Although the monochromation systems in EMs (electron microscopes) can significantly improve the energy resolution of the systems, they do have disadvantages such as a loss of beam current, enlarged probe size, high cost, and lack of retrograded potential. The origin of low beam current or enlarged probe size is that a reduction in brightness ([A/Str.m2]) is fundamentally unavoidable in a monochromatized illumination system.
In some laboratories, even though monochromators have been installed in the electron microscopes, the monochromators are turned off for routine work because:
i) The loss of beam current with monochromators on,
ii) High energy resolutions (e.g. < 0.5 eV) is not always necessary, for instance, in the case of core-loss spectroscopy, the fine structures in a core-loss spectrum are dominated by lifetime broadening and solid-state effects  that normally does not need high resolutions.
iii) The performance of the monochromator can be degraded by energy instability .
Therefore, a TEM without a monochromator may still be the most attractive for EELS.
 Mitterbauer, C., Kothleitner, G., Grogger, W., Zandbergen, H., Freitag, B.,
Tiemeijer, P., Hofer, F., 2003. Electron energy-loss near-edge
structures of 3d transition metal oxides recorded at high-energy
resolution. Ultramicroscopy 96, 469–480.
 Tiemeijer, P.C., Lin, J.H.A.v., Freitag, B.H., Jong, A.F.d., 2002.
Monochromized 200 kV (S)TEM. Microsc. Microanal. 8 (Suppl. 2),