Home > Tech Explained

How Do Fingerprint Scanners Work?

By Andrew Braun
Fingerprint Scanners Feature 2

Since fingerprint scanners first appeared on smartphones in 2011 they’ve become pretty much a standard feature. They’re fast, convenient, and relatively secure, since fingerprints are unique enough that the odds of anyone having a similar enough print to unlock your phone are very low, unless someone cares enough to design a convincing duplicate of your fingerprint.

There’s not just one type, though: some scanners rely on light, others on electricity, and still others on sound to map the ridges and valleys of your fingers. Capacitive (electronic sensors) are popular in smartphones because they’re accurate, small, and fast, but optical and ultrasonic technologies have the advantage of enabling in-display scanning. But what’s actually going on in those milliseconds right after you put your finger on a scanner?

Fingerprint Scanners Fingerprint

Optical fingerprint scanners

Fingerprint Scanners Optical

The most basic type of fingerprint scanner works along the same lines as a digital camera. To oversimplify the process a bit, it just takes a picture of your finger and sends that picture off for processing. The following is the general idea:

  1. The scanner shines a bright light onto your finger using LEDs and takes a picture.
  2. If the average pixel value of the photo is too dark or light, it adjusts the exposure and tries again.
  3. It also checks for good resolution by seeing if dark and light areas are alternating in a way that’s consistent with clearly-defined ridges and valleys.
  4. If the picture is clear, the image can be processed!

This type of scanner is most common in places like police stations, airports, and secured entrances, but isn’t often used in smartphones, which favor smaller, more secure capacitive sensors.

The exception is smartphones that have in-display fingerprint scanners, some of which use optical or ultrasonic sensors under the screen to get an image. Even here, though, optical sensors tend to lose out to ultrasonic scanners, which are slower but have the advantage of not being vulnerable to 2D images of fingerprints.

Also read: 4 Questions About Fingerprint Authentication Answered

Capacitive fingerprint scanners

Fingerprint Scanners Capacitive Diagram

When you put your finger on a capacitive scanner, it’s using tiny amounts of electricity to measure the distance between different parts of your finger and the scanner. Inside the scanner are rows of tiny capacitors, each of which can carry an electrical charge. If one of the capacitors is below one of your fingertip’s ridges, it can hold more charge since it’s coming into contact with your skin. If it’s below a valley, the capacitor will remain in contact with air, which doesn’t allow it to store much extra charge.

Fingerprint Scanners Capacitive Example

You can somewhat think of these capacitors as pixels in a camera. If one is holding more of a charge, that can be interpreted as a ridge area, while the others are read as valleys. Taken as a whole, this gets you an image of a fingerprint that’s just as accurate as one from an optical sensor and even more secure, since tricking it would require a 3D model of the fingerprint rather than just a 2D one. This, combined with its smaller size, makes it a popular choice among smartphone manufacturers.

Ultrasonic fingerprint scanners

Fingerprint Scanners Qualcomm Ultrasonic

If you can read fingerprints with light and electricity, why not sound? Ultrasonic scanners fire out a high-frequency pulse of sound that then bounces off the user’s fingertip back to the receiver, a sensor that can measure mechanical stress. That sensor is looking at the intensity of the returning pulse at different points on the finger and can use that data to calculate where the ridges and valleys are. It’s a little bit like the way a bat navigates by making high-pitched noises and listening to the echoes – the returning sound carries some important data about distance and size.

Because sound waves can measure the depth of the valleys in a fingerprint, the resulting map is 3D, which makes ultrasonic scanners even more precise and secure (if a bit slower) than capacitive models. They can also work as under-display fingerprint scanners, as the sound waves can easily travel through glass, making them a more secure alternative to optical versions.

Also read: What You Need to Know About USB Fingerprint Readers

Analyzing, storing, and using the fingerprint

Fingerprint Scanners Analysis Encoding.png

After the image is captured, whether through light, electricity, or sound, the software needs to check if the fingerprint matches with an authorized user. Figuring out fingerprint matches, whether you’re a human or a computer, is largely done by looking for things called “minutiae” – points of the fingerprint where something relatively interesting happens, like a place where ridgelines terminate or split.

