Infrared invisible ink?

I've seen invisible ink pens and things for sale that contain ink that show up in infrared, instead of ultraviolet light.  That's really appealing to me for a number of project ideas because digital cameras pick up IR exteremely well (especially with their filter removed).

The problem is I think I'm going to want a lot of it, and those dumb little pens are crazily expensive.

So any of you chemistry buffs out there have a secret formula I could make?  I'd prefer something invisible in regular light, but hey, whatever you've got.

I make IR ink. I also have the filters needed to see the ink. 

Anyone had luck with making the IR ink?

But that did get me hammering on wikipedia more, where I found this: http://www.invisibleinkmarkedcards.com/marking-cards-invisible-ink.shtml

An ink that is invisible in normal light, but shows up under IR light?

Or an ink that is invisible until observed through an IR camera?

Do you have a link?

See if you can find out what is in the luminius ink pens (might find an MSDS or similar on manufacturer's web site) and buy the material(s) in bulk.