The theoretical and experimental study of the diffraction efficiency of a volume transmission diffraction grating when illuminated by an arbitrary direction reconstruction beam was realized. It was found that on the contrary of what is usually expected, the diffraction efficiency could be increased by a rotation of the grating around its K vector, although the exact Bragg condition is no longer satisfied. Theoretically, the diffraction efficiency can be increased up to 100% for any grating, if we discard the reflection on the grating boundary, and absorption on the holographic media.