Non-isochromaticity of an omega filter in a 200 kV transmission electron microscope

Non-isochromaticity of a newly developed energy filtering transmission elec tron microscope (EFTEM), JEM2010FEF has been evaluated. The non-isochromati city is the difference in energy from position to position in an image of E FTEM. It depends on the magnification and the exit beam angle ao for the im age mode and depends on the camera length in the diffraction mode. The non- isochromaticity of 2 eV is achieved at magnifications greater than 50 000 t imes under the conditions of alpha(0) = 2 mrad and the final screen of 100 mm in diameter. If a smaller objective aperture is used to reduce alpha(0), it is possible to improve the non-isochromaticity further even at lower ma gnifications. The non-isochromaticity less than 10 eV is obtained in the di ffraction mode for camera length greater than 600 mm. Possible ways of impr oving the non-isochromaticity for short camera length are to reduce the sph erical aberration coefficients of the objective lens and to increase the fi lter dispersion.