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Q & A: seeing IR from remote control?

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Most recent answer: 04/12/2014
Q:
Visible light ranges from 380 to 750 nm. My TV remote control emits at 940 nm (according to specs) which is far beyon visible light. Under normal conditions, the LED doesn't appear to light up at all when I push buttons. However in a very dim lit room (like having my LCD screen turned on but showing black) I clearly see the LED lighting up and flickering rapidly with a very deep red hue when I push buttons. The light is best seen when I do not stare directly at the LED. I tried the experiment in a total dark room and the LED had less of a reddish hue and had more of a undescriptible hue (like a fuzzy mix of red, brown and gray). I also tried with another TV remote control of a different make, same results although the LED was a bit brighter. I am a early 30s young man with no family record of color blindness. I have now 2 questions : how is it that I'm able to see such a low frequency and why does the hue change according to the lighting of the room? Thank you!
- Anonymous
A:

That the hue changes depending on the background light is to be expected. Our eyes and brains do a very complicated sort of signal processing to create a sense of hue based not only on which of the color-sensitive cones are firing in some part of the retina but also on what the cones in surrounding areas are doing.

Now for the simpler question- why are you seeing anything at all? Published spectra of 950 nm LEDs have some width, but seem to fall to zero around 850 nm, still well above the nominal sensitivity of our eyes. 

So the question is whether the eye has a little sensitivity extending to lower frequencies, or the LED has a little emission extending to higher frequencies, or some combination. Here's how you might test. Get a few infrared low-pass filters with cutoff wavelengths in the 750-850 nm range. If there's a tail of emission with wavelengths under 850 nm, then viewing through a filter to block it will make the image much dimmer. If on the other hand your eye is actually sensing some light at wavelengths longer than 850 nm, the filter will make little difference. 

Mike W. 

 


(published on 04/12/2014)

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