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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There are many factors which can induce background contrasts in TEM and STEM images. For instance, objective astigmatism is one main factor that contribute background contrast to high resolution TEM images.

Furthermore, high-angle incoherently scattered TDS (thermal diffuse scattering) electrons are subject to significant phase shifts from the rapidly varying portion of the lens transfer function and thus will extremely contribute a constant background to both TEM and STEM images [1].

[1] C.B. Boothroyd, Journal of Microscopy 190 (1998) 99.