▁be ▁modified ▁into ▁a ▁sc anning ▁transmission ▁electron ▁mic ros cope ▁( ST EM ) ▁by ▁the ▁addition ▁of ▁a ▁system ▁that ▁r aster s ▁a ▁conver gent ▁beam ▁across ▁the ▁sample ▁to ▁form ▁the ▁image , ▁when ▁combined ▁with ▁suitable ▁detect ors . ▁Sc anning ▁co ils ▁are ▁used ▁to ▁def lect ▁the ▁beam , ▁such ▁as ▁by ▁an ▁electro static ▁shift ▁of ▁the ▁beam , ▁where ▁the ▁beam ▁is ▁then ▁collected ▁using ▁a ▁current ▁det ector ▁such ▁as ▁a ▁Far ad ay ▁cup , ▁which ▁acts ▁as ▁a ▁direct ▁electron ▁counter . ▁By ▁correl ating ▁the ▁electron ▁count ▁to ▁the ▁position ▁of ▁the ▁sc anning ▁beam ▁( known ▁as ▁the ▁" pro be "), ▁the ▁transm itted ▁component ▁of ▁the ▁beam ▁may ▁be ▁measured . ▁The ▁non - trans m itted ▁components ▁may ▁be ▁obtained ▁either ▁by ▁beam ▁til ting ▁or ▁by ▁the ▁use ▁of ▁ann ular ▁dark ▁field ▁detect ors . ▁ ▁Fund ament ally , ▁T EM ▁and ▁S TE M ▁are ▁linked ▁via ▁Hel m hol tz ▁reci pro city . ▁A ▁S TE M ▁is ▁a ▁T EM ▁in ▁which ▁the ▁electron ▁source ▁and ▁observation ▁point ▁have ▁been ▁switched ▁relative ▁to ▁the ▁direction ▁of ▁travel ▁of ▁the ▁electron ▁beam . ▁See ▁the ▁ray ▁diag rams ▁in ▁the ▁figure ▁on ▁the ▁right . ▁The ▁S TE M ▁instrument ▁effectively ▁re lies ▁on ▁the ▁same ▁optical ▁set - up ▁as ▁a ▁T EM , ▁but ▁oper ates ▁by ▁f li pping ▁the ▁direction ▁of ▁travel ▁of ▁the ▁electrons ▁(