Hidden Message Zombie Detector | Make:

This hidden message project uses a photography slave flash to briefly illuminate our message when triggered by a camera’s flash. The message is hidden behind colored gels so that it is not visible from the outside of the case. Since the slave flash lights up the message for such a brief period of time, you can’t really see the hidden message with your bare eyes. Fortunately, your camera not only triggers the slave flash to fire with it’s own flash, but it also takes a picture of the hidden message for you.