Julius von Bismarck uses his oddly named camera-mod to project images onto street furniture where they appear in the photos of strangers, but remain invisible to their eyes. How? It’s simple. The device has a slave unit on top which is triggered when it sees a flash fire. This triggers his own flash, which fires through the back of the camera, through a film slide containing his slogan and then on and out through the lens at the front. This works because a camera is pretty much a projector in reverse. And because the light-graffiti is fired at the exact same moment the unsuspecting victim takes a picture, it ends up in their photograph and paranoid mind ramblings result.
Check out the reactions to this project in this video:
Some examples guerrilla-art stunts by von Bismarck
Barack Obama's speech in Germany in front of Berlin's Siegessäule. July 24th 2008
"the Magritte Dove" on the Mao Zedong portrait at Tiananmen Square in Beijing