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Absorbing visible light!

  1. Mar 26, 2005 #1
    I've seen those filters for cameras (etc.) which look black (because they don't pass almost no visible light), but they allow infra-red light to go right thorough it.

    What is it made of? (i tried to find out on my own using google; I tried to find out what elements absorb what wavelenghts, but I couldn't find it...)

    What substance does that (passes whatever and infra-red, BUT NOT visible light)? Can some of substances commonly found in household or whereever do that?

    ?How to improvise?
     
  2. jcsd
  3. Mar 27, 2005 #2

    brewnog

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    The black bits on the end of an exposed and processed photographic film work pretty well in the manner you've described!

    A few weeks ago I saw a guy who had replaced the filters in his digital camera (rubbish webcam) with a few pieces of exposed (black) photographic film, he got some really eerie images with it, especially when he used a TV remote control as a torch to illuminate his subjects!
     
  4. Apr 4, 2005 #3
    Wow. OK
    I'll try it (...and come to tink of it that I have had it handy all the time...).
    I'd never have thought of it.

    (...but I was experimenting with black marker and find out that it works very well indeed - I painted a piece of transparent plastic with it - both sides, and multiple times, so you can't see, not even a white wall with sunlight shining directly - right upon it, but camera sees right thorough it (black and white)).

    Nevertheless - what molecules have that exact properties (wavelenght absorption of visible light and reflection or passing of the infra-red).

    ONE EXPERIMENT: If you paint a magnifying lense with such a coating, would you be able to burn with it when you focuse a sun light? Or, on the other hand - if you would paint a concave miror could you start a fire with it? (as I understand almost 50% of sunlight is in infra-red spectrum)
     
  5. Apr 4, 2005 #4
    While this could be accomplished with series of molecules (say porphyrins), it is most easily done with semiconductors. A semiconductor absorbs "all" light with energy above the bandgap. And in fact silver oxide (from the over exposed film referred to above) is a semiconductor (Ebg=1.1 eV or 1127 nm). So "all" visible light will be absorbed by it while the near ir will be transmitted (well most of it imho nir starts ~750 nm).

    The solar spectrum is shown below (AM0). You can see that with a filter such as silver oxide you would loose ~75 % of the total energy available (e.g. you're not going to be toasting bugs with that magnifying glass).
     

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