Spatial Resolution of EFTEM Mapping Affected by Spherical Aberration
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Similar to the effect of chromatic aberration, spatial resolution of EFTEM mapping is also affected by spherical aberration, given by,

          ΔdS = 2CSβ3 -------------- [1941a]

          β -- The collection angle,
          CS -- The spherical aberration.

For instance, assuming the spherical aberration constant is 0.7 mm, collection semi-angle 15 mrad, the ΔdS is ~3.8 nm. For thick specimens (with multiple scattering) the calculation in Equation 1941a predicts the CS-effect on the spatial resolution well, while for a thin specimen the characteristic inelastic scattering angle is much smaller, given by,

         θe = γΔE/2E0 ----------------------- [1941b]

          γ -- The relativistic correction factor (=(E0+mec2)/(E0+2mec2), e.g. =0.61 for 300 kV).

θe is normally ≤0.6 mrad. Substituting θe for β in Equation 1941a, we can know that ΔdS is normally negligible for most microscope configurations.




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