HIGH BRIGHTNESS THERMIONIC ELECTRON GUNS FOR ELECTRON MICROSCOPES.

Some of the factors are outlined which determine the performance of high brightness thermionic electron guns of the type commonly used in scanning electron microscopes and other electron beam instruments. The concept of brightness in electron beam systems is introduced together with some of the thermionic emissions principles involved. The basic form of the triode type electron gun is described together with methods of measuring electron-optical characteristics of narrow angle electron guns. Emphasis is placed on difference between the ″cross-over image″ and the ″beam intensity profile distribution″ , gun characteristics which may now be observed in some commercial instruments.