 For example the BIPS molecule(2H-l-benzopyran-2,2-indolines) can convert from trans to cis and back by absorbing a photon.The different forms are associated with different absorption bands.In a cis-form of BIPS, the transient absorption band has a value of 21050 cm-1, in contrast to the band from the trans-form, that has a value of 16950 cm-1.