Using a two-or multiple detector system for secondary electrons (SE) or backscattered electrons (BSE) the difference signal can be used to reconstruct the surface profile. Micrographs re-corded by these difference signals are more like shaded images obtained by illumination from one side than conventional micrographs using only one detector. Therefore, the concept of the shape-from-shading method developed for light illumination can be transferred to scanning electron microscopy as long as one considers the characteristic differences in signal detection and image formation. The surface tilt contrast causes signal differences A-B when using a two-detector system of opposite Everhart-Thornley or semi conductor detectors which are linearly to the surface gradient az/ax for SE and proportional to sin 𝛷 cos x for BSE in first order approximation, where 𝛷 denotes the surface til t angle relative to the electron beam and x an azimuth. This allows us to reconstruct the surface profile by analogue or digital image processing.
Plots of isodensities from a spherical specimen or in a gradient plane correspond to a parallel and gnonomic projection of a sphere, respectively, and are useful to compare different detector systems. The signals of SE and BSE can be self-shadowed by the specimen. The influence of the shadowing on the surface reconstruction can be reduced by an iterative correction method.
Reimer, L.; Böngeler, R.; and Desai, V.
"Shape from Shading Using Multiple Detector Signals in Scanning Electron Microscopy,"
Scanning Microscopy: Vol. 1
, Article 10.
Available at: https://digitalcommons.usu.edu/microscopy/vol1/iss3/10