I went into a dark room with a tv remote control. Then I waited a couple of minutes until my eyes adjusted to the dark. Then I pressed a button on the remote and I could see a very faint flickering brownish glow eminating from the IR LED. I'm assuming this was just a sub-harmonic frequency generated by the non-linear characteristic of a diode, and it just happened to fall in the border of visible light. Anyway, this was very neat. You guys can try it.