Why is off-axis holography more versatile than in-line holography?

Off-axis holography is considered more versatile than in-line holography primarily because it effectively separates the reconstructed images from one another.1

In the original in-line system, the light diffracted by the object and the collinear reference beam propagate in the same direction. This results in the formation of twin images—a real image and a virtual image—that overlap, causing the light from one image to appear as an out-of-focus component when viewing the other. Consequently, in-line holography is largely restricted to objects with significant transparent areas.12

In contrast, off-axis (or side-band) holography introduces a separate reference beam at an angle to the diffracted beam. When the resulting hologram is illuminated, the image-forming beams are inclined to each other, ensuring that the light associated with the real image is completely separated from the virtual image. This separation eliminates the interference caused by the overlapping twin images, allowing for clearer, more versatile imaging of a wider range of objects, including those viewed in reflected light.1