A holographic display is a display that uses coherent light, such as that created by laser, to create a three-dimensional (3D) image in space.
True holographic displays create images that conform to the six depth cues by which we recognize 3D views in the real world:
- Perspective: Objects farther away appear smaller.
- Occlusion: A closer object may obscure objects in the distance.
- Stereoscopic vision: Two viewpoints get a different view of an image, as human eyes do.
- Motion Parallax: Perspective changes as a viewpoint moves.
- Convergence: Two points cross on viewing closer objects, as human eyes do.
- Accommodation: Lenses change focus depending on an object’s distance within the image.
Originally developed in the 1940s by physicist Dennis Gabor, holographic displays remain the holy grail of 3D image display. In recording a hologram, a split laser simultaneously illuminates the subject and the recording photographic plate, as a reference beam. In a space with no other light, a laser of a controlled wavelength flashes on the subject and plate, and the hologram is captured as a light field. Close inspection of the plate does not reveal the contents. The holographic image is revealed by lighting the plate with the same wavelength of cohesive laser light.
The first true commercial holographic displays are only now reaching availability and are generally of lesser resolution. There are a number of 360-degree virtual reality headsets that fill almost all of the cues by which we recognize depth, however most of these can't be called true holographic displays as they do not have true depth and consequently require eye focus changes.
Commonly, images created by holographic displays as well 3D images that are recorded onto typically 2D holographic media are called holograms. Technically, however, a hologram is the recording of a holographic image on a medium and an apparently freestanding 3D image created by a holographic display is called a holographic image.