Prospects for extending the resolution limit of the electron microscope.

The factors that limit the performance of present-day high-resolution electron microscopes include the source brightness, the temporal coherence of the illumination, and phase contrast 'artefacts' due to spherical aberration and defocus. Further advancement of the instrumental performance might be accomplished in different ways, including the use of high coherence conditions combined with image restoration; the use of higher accelerating voltages; the use of aberration correctors; or the use of conditions for incoherent image formation. Current efforts in each of these directions are reviewed, from which it is evident that the combined use of higher voltages and improved coherence is the direction that is presently leading to the greatest degree of progress.