I can't help but wonder if the same ultra-short pulses of laser light can be used to vaporize dark pen ink, giving users the ability to erase or alter things like signatures.
There are other things for removing ink from paper, right? Counterfeiters do it, to forge cheques, I think.
I could see the laser maybe working on ballpoint ink, which AFAIK stays more on the surface due to its high viscosity, but I think like check washing techniques, it'd have trouble with the more liquid inks, since the paper absorbs them more readily.
While goofing around I discovered that I could vaporize the ink of dollar bills. The largest area per pulse was pretty small, 25 microns if I remember right. Maybe 100. So it wasn't very practical, but it worked.
I don't remember if I tried it with pen ink. It might work okay for ball-point as that mostly lays ink on top of the paper. You'd still have the impression in the paper from the pen though.
Ink that didn't soak in but lay on top of the paper would probably stand a good chance of working. Maybe gel ink?
Anyway, when I wrote that, I had an inkling that someone in HN would know probably something about the subject matter, but I never expected anyone to have real-world experience using pulse lasers to vaporize ink from a piece of paper, let alone dollar bills!
Awesome.
This is why I love HN.
I can imagine a group stealing a laser to remove the postmark. Luckily, UK stamps now have other security features.
(http://www.telegraph.co.uk/news/uknews/5400999/Royal-Mail-la...)