SOME GENERAL CONSIDERATIONS CONCERNING THE OPTICS OF THE FIELD EMISSION ILLUMINATION SYSTEM.

Due to its small size, the field emission source gives a very high brightness despite a relatively low current capability. Optimum application of the new source requires a review of the techniques used to match the illumination system optics to both the source and the application. Some performance estimates and design criteria are presented for conventional and scanning microscopy, electron diffraction and microprobe research, with special emphasis placed upon choosing the magnification to suit the application. Three optical configurations are introduced, each designed to avoid some of the difficulties inherent in the first and third order optics of the field emission system. Spherical aberration, defocus and environmental stability can each play decisive roles in achieving favorable illumination.