Optical properties of a symmetrical slit electron lens

Abstract

Optical properties, such as focal length, and defects of a symmetrical and parallel three-slit plane electrode lens are presented in diagrammatic form with aperture gap widths and applied potential differences as parameters. In calculating these properties as potential distributions on the optical axis, the analytical expression determined by G.D. Archard (1954) was applied. Calculated results show that the aperture gaps have considerable influence on the focal length, F, and spherical aberration coefficient, CS, values, but little effect on the chromatic aberration coefficient, CC. For example, typical values of CSF-1 and CCF-1 are of the order 4.2*102 and 2.5 respectively, for F=10 and infinite magnification, when the aperture gaps of the three electrodes are each equal to half the spacing between the outer electrodes.