The Fulgurator. By using a flash detector and a high-powered flash of its own the Fulgurator can project images that are only seen when photographed. "This procedure is very inconspicuous, since it takes place within a few milliseconds." A short video of the Fulgurator in action shows its usage and the results.

Looks like he rigged up an optical slave behind some sort of screen that would make it project an image when triggered by someone else's flash. This is a neat hack in that the biggest downside of an optical slave is everyone else's camera flash triggers yours. I want to do something like this. But make it less gun/weapon looking so I wouldn't get arrested.