Abstract
Axial coherent (parallel) illumination, commonly used for bright-field imaging, creates artifacts and strong structural noise. These parasitic structures are caused by an overlap of Fresnel fringes, which can be largely suppressed if hollow-cone (partially coherent) illumination is employed (1,2). However, it is commonly assumed that this imaging mode leads to a severe reduction in contrast. To obtain reliable information on the maximum contrast achievable, we have made extensive calculations of images of single atoms taking into account both phase contrast and scattering absorption contrast. The image contrast C = C(r, C3, Δf, U, θo, θ, Z) of an atom is a function of the image coordinate r, sperical aberration C3, defocus Δf, voltage U, aperture ΔO, cone angle Δ of the illumination, and the atomic number Z. At resolutions currently obtainable, the bright-field contrast of the objects can be described very accurately if the first- and second-order terms of the Born approximation of the scattering amplitude are considered.
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