During the pontifical mass at the 2011 World Youth Day in Madrid, Santiago Sierra used a photographic camera developed by Julius von Bismarck to project the letters NO onto the ongoing ceremony as well as pilgrims and policemen guarding the event against protestors. Projected as a flashlight, the word NO was visible only when the camera was triggered—and only to this camera and others triggered at the same time, but not, as a lasting phenomenon, to human eyes, and in particular not to those of the security team.