There is a quirk and it is called frequency doubling in optically
nonlinear substances. I understand that things are not that simple in
reality so it's more like in lasers, i.e. there is a third level involved,
but afaik blue diode lasers are one of the applications. Also Polaroid or
Kodak make some sort of lenses that convert IR to visible (yellowish)
light. They have been making them for a while now.

Peter

--
http://www.piclist.com hint: The PICList is archived three different
ways.  See http://www.piclist.com/#archives for details.