When you enroll your fingerprints, it keeps this set of information locally. The phone can then analyze the recorded data to create a map of your fingerprint. When your finger touches this plate, the ridges that make up your fingerprint will alter the stored charge in the capacitors underneath. Arrays of capacitor circuits that hold a charge are kept beneath a plate on top of the scanner. The capacitive sensor works very differently from its optical predecessor. 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 iPhone had a faster and more reliable scanner that was actually enjoyable to use, rather than making your phone a chore to unlock. The capacitive fingerprint scanner has been available on phones for a long time, but it was the iPhone 5S that made it popular. 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. 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. Thus the optical scanner saw a resurgence in popularity. The superior capacitive sensors can't go under your screen yet, and ultrasonic technology is still relatively new and expensive. 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. While optical fingerprint readers are ( usually) fast, they aren't as secure as the others on this list. A few obscure handsets in the early 2000s may have used them, but they were quickly replaced with capacitive hardware. Despite being the oldest technology on this list, it only recently started to make its way into smartphones. 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.