I'll go out on a limb as say that outside is a very faint -maybe-. The optitrak cameras are infrared detectors; the cluster around the camera lens are hi out IR leds. Your two biggest problems would be consistent light intensities, and bleaching out of IR sources by ambient light (obviously this wouldn't work for anything other than an open [and probably roofed at the least] area. For something like being able to walk in the woods or around actual objects, you would need a lot more cameras and IR spotlights, or something like the Gypsy Gyro suit. Which is, shall we say, costly?). It might work on something like a solidly overcast day, but I would be highly doubtful about any other kind of exterior lighting conditions...