An increasing number of phones including the Samsung Galaxy S10 seriesMate 20 ProOnePlus 6T and Xiaomi Mi 8 Pro all feature an in-display fingerprint reader. But how do they work, and where will we see them over the coming weeks?

Although the iPhone has moved to Face ID and others have experimented with facial and iris recognition, it seems that many of the big Android phones of the next year will stick with fingerprint ID as the main biometric entry point to your phone – even if they also hedge their bets in terms of supporting improved facial recognition as well.

Huawei and Oppo/OnePlus were first to seriously incorporate fingerprint sensors under the screen. They weren't first though - see Vivo’s Nex S. Let's look at the technologies involved in bringing under-display fingerprint readers to our phones. 

Optical vs ultrasonic

Most of the scanners we've seen so far - in phones like the Nokia 9 PureView - are optical scanners – these use some light to illuminate your finger. A tiny camera under the screen takes an image of your finger which is then compared to the stored image.

In-display fingerprint scanners will increasingly also be ultrasonic instead of optical and it's one of these that's used inside the new Galaxy S10.

These more expensive units work using ultrasound to build up an image of your fingerprint (yes, really) and work better with contaminated fingerprints - if they're wet or oily with sun cream, for example. They're essentially 'Face ID for your finger'. 

Qualcomm recently announced the third-generation version of its existing ultrasonic scanner at its annual Tech Summit. Called the 3D Sonic sensor, the tech is a new version of Snapdragon Sense ID which was originally shown off in 2015.

Why is ultrasonic better?

The road to ultrasonic fingerprint readers has been reasonably long. In 2013 Qualcomm acquired a company called Ultra-Scan, a small company with “very good IP for ultrasonic waveform” and with a background in producing ultrasonic readers for the US Government.

"We took that and found a way to manufacture in the millions to keep the costs down," explained Qualcomm's Alex Katouzian at its recent tech summit. "We don’t need to shine a light source through the display. The light source over time can degrade the LCD...it’s very similar to a photocopier.

Ultrasonic fingerprint readers are more accurate than their optical counterparts and more secure because of the 3D impression that the reader creates of your finger. It's not just an image as with optical. It's theoretically possible that you could fool an optical sensor with an image of a fingerprint. 

Current ultrasonics can penetrate through glass (up to 800 µm or microns thick) and metal (up to 400 µm). We expect Qualcomm's new sensor to be able to work through thicker materials but we haven't got confirmed specs as yet. 

Qualcomm certainly isn't the only name in fingerprint sensors though; Synaptics is another and their tech is behind Xiaomi's fingerprint sensor in the Mi 8 Pro (called Clear ID by Synaptics) as well as the previously-mentioned Vivo handset. 

Synaptics is a company that's been involved in computing and phone interfaces for more than 30 years; it developed the first computer touchpad and the touch tech for the click wheel on the iPod. Its tech is also used for many standard external fingerprint sensors on phones. 

Wider zone optical sensors

The game isn't over for optical: we're expecting plenty of phones to launch with cheaper optical sensors this year. In early January Oppo showcased a "wide zone" optical fingerprint sensor, which recognises an area up to 15 times bigger than current sensors. We're also expecting to hear more about this at next week's Mobile World Congress. 

That changes the game for optical since you can unlock or pay by being much less precise with your finger - and so use the phone one-handed. 

OppoIn-display Fingerprint Readers How Do They Work And What Phones Will We See Them In image 2

What are the latest fingerprint readers like to use? 

We do know that Huawei said its fingerprint reader features 10 level dynamic pressure sensing (dps) for improved accuracy. The Chinese handset maker says this can improve the unlock speed by up to 30 percent compared with the Mate 10 but it didn’t make any claims for improved accuracy.

In the Mate 20 Pro, our reviews editor Mike Lowe called it it the best fingerprint reader of its kind – it’s responsive and accurate. One thing that we did notice was tat it takes a while to set up; that’s because you need to enable it to read your fingerprint thoroughly to improve accuracy.

The OnePlus 6T fingerprint reader is equally impressive. As with the Huawei version, the setup feels a little involved as you need to enable the sensor to capture a lot of detail. But once done the experience is beautifully responsive - almost as much as a physical fingerprint reader. 

OnePlus told us: "The particular coloured light chosen has been tested and deemed the best for accuracy and the durability of the tech used has passed 300 thousand taps and has shown no sign of deterioration"