198 research outputs found
Coherence-resolution relationship in holographic and coherent diffractive imaging
No full textWe study by numerical simulation how spatial coherence affects the reconstruction quality of images in coherent diffractive x-ray imaging. Using a conceptually simple, but computationally demanding approach, we have simulated diffraction data recorded under partial coherence, and then use the data for iterative reconstruction algorithms using a support constraint. By comparison of experimental regimes and parameters, we observe a significantly higher robustness against partially coherent illumination in the near-field compared to the far-field setting.Bundesministerium für Bildung und Forschung (BMBF) http://dx.doi.org/10.13039/501100002347Deutsche Forschungsgemeinschaft (DFG) http://dx.doi.org/10.13039/50110000165
The fluence–resolution relationship in holographic and coherent diffractive imaging
No full textThis work presents a numerical study of the fluence–resolution behaviour for two coherent lensless X-ray imaging techniques. To this end the fluence–resolution relationship of inline near-field holography and far-field coherent diffractive imaging are compared in numerical experiments. To achieve this, the phase reconstruction is carried out using iterative phase-retrieval algorithms on simulated noisy data. Using the incident photon fluence on the specimen as the control parameter, the achievable resolution for two example phantoms (cell and bitmap) is studied. The results indicate the superior performance of holography compared with coherent diffractive imaging, for the same fluence and phase-reconstruction procedure.</jats:p
Divide and update: towards single-shot object and probe retrieval for near-field holography
No full textWe present a phase reconstruction scheme for X-ray near-field holographic imaging based on a separability constraint for probe and object. In order to achieve this, we have devised an algorithm which requires only two measurements - with and without an object in the beam. This scheme is advantageous if the standard flat-field correction fails and a full ptychographic dataset can not be acquired, since either object or probe are dynamic. The scheme is validated by numerical simulations and by a proof-of-concept experiment using highly focused undulator radiation of the beamline ID16a of the European Synchrotron Radiation Facility (ESRF).Open-Access-Publikationsfonds 201
Reconstructing mode mixtures in the optical near-field
No full textWe propose a reconstruction scheme for hard x-ray inline holography, a variant of propagation imaging, which is compatible with imaging conditions of partial (spatial) coherence. This is a relevant extension of current full-field phase contrast imaging, which requires full coherence. By the ability to reconstruct the coherent modes of the illumination (probe), as demonstrated here, the requirements of coherence filtering could be relaxed in many experimentally relevant settings. The proposed scheme is built on the mixed-state approach introduced in [Nature494, 68 (2013)], combined with multi-plane detection of extended wavefields [Opt. Commun.199, 65 (2001), Opt. Express22, 16571 (2014)]. Notably, the diversity necessary for the reconstruction is generated by acquiring measurements at different defocus positions of the detector. We show that we can recover the coherent mode structure and occupancy numbers of the partial coherent probe. Practically relevant quantities as the transversal coherence length can be computed from the reconstruction in a straightforward way.Open-Access-Publikationsfonds 201
Approximate Hagemann-Mitschke co-operations
No full textWe show that varietal techniques based on the existence of operations of a certain arity can be extended to n-permutable categories with binary coproducts. This is achieved via what we call approximate Hagemann-Mitschke co-operations, a generalisation of the notion of approximate Mal'tsev co-operation. In particular, we extend characterisation theorems for n-permutable varieties due to J. Hagemann and A. Mitschke to regular categories with binary coproducts
Solving the Phase Problem in X-Ray Near-Field Holography Beyond the Assumption of Weak Objects
No full textPhase retrieval for near-field X-ray imaging beyond linearisation or compact support
No full textX-ray phase contrast imaging based on free space propagation relies on phase retrieval to obtain sharp images of micro- and nanoscale objects, with widespread applications in material science and biomedical research. For high resolution synchrotron experiments, phase retrieval is largely based on the single step reconstruction using the contrast transfer function approach (CTF), as introduced almost twenty years ago [Cloetens et al., Appl. Phys. Lett. 75, 2912 (1999)]. Notwithstanding its tremendous merits, this scheme makes stringent assumptions on the optical properties of the object, requiring, in particular, a weakly varying phase. In this work, we show how significant the loss in image quality becomes if these assumption are violated, and how phase retrieval can be easily improved by a simple scheme of alternating projections. Importantly, the approach demonstrated here uses the same input data and constraint sets as the conventional CTF-based phase retrieval, and is particularly well suited for the holographic regime
Entwicklung neuer Studiengänge - Curricula kooperativ und kompetenzorientiert gestalten
No full textMit einer "kooperativen Curriculumsentwicklung" soll ein Beitrag zur Öffnung von Hochschulen geleistet werden: Neue Studiengänge werden nicht mehr nur von der Hochschule, sondern gemeinsam mit Unternehmen und Einrichtungen der beruflichen Bildung entwickelt. Die Fachhochschule der Diakonie in Bielefeld hat dazu ein Modell entwickelt und setzt dieses in einem Forschungsprojekt im Rahmen der BMBF-Initiative "Aufstieg durch Bildung – Offene Hochschulen" um. Der Werkstattbericht gibt einen Einblick in die konkreten Entwicklungsschritte und diskutiert Chancen und Hürden des Modells.
21.03.2014 | Miriam Schäfer, Michael Kriegel & Tim Hagemann (Bielefeld
Probe reconstruction for holographic X-ray imaging
No full textIn X-ray holographic near-field imaging the resolution and image quality depend sensitively on the beam. Artifacts are often encountered due to the strong focusing required to reach high resolution. Here, two schemes for reconstructing the complex-valued and extended wavefront of X-ray nano-probes, primarily in the planes relevant for imaging (i.e. focus, sample and detection plane), are presented and compared. Firstly, near-field ptychography is used, based on scanning a test pattern laterally as well as longitudinally along the optical axis. Secondly, any test pattern is dispensed of and the wavefront reconstructed only from data recorded for different longitudinal translations of the detector. For this purpose, an optimized multi-plane projection algorithm is presented, which can cope with the numerically very challenging setting of a divergent wavefront emanating from a hard X-ray nanoprobe. The results of both schemes are in very good agreement. The probe retrieval can be used as a tool for optics alignment, in particular at X-ray nanoprobe beamlines. Combining probe retrieval and object reconstruction is also shown to improve the image quality of holographic near-field imaging
The optical stretcher as a tool for single-particle X-ray imaging and diffraction
No full textFor almost half a century, optical tweezers have successfully been used to micromanipulate micrometre and sub-micrometre-sized particles. However, in recent years it has been shown experimentally that, compared with single-beam traps, the use of two opposing and divergent laser beams can be more suitable in studying the elastic properties of biological cells and vesicles. Such a configuration is termed an optical stretcher due to its capability of applying high deforming forces on biological objects such as cells. In this article the experimental capabilities of an optical stretcher as a potential sample delivery system for X-ray diffraction and imaging studies at synchrotrons and X-ray free-electron laser (FEL) facilites are demonstrated. To highlight the potential of the optical stretcher its micromanipulation capabilities have been used to image polymer beads and label biological cells. Even in a non-optimized configuration based on a commercially available optical stretcher system, X-ray holograms could be recorded from different views on a biological cell and the three-dimensional phase of the cell could be reconstructed. The capability of the setup to deform cells at higher laser intensities in combination with, for example, X-ray diffraction studies could furthermore lead to interesting studies that couple structural parameters to elastic properties. By means of high-throughput screening, the optical stretcher could become a useful tool in X-ray studies employing synchrotron radiation, and, at a later stage, femtosecond X-ray pulses delivered by X-ray free-electron lasers
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