RESOLUTION AND ILLUMINATION COHERENCE IN ELECTRON MICROSCOPY.

Resolution and coherence criteria in transmission electron microscopy are discussed, and a contrast transfer function (CTF) is derived for coherent spherical wave illumination. Assuming a disc source, the CTF for spherical wave illumination is identical to that for convergent illumination from an incoherent source. Assuming a Gaussian source, the CTF decays more rapidly for a spherical wave than for an incoherent convergent beam. The implications for high resolution microscopy using a field emission gun are considered and it is shown that, owing to both the reduced energy spread and the increased brightness of a field emission source, point-to-point resolutions of better than 1 A should be possible using existing technology. A simple method of incorporating the CTF in image calculations for perfect or imperfect crystals, which does not involve the usual Fourier transforming, is also given.