t ▁pol yc ry st all ine ▁film ▁taken ▁without ▁an ▁objective ▁a pert ure . ▁In ▁order ▁to ▁enh ance ▁the ▁contrast ▁in ▁the ▁T EM ▁image ▁the ▁number ▁of ▁scattered ▁be ams ▁as ▁visible ▁in ▁the ▁diff raction ▁pattern ▁should ▁be ▁reduced . ▁This ▁can ▁be ▁done ▁by ▁selecting ▁a ▁certain ▁area ▁in ▁the ▁back ▁foc al ▁plane ▁such ▁as ▁only ▁the ▁central ▁beam ▁or ▁a ▁specific ▁diff ract ed ▁beam ▁( angle ), ▁or ▁combinations ▁of ▁such ▁be ams . ▁By ▁intention ally ▁selecting ▁an ▁objective ▁a pert ure ▁which ▁only ▁perm its ▁the ▁non - diff ract ed ▁beam ▁to ▁pass ▁beyond ▁the ▁back ▁foc al ▁plane ▁( and ▁onto ▁the ▁image ▁plane ): ▁one ▁creates ▁a ▁Bright - Field ▁( BF ) ▁image ▁( c ), ▁whereas ▁if ▁the ▁central , ▁non - diff ract ed ▁beam ▁is ▁blocked : ▁one ▁may ▁obtain ▁Dark - Field ▁( DF ) ▁images ▁such ▁as ▁those ▁shown ▁in ▁( d - e ). ▁The ▁D F ▁images ▁( d - e ) ▁were ▁obtained ▁by ▁selecting ▁the ▁diff ract ed ▁be ams ▁indicated ▁in ▁diff raction ▁pattern ▁with ▁circles ▁( b ) ▁using ▁an ▁a pert ure ▁at ▁the ▁back ▁foc al ▁plane . ▁Gra ins ▁from ▁which ▁electrons ▁are ▁scattered ▁into ▁these ▁diff raction ▁sp ots ▁appear ▁br ighter . ▁More ▁details ▁about ▁diff raction ▁contrast ▁formation ▁are ▁given ▁further . ▁ ▁There ▁are ▁two ▁types ▁of ▁amplitude ▁contrast ▁– ▁mass – thick ness ▁and ▁diff raction ▁contrast . ▁First