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Fingerprint scanners have become ubiquitous in the last few years. With the exception of newer iPhones and Google's Pixel 4, the best smartphones of all shapes, sizes, and prices use a fingerprint reader to keep your phone secure. What you might not realize is that there are three commonly used types of fingerprint scanners found in phones — optical, capacitive, and ultrasonic. Let's break down how these technologies work and see why all three are still in use.

Optical sensors

As the name might suggest, an optical scanner is an imaging sensor that takes a 2D image of your fingerprint and uses algorithms to identify the unique patterns of your print. Despite being the oldest technology on this list, it only recently started to make its way into smartphones. A few obscure handsets in the early 2000s may have used them, but they were quickly replaced with capacitive hardware.

While optical fingerprint readers are (usually) fast, they aren't as secure as the others on this list. But if that's such a concern, why have they become so popular in recent years? Modern smartphones, whether you like it or not, tend to hide their fingerprint scanners away under the display. The superior capacitive sensors can't go under your screen yet, and ultrasonic technology is still relatively new and expensive. Thus the optical scanner saw a resurgence in popularity.

You can easily tell if your phone uses one of these readers by seeing if the screen lights up under your finger when unlocking the phone. The imaging hardware needs a lot of light to see your fingerprint in detail, so phones using this technology will crank up the screen brightness directly beneath your finger.

Capacitive sensors

The capacitive fingerprint scanner has been available on phones for a long time, but it was the iPhone 5S that made it popular. The iPhone had a faster and more reliable scanner that was actually enjoyable to use, rather than making your phone a chore to unlock. It took a while for other manufacturers to catch up, but after a year or two capacitive scanners were becoming standard equipment for flagship smartphones.

The capacitive sensor works very differently from its optical predecessor. Arrays of capacitor circuits that hold a charge are kept beneath a plate on top of the scanner. When your finger touches this plate, the ridges that make up your fingerprint will alter the stored charge in the capacitors underneath. The phone can then analyze the recorded data to create a map of your fingerprint. When you enroll your fingerprints, it keeps this set of information locally. From then on, every time a finger is pressed against the scanner, it'll compare the data against what it has stored, and if it matches, your phone will unlock.

Another advantage the capacitive scanner offers is tracking movement. As you drag your finger across the sensor, the phone can read the change in charge stored in the capacitors to track that movement. This capability allowed companies like Google to add gestures to their fingerprint readers, like swipe to pull down the notification shade.

Capacitive fingerprint readers are fast, reliable, and, most importantly, secure. The only downside is that you can't put them behind screens at this time, although some patents registered in 2020 suggests that might change one day.

Ultrasonic sensors

Ultrasonic fingerprint readers are the new kid on the block and are usually made by Qualcomm. Beneath the display sits an ultrasonic transmitter and receiver. When the phone detects a finger is touching the right part of the display, the transmitter emits an ultrasonic pulse against it, absorbing some of the pulses and bouncing the rest back to the receiver. It's similar to how a bat uses echolocation to navigate its surroundings or the sonar found on a submarine. The receiver isn't a microphone but a component that can detect the physical stress of the pulse bouncing back. Because the phone knows the characteristics of the pulse it sent and can measure what returns, it can piece this data together into a highly accurate 3D map of your fingerprint.

The 3D in that sentence is important, making these readers much harder to fool. It isn't impossible, as Samsung learned a few years ago, but by and large, these offer the most secure method of using fingerprints to authenticate your device. Another advantage is that these readers work better even if your hands are wet or oily. So long as your fingers aren't soaking, the scanner should work as usual, if slightly slower. This is a lifesaver in England, where raindrops on your screen are a common occurrence. One drawback is speed, especially on first-generation scanners found in phones like the Galaxy S10 and S20 series. We're only talking tenths of a second, but it was enough for people to notice.

Qualcomm mostly fixed the speed issue with the second-generation scanner found in the Galaxy S21, and the company significantly increased the size of scanner as well, making it easier to use without looking at your phone. It claims that the sensor can measure heart rate, BMI, and even blood-sugar levels using the scanner, although no device has made use of these capabilities just yet.

Despite the advantages ultrasonic sensors have over other in-display hardware, they aren't a common sight outside of Samsung phones, and even then, you'll only find them on the expensive S and Note series. Cost is the likely reason for this. Ultrasonic scanners are the newest of these technologies, and being new and bleeding edge incurs a cost. This is well highlighted by last year's Galaxy S20 FE. It kept a lot of the great specs of the regular S20 phones, but it didn't keep the ultrasonic scanner, instead switching to an optical reader. Hopefully, as the hardware matures and costs come down, we'll see them become more widespread.

Final thoughts

So, is there a winner? Is there one type of fingerprint tech better than the others? The truth is, they all have their strengths and weaknesses. Optical scanners are fast and reliable and can live under displays, but they're fooled rather easily. Capacitive readers are just as fast and are harder to fool, but currently can't be hidden underneath screens. Ultrasonic fingerprint readers like the ones Samsung use represent a good middle ground, with decent speeds and excellent security, all while working well sitting underneath glass — the perfect choice for any manufacturer if it wasn't for the high price.

Venn diagram of fingerprint scanners redux

With fingerprint scanners, you can currently only pick two

One thing that is clear is that fingerprint scanners aren't going anywhere. When Apple switched to Face ID on the iPhone X and Google gave it a go on the Pixel 4, it seemed like that could be the direction we'd be moving toward. But face unlock didn't take off in the way fingerprint biometrics did, and with rumors suggesting that Apple will eventually switch to an under-display fingerprint scanner, I doubt we'll be switching to another method of securing our devices any time soon. Hopefully, that means things will improve in the next few years, whether that's faster and cheaper ultrasonic hardware, more secure optical scanners, or some new technology we haven't seen yet.