1,720,964 research outputs found
Coherent diffraction imaging using laboratory light sources
No full textCoherent diffractive imaging, also called lenses imaging methods, are now a standard tool for Xray imaging on synchrotron devices [1,2,3]. The diffractive imaging methods can avoid limitations of imperfect optics in the Xray region and also limit the dose exposed to the sample. Therefore, these methods allow to reach higher resolution compared to the standard lens based methods. Moreover, lensless methods allow to extract full information about the wavefront behind the sample and hence relative phase and amplitude can be recovered. Finally, advanced lensless methods such as ptychography allow to extract information about the illumination that can be used for beam metrology purposes [3]. In recent years, the coherent diffractive imaging is getting to be used also with the laboratory sources and longer wavelengths such as EUV and visible light region. Limitation of the laboratory EUV sources is lower signal, worse coherence properties and generally lower time stability of the signal. The requirements for lateral and temporal coherence can be slightly relaxed if the reconstruction is sufficiently overconstrained [4,5,6]. In this work, we will introduce and compare numerical methods that can significantly help to improve convergence and reconstruction quality. We will introduce the methods using numerical examples and also real measured data from EUV and IR light experiments. We will use ptychography method for the reconstruction, although most of the proposed methods can be used also with other lensless imaging techniques
Coherent diffractive imaging with laboratory scale EUV sources
No full textThe ptychography method for coherent diffractive imaging (CDI) is gaining significant popularity over the last few years in the synchrotron science community because of its advantages over simpler single diffraction CDI schemes. However, for imaging applications in the extreme ultraviolet (XUV), the ability to perform ptychography with table top setups is limited by requirements on high coherence of illumination and overall beam quality. With the aim of improving imaging using coherent table-top EUV sources, we demonstrate that high harmonics generation (HHG) source at 29 nm wavelength with relative bandwidth (Delta.lambda/lambda) of ~15% can provide useful gain in available flux without deteriorating the reconstruction quality. The HHG spectrum was self-consistently measured by the Young double slit (Fourier) based EUV spectrometer using the same setup that used for ptychography. Our setup uses fewer monochromating optics, obtaining higher flux at the sample and thus reaching higher resolution for semi-opaque objects. High coherent photon flux density is more critical in the case of ptychography when a large number of diffraction patterns needs to be collected compared to the traditional coherent diffraction method with compactness constraints. Our improved reconstruction method was tested on an aperiodic grid sample with reconstructed area 40×40 µm and half period resolution 55 nm but also on natural samples consisting from dried neurons on a 50nm thick silicon nitride window with 90nm resolution
Coherent diffractive imaging with lower coherence laboratory sources
No full textCoherent Diffractive Imaging (CDI) also called "lens-less imaging" is an alternative to the classical brightfield, dark-field and phase contrast microscopy. CDI allows us to recover full information about the wave-field that passed sample ie. attenuation and relative phaseshift that can be used for estimation of the sample material propertie
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Full characterisation of a focussed extreme ultraviolet beam using a non-redundant array of apertures
No full textThis paper presents a novel technique for characterising wavefront curvature and M2, by utilising a non-redundant array (NRA) of apertures to define the plane of investigation through an experimental extreme ultraviolet (EUV) focus. Appropriately sampled, far-field EUV scattering from this NRA is captured on a CCD as the NRA is scanned along the beam axis through the focus. By taking the inverse Fourier transform (IFT), it is possible obtain the spatial autocorrelation functions, via the Wiener-Khinchin theorem, of the exit wave field. By observing the position of the first-order peaks in the autocorrelation as a function of grid translation, both the real and imaginary parts of the complex beam parameter can be determined and the M2 calculated, yielding full characterisation of the embedded Gaussian. Since the periodicity of the grid is known, the planar pixel resolution can be calculated, also allowing the translations movement to be confirmed due to the change in angular acceptance of the fixed CCD. This makes the technique self-calibrating. A high impact, easy to use, cross field technique for full profiling of the embedded Gaussian of probe beams using a non-redundant array of apertures is presented. The technique is experimentally verified in the highly absorbing EUV spectral regime, and is expected to play a significant role in other regimes, where experimental issues prevent the use of existing techniques
Coherent diffractive imaging with laboratory EUV sources
No full textLaboratory EUV (extreme ultraviolet) sources can be in future practical and cheaper alternative to the large facilities such as synchotrons. HHG (high harmonic generation) laser sources are getting to be common EUV source for CDI (coherent diffractive imaging). We demonstrate coherent imaging with HHG at 27nm and with a pinch plasma based EUV source at 17.3nm wavelength. We have successfully tested imaging of strongly scattering compact objects using the HIO and Shrinkwrap algorithms. In near future, we are going to use ptychography
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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