This book (Practical Electron Microscopy and Database) is a reference for TEM and SEM students, operators, engineers, technicians, managers, and researchers.
 

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A field emission gun (FEG) is needed for electron holography since the spatial coherence with a conventional electron gun extremely limits the resolution. Note that even though for a TEM with a conventional electron gun, a smaller illumination aperture can be used to increase the degree of coherence, the smaller aperture still cannot practically be used for holography measurement because of the smaller coherent beam current I_coh.