Vein viewer - GIF

[JTG]

Vein viewers usually use two lasers and a detector to build an image. First it scans an infrared laser across an area while a detector looks for any drops in the reflected intensity -- the blood in veins/arteries absorbs infrared light better than the surrounding tissue, so blood vessels within the first half-inch of tissue will show up on the detector. The scanner uses this information to build an image of the area, which it then projects back onto the same area using a visible red laser. You can look at veins in real time, and they're quite fun to play with.