ine ▁sample , ▁dispers es ▁electrons ▁into ▁discrete ▁locations ▁in ▁the ▁back ▁foc al ▁plane . ▁By ▁the ▁pla cement ▁of ▁a pert ures ▁in ▁the ▁back ▁foc al ▁plane , ▁i . e . ▁the ▁objective ▁a pert ure , ▁the ▁desired ▁B rag g ▁ref lections ▁can ▁be ▁selected ▁( or ▁ex cluded ), ▁thus ▁only ▁parts ▁of ▁the ▁sample ▁that ▁are ▁causing ▁the ▁electrons ▁to ▁scatter ▁to ▁the ▁selected ▁ref lections ▁will ▁end ▁up ▁project ed ▁onto ▁the ▁imag ing ▁appar atus . ▁ ▁If ▁the ▁ref lections ▁that ▁are ▁selected ▁do ▁not ▁include ▁the ▁un sc atter ed ▁beam ▁( which ▁will ▁appear ▁up ▁at ▁the ▁foc al ▁point ▁of ▁the ▁l ens ), ▁then ▁the ▁image ▁will ▁appear ▁dark ▁wherever ▁no ▁sample ▁scatter ing ▁to ▁the ▁selected ▁peak ▁is ▁present , ▁as ▁such ▁a ▁region ▁without ▁a ▁spec imen ▁will ▁appear ▁dark . ▁This ▁is ▁known ▁as ▁a ▁dark - field ▁image . ▁ ▁Modern ▁T EM s ▁are ▁often ▁equ ipped ▁with ▁spec imen ▁hold ers ▁that ▁allow ▁the ▁user ▁to ▁t ilt ▁the ▁spec imen ▁to ▁a ▁range ▁of ▁angles ▁in ▁order ▁to ▁obtain ▁specific ▁diff raction ▁conditions , ▁and ▁a pert ures ▁placed ▁above ▁the ▁spec imen ▁allow ▁the ▁user ▁to ▁select ▁electrons ▁that ▁would ▁otherwise ▁be ▁diff ract ed ▁in ▁a ▁particular ▁direction ▁from ▁entering ▁the ▁spec imen . ▁ ▁App lications ▁for ▁this ▁method ▁include ▁the ▁identification ▁of ▁lattice ▁defect s ▁in ▁cry st als . ▁By ▁carefully ▁selecting ▁the ▁orientation ▁of ▁the