EM s ▁appropri ately ▁equ ipped , ▁electrons ▁can ▁be ▁separated ▁into ▁a ▁spectrum ▁based ▁upon ▁their ▁velocity ▁( which ▁is ▁closely ▁related ▁to ▁their ▁kin etic ▁energy , ▁and ▁thus ▁energy ▁loss ▁from ▁the ▁beam ▁energy ), ▁using ▁magnetic ▁sector ▁based ▁devices ▁known ▁as ▁E EL ▁spect rom eters . ▁These ▁devices ▁allow ▁for ▁the ▁selection ▁of ▁particular ▁energy ▁values , ▁which ▁can ▁be ▁associated ▁with ▁the ▁way ▁the ▁electron ▁has ▁interact ed ▁with ▁the ▁sample . ▁For ▁example , ▁different ▁elements ▁in ▁a ▁sample ▁result ▁in ▁different ▁electron ▁energ ies ▁in ▁the ▁beam ▁after ▁the ▁sample . ▁This ▁normally ▁results ▁in ▁chrom atic ▁aber r ation ▁– ▁however ▁this ▁effect ▁can , ▁for ▁example , ▁be ▁used ▁to ▁generate ▁an ▁image ▁which ▁provides ▁information ▁on ▁element al ▁composition , ▁based ▁upon ▁the ▁atomic ▁transition ▁during ▁electron - elect ron ▁interaction . ▁ ▁E EL S ▁spect rom eters ▁can ▁often ▁be ▁operated ▁in ▁both ▁spect ro scop ic ▁and ▁imag ing ▁modes , ▁allowing ▁for ▁isol ation ▁or ▁re jection ▁of ▁el ast ically ▁scattered ▁be ams . ▁As ▁for ▁many ▁images ▁in el astic ▁scatter ing ▁will ▁include ▁information ▁that ▁may ▁not ▁be ▁of ▁interest ▁to ▁the ▁investig ator ▁thus ▁reducing ▁observable ▁signals ▁of ▁interest , ▁E EL S ▁imag ing ▁can ▁be ▁used ▁to ▁enh ance ▁contrast ▁in ▁observed ▁images , ▁including ▁both ▁bright ▁field ▁and ▁diff raction , ▁by ▁reject ing ▁unw anted ▁components . ▁ ▁Three - dimensional ▁imag ing ▁ ▁As ▁T EM ▁spec imen ▁hold