Double exposure planar transmission holograms recorded in nonlinear dichromated gelatin. 
A coupled wave model is used to describe the replay of two transmission gratings (G(1) and G(2)) which have been recorded in dichromated gelatin at high exposure energies.  As a consequence of material nonlinearities, a third grating G(3) , which has a spatial frequency equal to the difference of the two primary recordings, is also recorded.  The model assumes multiple interactions between each of the three gratings and is used to match experimental data where the recording angles were (a) far apart so as to minimize interactions and (b) close together to enhance grating interactions.  Good theoretical agreement was found with experiment for both cases.