Coherent Soft X-Rays in High Resolution Imaging

To illustrate the coherence problems in X-ray imaging we shall examine - scanning X-ray microscopy - holography, and - soft X-ray diffraction as illustrative examples. We show that only X-rays from within the emittance xx'yy' < (2.44λ)2 have the necessary spatial coherence to yield diffraction- limited images. The degree of temporal coherence required depends on the nature of the imaging system, and the size of the specimen. In conventional X-ray systems, spatial coherence is generated by collimation, while temporal coherence is achieved with monochromators. In these systems, then, one filters out a minute coherent fraction of an intense incoherent beam. Aside from the inefficiency of the procedure, there is a practical limitation of the temporal coherence length of about 103λ that one can generate this way in the soft X-ray regime.