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Mikroskopie und Tomographie Poster: Mi., 14:00–16:30 M-P110
Influence of Noise and Partial Coherence on Resolution in Coherent Diffractive
Imaging Experiments
Andreas Schropp 1 , Christian Schroer 2 , Ivan Vartaniants 1 , Edgar
Weckert 1 , Christian Mocuta 3 , Hartmut Metzger 3
1 HASYLAB at DESY, Notkestr. 85, D-22603 Hamburg, Germany – 2 Institut für
Strukturphysik, TU Dresden, D-01062 Dresden, Germany – 3 ESRF, BP 220, F-38043
Grenoble Cedex, France
Coherent X-ray diffraction imaging (CXDI) [1] is a suitable method to determine the
structure of small, non-periodic objects up to a resolution on nanometer length scale.
It can be considered as a microscopic technique providing information on physical
properties of materials in a variety of scientific fields, e.g. whole-cell imaging [2] and
mesoporous structures. The method, also named lensless imaging, is simply based on
measuring of the Fraunhofer X-ray diffraction pattern of an object. Instead of using
lenses or other optical components for image formation the structure of the object is
reconstructed with the help of numerical algorithms which are able to recover the phase
information in the diffraction pattern. From this view the numerical reconstruction is
simply acting as a lens in an optical system. The achievable resolution of this method
of structure determination depends in principle only on the used wavelength and the
largest recorded diffraction angle. Nevertheless, the latter requirement is limited by
the specific experimental arrangement, i.e. distance between sample and detector,
detector size, signal to noise ratio, and especially by the total incoming coherent photon
flux. As a result of a strong decrease of the diffracted photon intensity towards larger
diffraction angles, i.e. I(q) ∼ q −3 , the amount of photons at large q-values determines
the resolution. A systematic series of numerical simulations was carried out aiming
to investigate the limitations in resolution due to a restricted number of incoming
photons.
In further simulations we considered additionally a second important factor which is
the influence of partial coherent illumination. In this case, the visibility of the interference
fringes is reduced which leads to a certain degree of smearing in the diffraction
pattern. This effect is not taken into account by the numerical algorithms (Hybrid-
Input-Output Method [3]) that are usually used and thus, it can essentially affect the
result of a reconstruction using a partially blurred data set.
The results of these numerical simulations together with a comparison to the results
of a CXD-experiment carried out at ID01 (ESRF) [4] will be presented on a poster.
[1] J. Miao et al, Nature, 400, 342 (1999)
[2] D. Shapiro et al., PNAS, 102, 15343 (2005)
[3] J.R. Fienup, Appl. Opt. 21, 2758 (1982)
[4] A. Schropp et al., Proceedings of the 8th International Conference on X-ray Microscopy
(XRM2005), (2006) to be published