An estimate of the effect of chromatic aberration in electron microscopy

Abstract

On the assumption that inelastic electron scattering in a specimen is localized and gives rise to an incoherent beam, an estimate is made of the effect of chromatic aberration on an electron microscope image.A model specimen of an unstained biological material with a variation in mass thickness is used to calculate the incoherent inelastic image; the aberration function is applied to this image to give an image with spherical and chromatic aberration defects. From this procedure, an estimate is made of the loss in resolution for the inelastic image due to chromatic aberration for two specimens of very different mass thickness (40 and 200 mg m−2 of carbonaceous materials) and for objective aperture sizes corresponding to semi-angles of 0·0025, 0·005 and 0·01 rad (incident electron energy 100 keV). It is concluded that chromatic aberration results in a loss in resolution on a 5-10 Å scale within the approximations of this work. The deterioration in resolution due to chromatic aberration is not very dependent on the size of the objective aperture.The conclusions of the present work are consistent with recent experimental work on the effect of chromatic aberration on the electron microscope image.