Infrared retroreflectors for location marking

Posted January 12, 2006

I got my hands on the proceedings from the ISWC 2005 conference, and while not a lot blew me away in wearable computing developments, there were a couple of neat ideas worth mentioning. A poster from Nakazato, Kanbara, and Yokoya addressed the problem of indoor location-sensing and general tagging of objects.

Without a view of the sky, GPS is problematic, so the Media Lab group typically used little Squirt tags, which were small active infrared transmitters that periodically chirped a unique identification code and then went back to sleep to conserve power. If you were in the vicinity (generally they were mounted on the ceiling, so if you were standing under them), your wearable could easily read off your location from the known tag codes in a second or two.

The infrastructure for this was annoying, however: either you had to wire them into a permanent power system, or go around and replace the batteries every couple of weeks. Bad.

Nakazato et al. have a passive approach instead, putting patterned translucent retroreflectors on the ceiling. To the naked eye they're effectively invisible, but if your wearable has an upward-pointing infrared LED and camera, they shine like beacons in the reflected light, and some simple image-recognition can extract a tag value. No infrastructure costs beyond one-time plastering of the retroreflective placards. Nice.