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 wi thin a scanning transmission electron microscope (STEM). The microscop e employs a cold field emission gun. After acceleration to 100 keV, th e emitted electrons are focused to a cross-over at a crystalline speci men. The visibility of interference features between diffracted discs at the image plane are a measure of the ''effective source coherence f unction'' at the specimen plane. Experimental results are shown compar ing two designs of gun. It is seen that ambient stray fields in the im mediate area dominate our microscope's performance. It is intended to utilise this measurement technique to characterise modified electron g uns installed in the future.