Although the EELS spectrometer in this system delivers an energy resolution of about 50-130meV, the natural width of the room temperature Field Emission Gun (FEG) electron source is 280-350 meV, so a full utilization of the spectrometer capability is limited. This can be changed by addition of an electron monochromator, placed within the gun. A 3 cm long “fringe field” monochromator has been designed and tested for this system. [1, 2] This device disperses the FEG source electrons against a 200 nm diameter aperture. The monochromator-spectrometer combination has shown a 61 meV resolution, while preserving the high brightness characteristics of the FEG. The left illustration below shows the intensity of the gun as the source spot is scanned across the small energy selecting aperture.
 P.E. Batson, H.W. Mook, and P. Kruit, High Brightness Monochromator for STEM, in International Union of Microbeam Analysis 2000, edited by D.B. Williams and R. Shimizu (Institute of Physics, Bristol, 2000), Vol. 165, pp. 213-214.
 H.W. Mook, P.E. Batson, and P. Kruit, Monochromator for high brightness electron guns, in 12th European congress on electron microscopy, Vol. III, (2000) pp. 315 -316.