Anyway moving on…
“Hologram” has become the term now used in lingua franca to describe the creation of a “Pepper’s Ghost”. This is a very old theatrical technique for placing a “ghost” image on stage, or into a set piece, or indeed a real environment.
You can witness this effect on a daily basis when you see a reflection in a window or glazed door (any bit of glass for that matter) that makes an object or person appear to be somewhere else. I see it in my lounge when the TV screen appears to be in our garden!!
Anyway, the point is that holograms are the creation of a seemingly 3D image in a 2D plane, using lasers, not film or video projectors. In contrast, Pepper’s Ghost is the creation of a 2D image in a 3D environment. If you want to find out more about real holograms follow this link to good old Wikipedia.
The so-called holograms that are created for music concerts, plays, and in museums and heritage attractions, are in fact just a “virtual image” created by a simple reflection of that image projected onto a screen (or on a TV monitor). A Pepper’s Ghost.
I say “simple”, but it isn’t as straight forward as that in creation, or in delivery. Angles of view have to be worked out, so that the image appears in the right place to the visitors’ viewpoint. Size of image must be calculated to ensure life-like representation.
Once these and other technicalities have been solved, the “ghost” will then be able to be seen to interact with its environment.