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The SE signal current (I_{S}) received by SEM detector is approximately equal to the current (I_{SE}) of emitted SEs minus the current (I_{RE}) of SEs pulled back to the specimen surface by the local surface potential due to charging. Therefore, I_{S} is given by,
 [3827a]
The process of which SEs return to the surface is called redistribution of SEs. The redistribution rate α is the ratio of the amount of SEs collected by the surface and the amount of emitted SEs, given by,
 [3827b]
The incident current (I_{p}) of primary electrons is given by,
 [3827c]
where,
δ  SE yield (emission coefficient)
Substituting Equations 3827b and 3827c into Equation 3827a, therefore, the SE signal current (I_{S}) can be obtained and given by,
 [3827d]
For certain observation conditions in SEM, I_{p} and δ are constant and thus, the redistribution rate α is the only factor for determining the signal current as well as the local contrast.
