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?