Section 19.6.3.3. Required properties of the illuminating electron beam

The important properties of the image in terms of defocus, astigmatism and the presence and effect of amplitude or phase contrast are discussed below (Sections 19.6.4.4 and 19.6.4.6). The best-quality incident electron beam is produced by a field emission gun (FEG). This is because the electrons from a FEG are emitted from a very small volume at the tip, which is the apparent source size. Once these electrons have been collected by the condenser lens and used to produce the illuminating beam, that beam of electrons is then very parallel (divergence of ~10⁻² mrad) and therefore spatially coherent. Similarly, because the emitting tip of a FEG is not heated as much as a conventional thermionic tungsten source, the thermal energy spread of the electrons is relatively small (0.5 to 1.0 eV) and, as a result, the illuminating beam is monochromatic and therefore temporally coherent. Electron beams can also be produced by a normal heated tungsten source, which gives a less parallel beam with a larger energy spread, but is nevertheless adequate for electron cryomicroscopy if the highest resolution images are not required.