How do fingerprint scanners work?

Chris Smith received this question from Scientizscht on The Naked Scientists forum which sparked another question from fellow panellist, Adam Murphy. It was over to technology journalist Tim Revell with an answer...

Tim - You get them on mobile phones, you also get them in police stations and they all rely on the fact that everyone has a unique fingerprint. And if you look your fingers you can see that there's a series of ridges and valleys that are characterised uniquely to you, and the way that any fingerprint scanner works is it tries to take an image of that in various different ways. So, for example, some fingerprint scanners are basically just cameras, they take a photo of the edge of your finger and then they compare it to other photos of other peoples' fingerprints.

But on your phone, for example, most phones use something called a capacitance scanner. And what this is, is it's loads of little squares in a grid that when you put your finger on them they produce a little current and the current reacts slightly different than it does in the ridges and valleys, and that produces measurement that it can use to sort of uniquely identify you.

Chris - Adam?

Adam - Is there any way to fool them like can you tape a bit of sellotape to someone's finger and put it on the scanner and break in?

Tim - The thing is fingerprints are very intricate. In theory it is possible to do that absolutely and the software that analyses fingerprint scans is not perfect, it doesn't do a perfect image comparison between you and whatever fingerprint it’s looking at. Especially because, when you're putting your fingerprint down on a fingerprint scanner you don't always use the same bits. You do it bit at a funny angle, and that gives a slightly different measurement. So that means, in principle, yes you can trick them but is really quite hard to imitate someone else's fingerprint.