X-ray optics and imaging with hard coherent synchrotron radiation

Results obtained on the ID19 beamline at ESRF, where particularly high coherence is associated with the long source-to-sample distance and the small size of the x-ray source, illustrate the possibilities of imaging using coherence. These features make the imaging of phase objects extremely simple, since a 'propagation' technique, similar to the defocusing mode of electron microscopy and to in-line Gabor holography in optics, can be used. The physical principle involved is Fresnel diffraction. We used this 'propagation' technique both to measure, via the figures obtained from a fiber and a periodic grating, the source size, and to image objects with negligible absorption for hard x-rays but appreciable variations in optical path length. Examples of the latter ar two or three-dimensional images of light natural or artificial materials. The 3D reconstruction can be performed either with a filtered back- projection algorithm designed for attenuation tomography, which was shown to be a good approximation in some cases, or with a phase reconstruction procedure similar to that used for electron microscopy. The spurious images associated with beam line components, and the conditions for coherence preservation are also briefly discussed.