▁one ▁direction ▁versus ▁the ▁opposite ▁direction . ▁ ▁This ▁effect ▁was ▁first ▁discovered ▁in ▁the ▁ 1 9 6 0 s . ▁It ▁has ▁been ▁observed ▁in ▁l ith ium ▁n io b ate ▁( Li N b O 3 ), ▁b arium ▁tit an ate ▁( B a T i O 3 ) ▁and ▁many ▁other ▁materials . ▁ ▁The oret ical ▁calculations ▁using ▁density ▁functional ▁theory ▁or ▁other ▁methods ▁can ▁predict ▁the ▁extent ▁to ▁which ▁a ▁material ▁will ▁exhib it ▁the ▁bulk ▁phot ov olta ic ▁effect . ▁ ▁Simple ▁example ▁▁ ▁Sh own ▁at ▁right ▁is ▁an ▁example ▁of ▁a ▁simple ▁system ▁that ▁would ▁exhib it ▁the ▁bulk ▁phot ov olta ic ▁effect . ▁There ▁are ▁two ▁electronic ▁levels ▁per ▁unit ▁cell , ▁separated ▁by ▁a ▁large ▁energy ▁gap , ▁say ▁ 3 ▁e V . ▁The ▁blue ▁ar rows ▁indicate ▁radi ative ▁trans itions , ▁i . e . ▁an ▁electron ▁can ▁abs orb ▁a ▁U V ▁phot on ▁to ▁go ▁from ▁A ▁to ▁B , ▁or ▁it ▁can ▁emit ▁a ▁U V ▁phot on ▁to ▁go ▁from ▁B ▁to ▁A . ▁The ▁pur ple ▁ar rows ▁indicate ▁non rad i ative ▁trans itions , ▁i . e . ▁an ▁electron ▁can ▁go ▁from ▁B ▁to ▁C ▁by ▁em itting ▁many ▁ph on ons , ▁or ▁can ▁go ▁from ▁C ▁to ▁B ▁by ▁absor bing ▁many ▁ph on ons . ▁ ▁When ▁light ▁is ▁sh ining , ▁an ▁electron ▁will ▁occasionally ▁move ▁right ▁by ▁absor bing ▁a ▁phot on