These are some pictures that I took with a cheap webcam that I modified to work in infrared wavelengths. More information on how to build your own is available at the bottom of the page.
To my eye, I can't even see through most of the blue pen. The camera I'm using to take this picture must be more sensitive in the infrared than my eye is.
This is the same image in the infrared. The blue pen completely disappears. (The original picture was fuzzy and low-contrast to begin with, by the way.)
Can you decode the secret message here?
It's perfectly clear if your eyes are sensitive to the infrared. It was written with two different black pens which are indistinguishable to the naked eye, but have different infrared characteristics. (Has anyone looked at Kryptos in the infrared?)
How much fun would this be, really? This is the same "card" in visible and infrared light. Even if you could put a tiny mark on a card in an ink with different infrared properties, you could Stick It To The Man.
If you're going to make your own bills, be sure to check your work in the infrared. New U.S. bills use infrared-invisible ink in parts.
I made the infrared camera from a crappy old $20 webcam and a piece of ordinary film negative. It took me about 5 minutes. I learned how to do it from Geoff Johnson's Site. Make one. It's fun. Good science fair project, too.