How do fingerprint scanners work?

In the 2012 James Bond blockbuster Skyfall, our protagonist 007 is issued with a Walther PPK pistol that will only fire when he is holding it. Using a palm scanner, it can detect if Mr Bond is holding the gun or if it is an adversary, thus preventing it firing if used against him.

This type of technology isn’t restricted to Hollywood. Biometric scanners are found in laptops, police stations and mobile phones. These systems are able to verify your identity using a password that you can never forget, lose or guess – your fingerprint.

But how do they work?

Your fingerprint is like a one-of-a-kind roadmap. Formed by random genetic and biological processes during your development in the womb, your fingerprint is unique, even if you have an identical twin.

Up close, the finger looks like a topographic map of a mountainous region. It has long ridges, diverging at multiple junctions and spreading in multiple directions with valleys between.

The points where these ridge lines split are known as bifurcations, while specific shapes and features are known as minutiae. When a fingerprint scanner reads your finger, it is looking for these minutiae, which are then analysed using complex algorithms to determine if the fingerprint you just gave matches the identify you have provided.

How these scanners “see” your finger can vary. Two main types are used commercially: optical and capacitive scanners.

Optical scanners use a core component known as the charged coupled device, which is made of multiple light-sensitive diodes known as photosites. Each one of these “photosites” records the amount of light reflected by the ridges and valley of your finger over a tiny area.

Each photosite records one pixel of information, which is then translated into an electrical signal for a computer to process into a digital image. It’s the same technology used in digital cameras and recorders.

Once this digital image is taken, a computer will verify if the picture is worth analysing by checking the average darkness of the image, which can’t be too low or too high, and its sharpness. Sharpness, also known as definition, is determined by examining light variation in a series of crisscrossing vertical and horizontal lines.

The other type of scanner, known as a capacitive scanner, works in an entirely different way but produces the same digital result.

Our bodies naturally generate a tiny electrical charge, as anyone who’s ever rubbed a balloon against their hair will know.

When you press your finger on a flat surface, fingerprint ridge tops make first contact, followed by the valleys. When the surface is conductive, varying levels of charge in ridges and valleys are measured. This creates an electrical fingerprint.

Capacitive scanners are generally smaller than their optical counterparts, allowing them to be used in devices such as mobile phones.

One of the biggest drawbacks of optical scanners is that they struggle to differentiate between an image of a fingerprint and the actual finger. But capacitive scanners rely on electrical conductivity, making them much harder to fool.