Optical or ultrasound scanners: how they work on phones


Before Apple introduced us to fingerprint unlocking with the iPhone 5S in 2013, we all relied on a PIN or long passwords. The Android world quickly embraced the trend and it quickly became a staple feature even on budget smartphones. Then came the iPhone X with Face ID to trigger conflicts of interest.

Other phone makers have started looking for alternatives due to the higher price, complexity, and the fact that Face ID setup requires a notch in a device. This gave birth to the trend of the in-display fingerprint scanner. There are generally two types of in-display fingerprint scanners that we’ll be looking at; Optical and ultrasound scanners.

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On the surface they work the same but below the two systems and their protection are radically different in terms of unlocking. Let’s take a look at each of them so that you can choose what is right for your use.

Read also: Galaxy S10 ultrasonic fingerprint sensor has a BIG flaw

How they actually work

The whole process for both sensors is simple and similar. First, a light tap on the screen is used to finish capturing fingerprint data during setup. The processing time on the two sensors is generally the same, but the underlying function is entirely different. The optical sensor captures the 2D image of the fingerprint during setup and stores the data on the device.

The screen will light up to illuminate the fingerprint when you tap the screen to authenticate your identity. The fingerprint image of your screen is taken by a small camera behind the screen. The image is then compared to the recorded image. On the other hand, as the name suggests, the ultrasonic fingerprint scanner uses very high frequency sound.

Sound waves are used to map the fingerprint data of users. The complete kit contains a transmitter as well as a receiver. Some of the pulse pressure is absorbed when a user sweeps their finger across the glass, but some bounces back toward the sensor. The data captured is made up of ridges, pores, and other characteristics unique to each fingerprint.

The mechanical stress can be measured by the sensor via the force of the ultrasonic pulse returning to various points of the scanner. This results in a very detailed 3D reproduction of the scanned fingerprint. This data is used to match existing fingerprint data captured during setup to authenticate and unlock the device.

Read also: Why You Shouldn’t Use Fingerprint / Touch ID and Face ID

Optical or ultrasonic scanners: speed and precision

First, let’s highlight the fact that these in-display fingerprint scanners lag behind their physical (capacitive) counterparts. These are mostly found on the back or side of older devices. Ultrasound has a higher edge compared to optics. Remember that the optical fingerprint scanner must produce an accurate 2D fingerprint image.

As a result, more pressure should be exerted on the screen than normal. It’s not disappointing but coming from a physical scanner it might seem slow. The ultrasonic fingerprint reader only needs pulse data and therefore only a light touch is needed to validate the identity of the user. Qualcomm claims there is a 250 millisecond delay to activate their ultrasound scanners.

The optical scanner can achieve several successes or failures when it comes to accuracy. The user may have pressed less or more and the camera may have failed to create a detailed 2D image to unlock the device. Often times, the optical scanner will not work with wet fingers because the reproduced 2D image will not be as accurate with the moisture in it.

The ultrasound scanner, on the other hand, is a bit more powerful; it works great with wet fingers. Qualcomm says its sensor has an error rate of around 1%, which is perfectly acceptable with current standards.

Read also: Palm Unlock: Another Tech Not Worth Exciting?

Optical or ultrasonic scanners: safety

The optical scanner uses a 2D fingerprint image to authenticate the user, which poses security concerns as it can be bypassed without much effort. The ultrasound scanner only confirms to the user with the detailed 3D model of the fingerprint. Which is based on the pulse, ridges and pores. It is as secure as Face ID.

Read also: Face ID is not yet fully secure. Here’s why your brother / sister can probably unlock your phone

What devices use on-screen scanners

Because it is rather a cheaper alternative to the ultrasound scanner, there is a wide range of devices that switch optical scanners. Almost all Chinese smartphone makers, including VIVO, Oppo, Xiaomi, OnePlus, and Huawei, ship their flagship phones with optical scanners. Even the upper midrange models from Oppo, Vivo, and Samsung have built-in optical scanners.

Optical or ultrasound scanners

The ultrasonic fingerprint scanner only starts to support the Snapdragon 855 platform. And because it’s complicated and expensive, only high-end flagships use it. Typical examples of flagships that use ultrasonic scanners are the Samsung Galaxy S20 and Note 20 series.

Read also: Complete guide to biometric authentication of smartphones (fingerprints, iris, voice, face)


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