IMPROVING IMAGE QUALITY BY ZERO-LOSS ENERGY-FILTERING - QUANTITATIVE ASSESSMENT BY MEANS OF IMAGE CROSS-CORRELATION

Fourier ring correlation and root-mean-square contrast of pairs of ima ges, taken under identical conditions, were used as criteria of image quality for comparing unfiltered with zero-loss energy-filtered imagin g using a TEM equipped with a post-column energy filter. For three dif ferent specimens (amorphous carbon film, macromolecules in light negat ive stain, virus particles in deep negative stain) the dependence of t hese quantities on electron dose, specimen thickness and defocus was i nvestigated. A model, based on simple assumptions, was used to describ e quantitatively their dependence on electron dose and specimen thickn ess. It was found that energy filtering is most advantageous for low-d ose imaging and small defocus values. The gain due to energy filtering strongly increases with specimen thickness, whereby the dependence is linear for light scattering elements. For thick specimens, the gain b y energy filtering is more pronounced in the resolution range between 4 and 2 nm than for lower spatial frequencies.