, ▁let ' s ▁consider ▁mass – thick ness ▁contrast . ▁When ▁the ▁beam ▁ill umin ates ▁two ▁neighbour ing ▁areas ▁with ▁low ▁mass ▁( or ▁thick ness ) ▁and ▁high ▁mass ▁( or ▁thick ness ), ▁the ▁heav ier ▁region ▁sc at ters ▁electrons ▁at ▁bigger ▁angles . ▁These ▁strongly ▁scattered ▁electrons ▁are ▁blocked ▁in ▁B F ▁T EM ▁mode ▁by ▁objective ▁a pert ure . ▁As ▁a ▁result , ▁heav ier ▁regions ▁appear ▁dark er ▁in ▁B F ▁images ▁( have ▁low ▁intensity ). ▁Mass – thick ness ▁contrast ▁is ▁most ▁important ▁for ▁non – c ry st all ine , ▁am orph ous ▁materials . ▁ ▁D iff raction ▁contrast ▁occurs ▁due ▁to ▁a ▁specific ▁cry stal log raph ic ▁orientation ▁of ▁a ▁gra in . ▁In ▁such ▁a ▁case ▁the ▁cry stal ▁is ▁in ▁a ▁so - called ▁B rag g ▁condition , ▁where by ▁atomic ▁plan es ▁are ▁orient ed ▁in ▁a ▁way ▁that ▁there ▁is ▁a ▁high ▁probability ▁of ▁scatter ing . ▁Thus ▁diff raction ▁contrast ▁provides ▁information ▁on ▁the ▁orientation ▁of ▁the ▁cry st als ▁in ▁a ▁pol yc ry st all ine ▁sample . ▁Note ▁that ▁in ▁case ▁diff raction ▁contrast ▁exists , ▁the ▁contrast ▁cannot ▁be ▁interpreted ▁as ▁due ▁to ▁mass ▁or ▁thick ness ▁variations . ▁ ▁D iff raction ▁contrast ▁ ▁Sam ples ▁can ▁exhib it ▁diff raction ▁contrast , ▁where by ▁the ▁electron ▁beam ▁under go es ▁B rag g ▁scatter ing , ▁which ▁in ▁the ▁case ▁of ▁a ▁cry st all