SE (secondary electron) yield depending on the incident angle of primary beam can be given by,
where n is about 0.8 for heavy elements while is about 1.2 for light elements and thus, this difference gives the SEM contrast difference between heavy and light elements.
At short working distance, the major contribution to the signal collected using the lateral detector is that of SE3 electrons, which are backscattered by the pole piece as shown in Figure 4571. The corresponding contrast is a Z contrast.
SE3 can be enhanced with a shield to increase atomic number contrast. In this case, BEs that strike the shield can he converted into SEs when the shield is positively biased or covered with a material that emits more secondary electrons. Refer to discussions provided by Reimer  and Peters .
Figure 4571. Source of Secondary Electrons in SEM. BS1 and BS2 are
backscattering electrons; SE1 - SE4 are secondary electrons.
 Reimer, L., Scanning Electron Microscopy, Springer Verlag, New
 Peters, K. R., "Conditions Required for High Quality High Magnification
Images in Secondary Electron-I Scanning Electron
Microscopy," SEM 1982, Vol. IV, SEM, Inc., Chicago, 1982, pp.