You can buy an infrared ink pen for $18.43. To see the writing use your smartphone as an IR camera, which will reveal IR signals. On an iPhone 4 you can do that by using the front-facing camera.
Proposed uses, from the vendor of this pen, for IR ink:
- Marking items or bags that can be seen by security personnel without the target knowing. The target will not see the ink and will not see any special light such as a blacklight used in the more typical UV invisible inks. A security monitor can view marked items on a separate monitor discretely and securely.
- Writing and marking papers, maps, special correspondence and letters in a very secure method. It is highly unlikely that a third party will discover any writing of IR1 invisible ink.
- Marking items which need to be preserved in normal visual appearance but also need to marked for some other security concern.
The basic deal seems an improvement to the classic dumb kid’s toy “invisible ink”. It’s not secret writing, though, because anybody with a detector can read it.
You could use an augmented reality app on a smartphone or something like Google Glass app to show you an IR view of the world, and it would reveal things written in IR ink.