Mechanisms in electrostatic charging of insulating materials during electron irradiation are very difficult to predict because they involve cross-correlated parameters: the surface potential, Vs and the secondary electron yield,δ, for bare specimens; the possible change of the bulk composition induced by the internal electric field for ground coated specimens. A better understanding of these mechanisms requires efforts from both the theoretical and the experimental points of view.

From the theoretical point of view, a two-layer model for distribution of the trapped charges (Q+ over the escape depth of the secondaries, s; Q‐ over the maximum penetration depth R of the incident electrons) often permits to account for many charging effects observed on bare specimens. For instance, the failure of the total yield approach may be partly explained by the fact that the charge distribution expected at the critical energy Ec2 (where δ+ η=l) corresponds to Q+=‐Q‐ but also to R ≫ s at the early beginning of the SEM and EDS investigation.