A method for measuring the effective source coherence in a field emission transmission electron microscope

A method is described to measure the coherence of the electron beam within a scanning transmission electron microscope (STEM). The microscope employs a cold field emission gun. After acceleration to 100 keV, the emitted electrons are focused to a cross-over at a crystalline specimen. The visibility of interference features between diffracted discs at the image plane are a measure of the ''effective source coherence function'' at the specimen plane. Experimental results are shown comparing two designs of gun. It is seen that ambient stray fields in the immediate area dominate our microscope's performance. It is intended to utilise this measurement technique to characterise modified electron guns installed in the future.