There’s no doubt that the Galaxy S10 is a sexy phone, and while the new hole-punched display has everyone talking, the in-display fingerprint sensor is also something to marvel at.

While all fingerprint sensors have one goal in common—to read the ridges and valleys on your fingerprint—each type attains that goal in vastly different ways. Unlike other in-display fingerprint sensors (like on the OnePlus 6T and Vivo X21, which use optical sensors), the Galaxy S10 and S10+ use a better, more reliable ultrasonic fingerprint sensor.

But before we dive deep into how this interesting technology works, let’s first take a look at some common fingerprint sensing technology that you’re already used to and see how they compare.

Capacitive and Optical Fingerprint Sensors

On most modern phones, you’ll either find a capacitive sensor or an optical camera sensor. The former is way more common because it’s much more difficult to fool than an optical sensor—pretty much every fingerprint sensor that isn’t built into the screen is a capacitive sensor.

Capacitive sensors use electrical current to read a fingerprint using an array of extremely tiny capacitors (hence the name) that each store a very small electrical charge. Think of this array like squares on a checkerboard, only on a minuscule level where each capacitor is smaller than the width of a fingerprint ridge.

iPhone home button fingerprint sensorEach capacitor includes two conductor plates. Wherever the ridges of your fingerprint rest on the sensor, the two conductor plates are connected, changing the amount of charge that that particular capacitor has. And wherever there’s a valley in your fingerprint, the charge in that specific capacitor remains unchanged. From there, the sensor can get an overall view of what your fingerprint looks like based on which capacitors have a specific amount of charge and which ones don’t. It’s nearly identical to how touchscreens work, but on a much more detailed level.

Optical fingerprint sensors use a simpler method that’s a bit easier to understand—they take a picture of your fingerprint. The sensor first shines a small LED light onto your fingerprint, and then a camera takes a quick snapshot capturing the light and dark areas which denote the ridges and valleys in your fingerprint, respectively.

Galaxy S10 ultrasonic fingerprint sensorWhile optical sensors are less secure than capacitive ones (since a simple photo can trick the sensor) they can be integrated behind a smartphone’s display, allowing for a bezel-less experience where the screen can take up the whole surface area of the phone.

The Latest Fingerprint Tech: Ultrasonic Sensors

The problems with optical sensors spurred Samsung and their technology partner Qualcomm in a new direction. After a lot of challenging R&D, Samsung thinks it’s found a way to make the experience better using ultrasonic technology.

Bats navigate quickly and precisely by timing how long their high-pitched sound waves take to return after bouncing off an object to figure out how far away it is. Ultrasonic technology mimicking this “feature” has been common in medical sonograms and underwater navigation for decades.

Galaxy S10 ultrasonic fingerprint sensorThe Galaxy S10 and S10+’s ultrasonic fingerprint sensor works in a similar fashion: it emits a high-pitched noise undetectable by human ears, bouncing sound waves off your finger and back into the sensor. The sensor measures how long the sound waves take to return from various locations—they’ll take longer to return if they bounce off a valley than a ridge in your fingerprint.

From there, the sensor creates a 3D topographic map of your fingerprint and use that to match it against the 3D fingerprint map that the phone has on file. If it’s a match, the phone unlocks.

While Apple has deemed fingerprint sensors old news, they’re still very much alive and well on Android devices, and Samsung wants to take it to the next level. While in-display fingerprint sensors—as a whole—seem to be fussier than capacitive sensors, in-display sensors may very well take over in the future, and ultrasonic technology could lead the way.

Craig is a staff writer at iFixit, as well as a general tech nerd, maker, fixer, DIYer, motorcyclist, party game enthusiast, baseball fan, crazy cat dad, and proud introvert.

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