Many people think any 3D image is a hologram – but most 3D images are not holograms at all.
Holograms are a specific method of storing light wave interference patterns on, say, 2D film, in a such a way that when a light passes through the film, the original true 3D image is recreated. A 3D hologram can viewed – glasses free – from any position. It’s not a stereoscopic image.
Researchers are developing true holographic displays based on acoustically modulated crystals that change their refraction. It’s a very clever idea.
Cheap Holograms Could Give Rise to Glasses-Free 3D TVs.