This is why I decided to buy 850nm IR lasers instead of a different wavelength:
Color Film Negative Makes Visible Light Blocking Filter: http://www.discovercircuits.com/H-Corner/color-neg.htm
Since the camera in my touch table needs to only see IR light, I had to first remove its IR filter so it can see EVERYTHING, then use DEVELOPED film negatives to block most of the visible light. This is a common tactic in the community, but now behold… a graph.
The film negative has to be exposed and then developed. I held it up to a fluorescent light for 30ish seconds to effectively take a picture that was all white. Then when you get it developed, the negatives will be all black. I had no luck at Wal-Mart, they don’t have a film processing machine anymore, at least not the one I went to. CVS did it, but they did not understand why I didn’t want any pictures and only negatives.