This paper is intended to introduce ptychography, a novel and very promising phase retrieval technique. It is based on
the lens-less recording of a series of diffraction patterns caused by coherent object illumination. In the visible region of
light, ptychography has successfully been implemented for visible light microscopy and optical metrology.
Ptychography has also successfully been applied to X-ray microscopy where it is difficult to manufacture good quality
lenses and where, at high X-ray energies, absorption contrast is low but where phase contrast is significant. In the course
of this paper theoretical fundamentals of ptychography are explained, advantages in comparison to traditional optical
techniques are represented and applications are shown.
In the field of diffraction microscopy, a coherent illuminating beam of finite extent impinges on a specimen and the
resulting diffraction pattern is recorded. The complex transmission function of the specimen is recovered using iterative
algorithms that exploit redundancies in the measured data. This is normally oversampling of the diffraction pattern when
it is known the object or illumination is of the finite size. In the case of curved illumination, there is no direct relationship
between the collection angle and the resolution of the recovered image. The result is a recovered image with varying
resolution over the field of view as different parts of the object are illuminated by different wave-vectors. An extension
of the Coherent Diffractive Imaging (CDI) technique (employing a single diffraction pattern) is to use multiple
diffraction patterns collected from adjacent parts of the object and is called ptychography. In ptychography, translation of
the illuminating wave across the specimen introduces translational diversity that leads to faster convergence of iterative
phase retrieval as well as extending the field of view. In this paper we investigate the expression of resolution
information in the diffraction pattern using curved illumination in order to facilitate specimen recovery with uniformly
improved resolution over the entire field of view.
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