How does off-axis holography separate the two reconstructed images?

In off-axis holography, the two reconstructed images—the real (conjugate) image and the virtual (primary) image—are separated by introducing the reference beam at an angle to the beam diffracted by the object.1

In contrast to in-line holography, where the images propagate in the same direction and overlap, the off-axis method ensures that the image-forming beams are inclined to each other. Specifically, these beams are separated by an angle that is twice the angle between the original diffracted beam and the reference beam used during recording. Because the light associated with each image propagates in different directions, the two images are completely separated, preventing the deleterious effects of one image appearing as an out-of-focus component over the other.1