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Visibility of IR light from remote controls

  1. May 22, 2010 #1
    So, I was playing around with remote controls last night and realized that, under the right conditions, I could see a very faint, blinking red glow from the LED that transmits signals to the TV when I held down a button. Looking this up, it appears that most such LED's shine at 940 nm, which is far outside the visible spectrum. What's going on here? Are those LED's not very monochromatic, or is it actually possible to see (however poorly) well into the near IR?
     
  2. jcsd
  3. May 22, 2010 #2
    The cones in your eyes have a relatively large response bell curve. The L type cone in your eye peaks at orange but detects a bunch of redder wavelengths. Humans can see up to 700 nm which we will see as dark red (this would be the edge of the L type cone response curve). Looking at some data sheets for IR LED's, there is also a bell curve of emitted wavelengths. Even though this curve peaks at non-visible near IR, the edge of the curve overlaps with the edge of the human L type cone response curve. So if you’re in a dark room with your eyes adjusted and the IR led is bright, the net result is you see a dim red light in the LED. I have seen this before so I know what you're talking about.
     
  4. May 22, 2010 #3
    Look at it through your camcorder; you'll see it VERY clearly....its very IR sensitive.
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