If you went to college around the same time that I did, you may have caught the last few professors that did not use power point for slides. Instead they used an ancient device called overhead projector which took letter (or A4) sized transparencies.
They were a behemoth of a device, capable of throwing an image across a huge auditorium. And a size to match. So did the heat produced and the noise from the fan. But in the hands of the skilled professor with a few markers those projectors were an efficient tool in hammering infinitesimal math into our plugged heads.
Those devices are obsolete now. Power point presentations and DLP projectors took their place. This is why hacking one into a pattern projector feels a whole lot better than killing an SLR. This is exactly what flickr user haristobald (blog) did and worked into a super heroes series. A strobe replaced the powerful lamp to throw a superhero icon on a wall, ceiling or even a person.
you can see a short BTS footage here
The device here
and more of the series here:
UPDATE: 2 more BTS videos from haristobald showing the use of the projector