Light interacting with matter forms the physical basis for most microscopic imaging processes. Thus it should come as no surprise that transparent objects (phase objects) – i.e. objects which only interact to a minimum degree with light – pose a quandary when creating an image. Making pure phase objects visible is not a new problem, and the search for better and innovative solutions continues.
Ptychography is one new approach from the field of computational imaging. In ptychography, a sample is illuminated using a coherent beam (e.g. a laser) and the diffraction image is observed at a large object distance. The beam is scanned over the sample with a large overlap of the illuminated area. To the naked eye, this single image is a completely useless diffraction image which looks like coherent noise (speckle). Thus the algorithmic evaluation of the data is essential – as this decodes the information contained in the diffraction patterns.
The unknown phase, i.e. the refractive properties of the object, is determined using the observed intensities of the speckle pattern. With modern enhancements, the exact illumination properties of the light source and the 3D structure of the object can also be determined using suitable physical modeling of this imaging process.
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Lars Omlor and Ivo Ihrke
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