront reconstruction from five different shears using a gradient-based iterative process.Wissenschaftlicher ArtikelBand:61Heft:1137 59 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Lens-based phase retrieval under spatially partially coherent illumination - Part I: TheoryWe present a novel approach of phase retrieval using a 4f-imaging system, for the specific case of spatially partially coherent illumination. The theoretical investigation presented in Part I is based on scalar diffraction theory in combination with a heuristic approach. We find that the intensity of the de-focused speckle fields across the image plane shows sufficient contrast for phase retrieval, as long as the radius of the point spread function of the imaging system is well below the radius of the correlation area (spatial coherence) in the object plane. We also derive a useful relation, which allows to determine the requirements towards the essential parameters of an experimental setup, such as maximum propagation distance, diameter of the source, illumination distance and the numerical aperture of the imaging system. The consecutive paper, Part II, describes shape measurement based on the present treatise.Wissenschaftlicher ArtikelBand:13925 62 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Lens-based phase retrieval under spatially partially coherent illumination - Part II: Shape measurementsWe experimentally demonstrate the shape measurement of micro objects by means of phase retrieval under spatially partially coherent illumination. This approach offers an extended depth of focus, vibration insensitivity, short measurement time and precise shape measurements, enables the use of spatially extended light sources such as LEDs and thus significantly improves the eye safety of the method. The employed setup is based on a telecentric -imaging system with a tunable lens across the common Fourier plane. Based on the theoretical investigation presented in Part I, we verify that -based phase retrieval is feasible if the radius of the point spread function of the imaging system is smaller than that of the correlation (spatial coherence) area across the object plane. In this case, a light field with a defined complex amplitude can be assigned to the measured intensity distributions. As a proof of principle towards industrial applications, the 3D shape of a cold-formed micro-cup is reconstructed by combining phase retrieval with two-wavelength contouring. A depth of focus in the range of few millimeters and the measurement time and uncertainty of 0.4 s, and m, respectively, can be achieved.Wissenschaftlicher ArtikelBand:13934 29