A computer program has been developed to compute the emission of cathodes from the fully space charge limited diode equation. The emission of four different gun arrangements with thermionic cathodes of the type used in electron microscopes is investigated. Microflat cathodes with a small (≤50m in diameter) fixed emitting area show several advantages for high brightness possibilities whereas other types of cathodes for which the emitting area increases with the brightness are usually limited by too great a total emission
