Tracking fingertips with lasers

Posted August 20, 2005

I held off on posting this for a few days to see whether it would burst across the gadget blogosphere, but it looks like it fell below the radar after all. That's really a shame, because this project scores quite highly on my cleverness scale.

The general problem here is how to track objects (such as people's fingers) in space to use them as user input. Conventional systems rely on cameras and a boatload of processing power and special algorithms to pick out a finger from a sometimes cluttered visual field. Throwing all that away, these guys did the same thing using just one photodetector — not even an imaging detector, just a brightness sensor!

To do this, they have a steerable laser which continually illuminates the fingertip, with the sensor recording the reflected brightness. The laser executes tiny, very fast circles around the fingertip, which means that when the finger moves a little bit, the laser will start missing it for some of the arc. The brightness sensor detects that dimmer reflection, and based on where in the circle the gap took place, it knows what direction the finger moved in: steer the laser to compensate, and bingo, you're tracking a moving object continually with just a brightness sensor.

This is, in my book, a massively clever trick, both for the particular application and as a general technique to file away in the back of your brain... what other situations might you be able to pull off the same sort of conversion between a complicated 2D problem and a simpler feedback problem in the time domain?