In-display fingerprint readers: How they work plus optical vs ultrasonic

An increasing number of phones feature an in-display fingerprint reader. But how do they work and how is the tech evolving?

Although the iPhone has moved to Face ID on new models, all of the big Android phones offer 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 and they remain some of the key advocates. 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 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.

We had thought under-display fingerprint scanners will increasingly also be ultrasonic instead of optical and it’s one of these that is used inside the Galaxy S10 series. But the lack of takeup of these sensors does rather point toward the fact they’re too expensive to use.

Ultrasonic sensors work using ultrasound to build up an image of your fingerprint (yes, really) and work better with messy fingerprints – if your hands are wet or oily with sun cream, for example. They’re essentially ‘Face ID for your finger’. 

Qualcomm announced the third-generation version of its existing ultrasonic scanner – 3D Sonic – at its annual Tech Summit in late 2018. And it’s just revealed another generation on from that, called 3D Sonic Max (yes, another company using the well-trodden ‘Max’ suffix).

Alex Katouzian, head of mobile at Qualcomm, announced on stage that 3D Sonic Max offers a 17x larger recognition area than the previous generation, meaning there can be simultaneous two-finger authentication for increased security as well as being easier to use because you don’t need to be so precise on placement. Yes, you read that right – some phones will enable you to restrict access with dual fingerprints. 

The tech is the latest version of Snapdragon Sense ID which was originally shown off in 2015. Here’s the 3D Sonic Max sensor: 

Is ultrasonic better?

On the surface of it, the answer is yes. But it won’t matter if more handsets don’t take them up becuase of the extra cost. 

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 Katouzian. “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.

Qualcomm certainly isn’t the only name in fingerprint sensors though; Synaptics is another while Goodix also makes sensors for a huge number of Android devices including under-display sensors from Huawei, Vivo, Oppo, OnePlus and Xiaomi. Its tech can work under both LCD and OLED displays.

Indeed, there are around 100 devices currently using Goodix tech in their readers. 

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 (such as those on buttons). 

Wider zone optical sensors

We’re expecting plenty more phones to launch with optical sensors – the tech certainly isn’t going away. In early January Oppo showcased a “wide zone” optical fingerprint sensor, which recognises an area up to 15 times bigger than current sensors. 

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. 

Could we see this tech launch on a handset soon? 

  • Our guide to all the upcoming phones this year

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