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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As discussed in Section Source of Secondary Electrons in SEM, there are two types of secondary electrons, SE1 and SE2, generated from the specimen. SE1s are induced by the primary electron beam and SE2s are induced by the backscattered electrons (BSE). The energy change of the primary electron beam changes the ratio between the two types of SE. The total SEs from the specimen (δ), is given by,
          δ = δ₁ + δ₂ ---------------------------------- [4577a]
where δ₁ is related to SE1s and δ₂ is related to SE2s. δ2 is often given by,
          δ₂ = βηδ₁ ---------------------------------- [4577b]

          δ₁ = δ₂/(βη) ---------------------------------- [4577b]

where η is the backscattering coefficient. β factor is of a few unities when E₀ > 1-3 keV and is less than unity when E₀ < 1 keV [1]. The contribution of SE1 is higher when E₀ is smaller.

In 1940, before the first SEM images were collected, yon Ardenne predicted that SE2 would limit resolution, saying because SE2 cannot be detected separately from SE1 [2].

[1] Seiler H. (1983) Secondary electron emission in the scanning electron microscope. J. Appl. Phys. 54, R1–R18.
[2] McMullan, D., "SEM-Past, Present and Future," Journal of Microscopy, Vol. 155, No. 3, 1989, pp. 373-392.