A Discussion of the Fundamental Limit of Performance of an Electron Microscope

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Abstract
In the case of a given electron microscope objective the diffraction defect diffuses the image of a geometrical point over a finite area. The contrast which can be obtained in the image of a single atom is limited by this phenomenon because the actual image of an atom extends over many times the area of its geometrical image. If a ten percent change in intensity is the minimum change that can be detected in the electron image by means of a photographic emulsion, it is shown that a single atomic nucleus can produce a discernible image only if it is of atomic number greater than 25. With the optimum objective aperture the addition of the atomic electrons appears to produce only a slight change in this lower limit of atomic number. Its value is increased by the effects of neighboring atoms, non-parallel illumination, and lens aberrations.

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