The depth sensitivity of REELS can be varied by changing the primary-beam energy  or the incidence and collection angles. The sampling depth probed in REELS depends on the primary electron beam energy, can be described based on the first-order term in the Poisson distribution, and is given by the probability of the occurrence of one inelastic collision,
d -- The depth;
λ -- The inelastic mean
free path, depending on the primary electron beam energy;
p1 -- The probability that one inelastic collision
occurs in depth d.
The EELS sampling depth is given by,
α -- The incidence angle;
β -- The collection angle.
In REELS measurements, d is normally defined as the depth from which 95% of the inelastically scattered electrons are collected. For many materials, the d is often in the range of less than 5 nm at the beam energies of less than 1 keV.
 D. F. Cox and G. B.Hoflund, Surf. Sci. 151,202 (1985).
 P. Sen, M. S. Hegde, and C. N. R. Rao, Appl. Surf. Sci. 10, 63 (1982).