Although, critical performance criteria of TEM objective lenses, such as spherical and chromatic aberration and, more recently, 3-fold astigmatism, are of common concern, the important properties of TEM energy filters are less well-known and not often measured or quoted. In an effort to bring these to light, we have developed methods for automatically measuring and tuning three optical properties important to practical filter performance: non-isochromaticity, geometric distortion, and non-achromaticity.

A filter is perfectly isochromatic if the range of electron energies that contribute to an energy-filtered image is the same across all image pixels. In practice, energy filters display some degree of nonisochromaticity, i.e. the energy-loss range forming the periphery of the image differs by anywhere from a fraction of an eV to as much as few eV from that forming the center of the field of view.