An imaging filter for biological applications.

A recently developed post-column imaging filter optimized for biological electron microscopy is described. The filter uses a single magnetic prism, four strong quadrupole lenses and five sextupole lenses to form images and spectra free of all important aberrations and distortions of first and second order. The performance of the filter is illustrated with practical applications. It is shown that the filter offers several important advantages over currently existing in-column filters, particularly in the areas of imaging at low magnifications, attainable collection efficiency, energy resolution, and general spectroscopic operation.