Selectively Visible Ink

This might be a hard one to pull off. How do you create ink that only some people, equipped with a certain gadget can see? If you're on a plane and you get an idea and want to write something, you don't want people watching. The very idea that someone might be able to see any of what you write is a distraction more than anything else. If you're putting a fancy head mounted display on the person, then it's not so hard at all.