Why must reconstruction light strike a hologram from the reference beam direction?

In holography, the reconstruction light must strike the hologram from the same direction as the original reference beam to correctly reverse the diffraction process that created the interference pattern.12

The hologram acts as a complex diffraction grating that contains the phase and amplitude information of the original object. When the coherent reconstruction beam illuminates the hologram at the same angle as the original reference wave, the fringes on the plate bend the light to recreate the original optical field. If the reconstruction beam were to strike from a different angle, the diffracted light would not accurately reconstruct the wavefronts, leading to distorted or improperly formed images. By matching the original reference beam's direction, the system ensures that the light associated with the virtual and real images is properly separated and reconstructed in three-dimensional space.12