Absorbing visible light!

[SAZAR]

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?

[brewnog]

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!

[SAZAR]

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)

[DrMark]

Wow. OK


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



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).

Wow. OK


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)

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).