- R aman ▁spect ro sc opy . ▁ ▁In ▁spatial ▁filtering , ▁a ▁l ens ▁is ▁used ▁to ▁focus ▁the ▁beam . ▁Because ▁of ▁diff raction , ▁a ▁beam ▁that ▁is ▁not ▁a ▁perfect ▁plane ▁wave ▁will ▁not ▁focus ▁to ▁a ▁single ▁spot , ▁but ▁rather ▁will ▁produce ▁a ▁pattern ▁of ▁light ▁and ▁dark ▁regions ▁in ▁the ▁foc al ▁plane . ▁For ▁example , ▁an ▁imper fect ▁beam ▁might ▁form ▁a ▁bright ▁spot ▁surrounded ▁by ▁a ▁series ▁of ▁concent ric ▁rings , ▁as ▁shown ▁in ▁the ▁figure ▁to ▁the ▁right . ▁It ▁can ▁be ▁shown ▁that ▁this ▁two - dimensional ▁pattern ▁is ▁the ▁two - dimensional ▁Fourier ▁transform ▁of ▁the ▁initial ▁beam ' s ▁trans verse ▁intensity ▁distribution . ▁In ▁this ▁context , ▁the ▁foc al ▁plane ▁is ▁often ▁called ▁the ▁transform ▁plane . ▁Light ▁in ▁the ▁very ▁center ▁of ▁the ▁transform ▁pattern ▁corresponds ▁to ▁a ▁perfect , ▁wide ▁plane ▁wave . ▁Other ▁light ▁corresponds ▁to ▁" structure " ▁in ▁the ▁beam , ▁with ▁light ▁further ▁from ▁the ▁central ▁spot ▁corresponding ▁to ▁structure ▁with ▁higher ▁spatial ▁frequency . ▁A ▁pattern ▁with ▁very ▁fine ▁details ▁will ▁produce ▁light ▁very ▁far ▁from ▁the ▁transform ▁plane ' s ▁central ▁spot . ▁In ▁the ▁example ▁above , ▁the ▁large ▁central ▁spot ▁and ▁rings ▁of ▁light ▁surrounding ▁it ▁are ▁due ▁to ▁the ▁structure ▁resulting ▁when ▁the ▁beam ▁passed ▁through ▁a ▁circular ▁a pert ure . ▁The ▁spot ▁is ▁en larg ed ▁because ▁the ▁beam ▁is ▁limited ▁by ▁the ▁a pert ure ▁to ▁a ▁finite ▁size , ▁and