Each of these features is assigned a position relative to the other detected minutiae, and using the distance and angle between each item, the scanner software can make sort of a map that can be represented as a number. That number is essentially the encoded fingerprint.

Any data that your phone stores about your fingerprint, even if it’s just the maps of minutiae, is generally locked away in a secure environment, isolated from other apps that might try to get access to that information. Any apps or websites you use your fingerprint to access won’t ever actually receive your fingerprint data. They’ll only get a confirmation from your device that the fingerprint that was scanned matches one of those in storage.

Are fingerprint scanners secure?

Fingerprint scanners aren’t the most foolproof way to control access to something since they can often be tricked by good models, anything from a 2D picture to a 3D prosthetic. They are improving, and the security protocols for handling fingerprint data are fairly strong (as long as they’re being followed), but if you’re really worried about someone getting physical access to your phone, fingerprints aren’t the strongest lock. As ultrasonic sensors improve, though, they’ll probably get faster and more secure, and having them embedded under the display is a very neat perk.

Image credits: Fingerprint (PSF), Crime scene fingerprint, capacitive fingerprint sensor, Xiaomi Redmi Note 3, fingerprint reader, Qualcomm

Subscribe to our newsletter!

Our latest tutorials delivered straight to your inbox

Andrew Braun Avatar
Andrew Braun
Andrew Braun is a lifelong tech enthusiast with a wide range of interests, including travel, economics, math, data analysis, fitness, and more. He is an advocate of cryptocurrencies and other decentralized technologies, and hopes to see new generations of innovation continue to outdo each other.

Read next

Bright modern laboratory with computers and technical equipment for research and analysis.
ARPANET sent its first message on 29 October 1969 from a lab at UCLA to a machine at Stanford, and the message was supposed to read ‘LOGIN’ — but the system crashed after the L and the O, meaning the first word ever transmitted over the network that became the internet was, by accident, ‘LO’.
In 1995, Microsoft shipped a cartoon-house interface called Bob, led by Melinda French, who married Bill Gates while it was in development — it demanded twice the memory of a typical home PC, sold roughly 30,000 copies, and was dead within a year, leaving behind the font Comic Sans and the animated assistant that became Clippy.
Stunning underwater shot of tiger sharks swimming among fish in the ocean depths.
The Greenland shark grows about one centimetre a year, does not reach sexual maturity until around age 150, and a specimen carbon-dated by Danish researchers in 2016 was estimated to be at least 272 years old, meaning it was already swimming the North Atlantic when Mozart was composing symphonies.
A person using a smartphone with a green screen surrounded by fresh fruits and vegetables in the kitchen.
When Apple shipped iOS 12 in June 2018, a small feature called Screen Time slipped onto every iPhone with a counter nobody had quite prepared for — a tally of pickups — and within a day Tim Cook was telling CNN the number of times he picked up his own phone was simply too many
Starry night with radio towers and the Milky Way galaxy visible overhead.
When NASA lost contact with the IMAGE satellite in 2005, an amateur radio operator in Canada named Scott Tilley picked up its signal in January 2018 while hunting for a classified spy satellite, and the spacecraft turned out to be still spinning, still powered, and still trying to phone home after 13 years of silence.
Close-up of a smartphone highlighting its modern design on a dark background.
The original iPhone Steve Jobs unveiled in January 2007 could not record video, could not copy and paste text, could not run a single third-party app, and could only reach the internet over 2G — and Jobs spent ninety minutes on stage at Macworld arguing, one missing feature at a time, that every absence was actually a design decision.
In 1965, Joe Sutter’s Boeing team began shaping the 747 around a future they thought would belong to supersonic jets, lifting the cockpit onto a hump so the nose could open for cargo once the giant subsonic passenger plane had outlived its brief moment
Vintage keyboard with tactile buttons paired with a modern digital interface on screen.
Apple’s original 1984 Macintosh keyboard had no arrow keys, no function keys, and no numeric pad because Steve Jobs wanted users to reach for the mouse first. Then Apple quietly sold the missing keys as an accessory.