. ▁ ▁A pert ures ▁are ▁either ▁a ▁fixed ▁a pert ure ▁within ▁the ▁column , ▁such ▁as ▁at ▁the ▁cond enser ▁l ens , ▁or ▁are ▁a ▁mov able ▁a pert ure , ▁which ▁can ▁be ▁inserted ▁or ▁with dra wn ▁from ▁the ▁beam ▁path , ▁or ▁moved ▁in ▁the ▁plane ▁per pend icular ▁to ▁the ▁beam ▁path . ▁A pert ure ▁assemb lies ▁are ▁mechanical ▁devices ▁which ▁allow ▁for ▁the ▁selection ▁of ▁different ▁a pert ure ▁sizes , ▁which ▁may ▁be ▁used ▁by ▁the ▁operator ▁to ▁trade ▁off ▁intensity ▁and ▁the ▁filtering ▁effect ▁of ▁the ▁a pert ure . ▁A pert ure ▁assemb lies ▁are ▁often ▁equ ipped ▁with ▁mic rom eters ▁to ▁move ▁the ▁a pert ure , ▁required ▁during ▁optical ▁cal ibration . ▁ ▁Im aging ▁methods ▁Im aging ▁methods ▁in ▁T EM ▁use ▁the ▁information ▁contained ▁in ▁the ▁electron ▁waves ▁exit ing ▁from ▁the ▁sample ▁to ▁form ▁an ▁image . ▁The ▁project or ▁l enses ▁allow ▁for ▁the ▁correct ▁position ing ▁of ▁this ▁electron ▁wave ▁distribution ▁onto ▁the ▁view ing ▁system . ▁The ▁observed ▁intensity , ▁I , ▁of ▁the ▁image , ▁assuming ▁sufficiently ▁high ▁quality ▁of ▁imag ing ▁device , ▁can ▁be ▁approxim ated ▁as ▁proportional ▁to ▁the ▁time - aver aged ▁squ ared ▁absolute ▁value ▁of ▁the ▁amplitude ▁of ▁the ▁electron ▁wave functions , ▁where ▁the ▁wave ▁that ▁forms ▁the ▁exit ▁beam ▁is ▁denoted ▁by ▁ Ψ . ▁ ▁D ifferent ▁imag ing ▁methods ▁therefore ▁attempt ▁to ▁modify ▁the ▁electron ▁waves ▁exit ing ▁the ▁sample ▁in