You’ve seen it in holographic displays in places like Disneyland’s Haunted Mansion, but home haunters are now using them for party and horror effects for the kiddies. These are so cool. It’s an interesting illusion that makes a ghostly image appear. I very much wanted to use one for my Halloween party for outside the house so it looked like a ghost inside was banging on the windows to get out and pleading and screaming, but I had no way to build up the necessary addition to house the effects.
This article gives you a great explanation of the process.
Now, if you haven’t been to Disneyland’s Haunted Mansion—enjoy the ride. Someone videotaped it and put it on YouTube.