The Zombie Detector is another of my hidden message projects. This is a flash hidden message. A slave flash (used in photography) is hidden inside a prop "Zombie Detector" and held by the participant. When the test subject gets their picture taken the slave flash illuminates the "results" declaring them either Human or Zombie.
The pictures are posted to my Flickr account: http://www.flickr.com/photos/zombiedetector/
This project was also written up for Make Projects with new pictures and details:
Hidden Message Zombie Detector