Limitations to Resolving Power in the Thermionic Emission Microscope†

Abstract

ABSTRACT Formulation of resolving power as a function of electron chromatism at the source only cannot account for the large deviation from practical resolution figures measured for emission microscopes, not even when best corrected lens systems and very high accelerating potentials are employed. Considering electron diffraction only as the decisive resolution limiting factor, too low a value is obtained. Hence, it appears, that the optimum practical resolution of 800 Å would lie between the two functions. An approximation is given for computing the effective resolving power. It demonstrates the little influence of the field strength at the emitter surface on overall resolution. This then also implies that the use of very high first anode voltages add little only to enhance image quality. It is shown how severe the resolution degrading property of surface irregularities is as evidenced by photographs taken from a 21 in. directly viewed screen. Surface depressions, elevations and pores may lead to serious misinterpretations of the actually existing emission mechanism due to electron ray refraction by distorted equipotential planes which shape the trajectories in peculiar ways. Bunching, focusing and divergence effects are shown to influence the emission patterns on the screen, also preparing the way for a proper interpretation of observations on more difficult surfaces such as oxide cathodes or any other irregular surface. Special situations are also discussed which produce electron-optical illusions on the screen such as loss of surface detail, splitting of electron sheets, multi-helical paths and multi-circular patterns.