A model is presented in which a recorded and processed volume holographic grating and an 'effective diffraction grating.' The effective grating is determined by the grating vector after processing and has an effective thickness. This effective thickness can then be used to express the relation that exists between the components of the grating vector before and after processing. This thickness determines the direction of the reconstruction beam when Bragg's Law is complied with. Experimental results with holographic gratings are presented.© (1991) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.