In　this　paper,　it　is　experimentally　verified　that　the　refractive-index　modulation　of　the　existing　gratings　continues　to　increase　with　uniform　exposure　or　when　a　current　grating　is　being　recorded.　And　it　is　reasonable　to　assume　that　the　effect　of　a　sequential　recording　on　an　existing　grating　can　be　equivalent　to　the　uniform　exposure.　Our　simplified　model　for　grating　formation　in　photopolymers　is　extended　to　describe　the　effects　of　uniform　post-exposure　on　an　existing　hologram,　and　further　to　deduce　a　new　model　of　holographic　multiplexing　in　detail　for　calculating　the　exposure　schedule　for　multiplexed　gratings　with　uniform　diffraction　efficiency.　Twenty　gratings　were　multiplexed　using　the　exposure　schedule　calculated　with　the　uniform　post-exposure　model.　©　2009　SPIE.