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Assuming a flat sample surface is irradiated at normal incidence with an incident electron beam of energy E0. The secondary electron emission angle can be defined by the electron emission angle with respect to this normal. This angular distribution depends upon the transmission probability of SE across the surface potential barrier.
SE yield depending on the incident angle of primary beam can be given by,
 ------------------------ [4823]
where n is about 0.8 for heavy elements while is about 1.2 for light elements.
Equation 4823 indicates that the SE yield increases with θ. As θ increases, the primary-electron scattering occurs mostly near the surface so that more SEs which are generated by ionization reach and escape from the surface to the vacuum. The surface topographic contrast is utilized most frequently in SEM observations comparing with other SEM contrasts. This contrast originates from the difference of the SE yield for different incident angles of primary electrons irradiating a specimen surface.
Solid angle of detection can be defined by the angular interval of α from αmin to αmax. Not all emitted SEs are collected by the detectors in SEM and the collection efficiency of the detectors is a function of the solid angle of the detector relative to the successive analyzing locations.
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