It is well known that a thin phase grating holographically produced exhibits Raman-Nath behavior and a thick phase grating shows Bragg behavior in the diffraction process. In the Raman-Nath regime several diffracted waves are produced. Usually, in a photorefractive material such as a sillenite BSO crystal, volume phase holograms are stored by means of the interference of two coherent beams intersecting inside the crystal. In our work, we analyze the diffraction properties of gratings incoherently stored in a photorefractive medium. To this purpose, an input Ronchi grating is incoherently imaged in a thick BSO crystal. The grating is stored on a birefringence modulation basis. In the reconstruction step, when a collimated beam impinges perpendicularly to the crystal input face several diffracted orders appear in accordance with the Raman-Nath regime. As far as the read-out beam direction is rotated the diffraction efficiency of each order changes. The angular selectivity behaviour of the grating in terms of the crystal thickness and the grating period is analyzed. The adequate selection of the write-in parameters allow to highlight a determined order and to achieve multiple storage, without cross-talk.