ann ers ▁ ▁Las er ▁sc ann ers ▁work ▁the ▁same ▁way ▁as ▁pen - type ▁readers ▁except ▁that ▁they ▁use ▁a ▁las er ▁beam ▁as ▁the ▁light ▁source ▁and ▁typically ▁employ ▁either ▁a ▁reci proc ating ▁mirror ▁or ▁a ▁rot ating ▁pr ism ▁to ▁scan ▁the ▁las er ▁beam ▁back ▁and ▁forth ▁across ▁the ▁bar code . ▁As ▁with ▁the ▁pen - type ▁reader , ▁a ▁photo - di ode ▁is ▁used ▁to ▁measure ▁the ▁intensity ▁of ▁the ▁light ▁reflected ▁back ▁from ▁the ▁bar code . ▁In ▁both ▁pen ▁readers ▁and ▁las er ▁sc ann ers , ▁the ▁light ▁em itted ▁by ▁the ▁reader ▁is ▁rapidly ▁varied ▁in ▁bright ness ▁with ▁a ▁data ▁pattern ▁and ▁the ▁photo - di ode ▁receive ▁circuit ry ▁is ▁designed ▁to ▁detect ▁only ▁signals ▁with ▁the ▁same ▁mod ulated ▁pattern . ▁ ▁C CD ▁readers ▁( also ▁known ▁as ▁LED ▁sc ann ers ) ▁ ▁C CD ▁readers ▁use ▁an ▁array ▁of ▁hundreds ▁of ▁tiny ▁light ▁sens ors ▁l ined ▁up ▁in ▁a ▁row ▁in ▁the ▁head ▁of ▁the ▁reader . ▁Each ▁sensor ▁measures ▁the ▁intensity ▁of ▁the ▁light ▁immediately ▁in ▁front ▁of ▁it . ▁Each ▁individual ▁light ▁sensor ▁in ▁the ▁C CD ▁reader ▁is ▁extremely ▁small ▁and ▁because ▁there ▁are ▁hundreds ▁of ▁sens ors ▁l ined ▁up ▁in ▁a ▁row , ▁a ▁voltage ▁pattern ▁identical ▁to ▁the ▁pattern ▁in ▁a ▁bar code ▁is ▁generated ▁in ▁the ▁reader ▁by ▁sequ entially ▁meas uring ▁the ▁volt ages ▁across ▁each ▁sensor ▁in ▁the ▁row . ▁The ▁important ▁difference ▁between