This book (Practical Electron Microscopy and Database) is a reference for TEM and SEM students, operators, engineers, technicians, managers, and researchers.
 

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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 [1] that normally does not need high resolutions.
          iii) The performance of the monochromator can be degraded by energy instability [2].

Therefore, a TEM without a monochromator may still be the most attractive for EELS.

[1] 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.
[2] 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), 70–71.