page_title: Ptychography
section_title: Near-field ptychography
hierarchical_section_title: Ptychography / Optical configurations / Near-field ptychography
caption_attribution_description: Diagram showing the setup for near-field ptychography.
context_page_description: Ptychography is a computational method of microscopic imaging. It generates images by processing many coherent interference patterns that have been scattered from an object of interest. Its defining characteristic is translational invariance, which means the interference patterns are generated by one constant function moving laterally by a known amount with respect to another constant function. The interference patterns occur some distance away from these two components, so that the scattered waves spread out and ‘fold’ into one another as shown in the figure.
  Ptychography can be used with visible-light, X-rays, extreme ultraviolet or electrons. Unlike conventional lens imaging, ptychography is unaffected by lens-induced aberrations or diffraction effects caused by limited numerical aperture. This is particularly important for atomic-scale wavelength imaging, where it is difficult and expensive to make good quality lenses with high numerical aperture. Another important advantage of the technique is that it allows transparent objects to be seen very clearly.
context_section_description: This uses a wide field of illumination. To provide magnification, a diverging beam is incident on the specimen. An out-of-focus image, which appears as a Fresnel interference pattern, is projected onto the detector. The illumination must have phase distortions in it, often provided by a diffuser that scrambles the phase of the incident wave before it reaches the specimen, otherwise the image remains constant as the specimen is moved, so there is no new ptychographical information from one position to the next. In the electron microscope, a lens can be used to map the magnified Fresnel image onto the detector.
caption: Optical configuration for near-field ptychograhy.
exif: {}