Abstract
Point-projection shadow images (PPM) are obtained by placing a nanotip field emitter at a potential of a few hundred volts close to a grounded sample, which acts as the anode. It can be shown that the shadow image is identical to an conventional out-of-focus TEM image with a defocus equal to the tip-to-sample distance z1, and is an in-line hologram. Recent work shows that most objects used for PPM may be treated as opaque masks containing holes, since the inelastic mean free path (IMF) for 100 eV electrons is 6-8Å in carbon. The question arises if the mask shape can be reconstructed from the shadow image, i.e. if the image can be brought back into focus. For masks, Fresnel edge fringes constitute most of the hologram (apart from interference between different holes). Fresnel fringes and shadow images were first observed by Boersch, prior to Gabor's demonstration that, for small opaque objects, they could be removed by holographic reconstruction or “focus restoration”.
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