▁In ▁both ▁cases ▁the ▁spec imen ▁is ▁ill umin ated ▁with ▁the ▁parallel ▁beam , ▁formed ▁by ▁electron ▁beam ▁sh aping ▁with ▁the ▁system ▁of ▁Cond enser ▁l enses ▁and ▁Cond enser ▁a pert ure . ▁After ▁interaction ▁with ▁the ▁sample , ▁on ▁the ▁exit ▁surface ▁of ▁the ▁spec imen ▁two ▁types ▁of ▁electrons ▁exist ▁– ▁un sc atter ed ▁( which ▁will ▁correspond ▁to ▁the ▁bright ▁central ▁beam ▁on ▁the ▁diff raction ▁pattern ) ▁and ▁scattered ▁electrons ▁( which ▁change ▁their ▁traject ories ▁due ▁to ▁interaction ▁with ▁the ▁material ). ▁ ▁In ▁Im aging ▁mode , ▁the ▁objective ▁a pert ure ▁is ▁inserted ▁in ▁a ▁back ▁foc al ▁plane ▁( B FP ) ▁of ▁the ▁objective ▁l ens ▁( where ▁diff raction ▁sp ots ▁are ▁formed ). ▁If ▁using ▁the ▁objective ▁a pert ure ▁to ▁select ▁only ▁the ▁central ▁beam , ▁the ▁transm itted ▁electrons ▁are ▁passed ▁through ▁the ▁a pert ure ▁while ▁all ▁others ▁are ▁blocked , ▁and ▁a ▁bright ▁field ▁image ▁( BF ▁image ) ▁is ▁obtained . ▁If ▁we ▁allow ▁the ▁signal ▁from ▁a ▁diff ract ed ▁beam , ▁a ▁dark ▁field ▁image ▁( DF ▁image ) ▁is ▁received . ▁The ▁selected ▁signal ▁is ▁magn ified ▁and ▁project ed ▁on ▁a ▁screen ▁( or ▁on ▁a ▁camera ) ▁with ▁the ▁help ▁of ▁Inter mediate ▁and ▁Project or ▁l enses . ▁An ▁image ▁of ▁the ▁sample ▁is ▁thus ▁obtained . ▁ ▁In ▁D iff raction ▁mode , ▁a ▁selected ▁area ▁a pert ure ▁may ▁be ▁used ▁to ▁determine ▁more ▁precisely