How does a smartphone ‘read’ my fingerprint?

Phones use one of three different technologies to read your fingerprint: optical, capacitive or ultrasonic. An optical fingerprint reader is the oldest of the three. It uses a specialised miniature camera to take a picture of your finger, often backlit with little LEDs or the phone’s screen.

Unfortunately, these sensors are easy to fool – even a good photo can trick them, so it may be combined with a capacitive sensor, the second technology, to check there is really a finger there.

A capacitive fingerprint sensor uses a grid of tiny capacitors that store electricity, which is discharged only at the points where your fingerprint ridges touch. An array of thousands of capacitors can then be used to map the pattern of your fingerprint. Sometimes these sensors also support swipes or force sensing.

The third and most advanced form of fingerprint sensing uses ultrasonics. Much like the ultrasonic scanners used for medical purposes, an ultrasonic sound pulse is transmitted to your finger and the reflected pulse is measured. Bats, whales and dolphins use ultrasonic to understand the shape of their surroundings; the sensors on a smartphone use it to understand the 3D shape of the ridges in your fingerprint. It can even work through the phone screen.

Read more:

• How does the phone network know where to send the signal when someone rings me?
• Is there any point turning my phone to ‘flight mode’ on a plane?
• Are hackers monitoring me through my phone?
• Do phones get heavier with each download?

Asked by: Hamish Anderson, Shrewsbury

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