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How do fingerprint scanners work?

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

However, there isn't just one type:some scanners rely on light, others on electricity, and still others on sound to map the ridges and valleys of your fingers. Capacitive sensors (electronic sensors) are popular in smartphones because they are precise, small and fast, but optical and ultrasonic technologies have the advantage of enabling on-screen scanning. But what actually happens in those milliseconds right after you put your finger on a scanner?

The most basic type of fingerprint scanner works similarly to a digital camera. To simplify the process a bit, just take a picture of your finger and send it for processing. Here is the general idea:

  1. The scanner shines a bright light on your finger using LEDs and takes a photo.
  2. If the average pixel value of the photo is too dark or too bright, it adjusts the exposure and tries again.
  3. It also checks for proper resolution by checking if dark and light areas alternate in a consistent fashion with clearly defined peaks and valleys.
  4. If the image is clear, the image can be processed!

This type of scanner is most common in places like police stations, airports, and secure entrances, but it's not often used in smartphones, which favor smaller, more secure capacitive sensors.

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

Capacitive Fingerprint Scanners

When you place your finger on a capacitive scanner, it uses 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 under one of the ridges on your finger, it may hold more charge as it comes into contact with your skin. If it is below a valley, the capacitor will remain in contact with air, which does not allow it to store much additional charge.

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

Ultrasonic Fingerprint Scanners

If you can read fingerprints with light and electricity, why not sound? Ultrasound scanners emit a high-frequency sound pulse that then bounces from the user's fingertip to the receiver, a sensor capable of measuring mechanical stress. This sensor looks at the intensity of the return pulse at different points on the finger and can use this data to calculate where the peaks and valleys are. It's a bit like the way a bat navigates by emitting high-pitched noises and listening for echoes – the returned sound contains important data about distance and size.

Because sound waves can measure the depth of valleys in a fingerprint, the resulting map is 3D, making ultrasound scanners even more accurate and secure (albeit a bit slower) than capacitive models. They can also function as under-screen fingerprint scanners, as sound waves can easily pass through glass, making them a safer alternative to optical versions.

Analysis, storage and use of the fingerprint

Once the image is captured, whether by light, electricity, or sound, the software must check whether the fingerprint matches an authorized user. Determining fingerprint matches, whether you're a human or a computer, is largely done by looking for things called "minutiae" - points in the fingerprint where something relatively interesting happens, like a location. where the ridge lines end or divide.

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

Any data your phone stores about your fingerprint, even if it's just minutiae cards, is usually locked away in a secure environment, isolated from other apps that might try to access that information. Applications or websites that you use your fingerprint to access will never receive your fingerprint data. They will only receive confirmation from your device that the fingerprint that was scanned matches one of the stored ones.

Are fingerprint scanners secure?

Fingerprint scanners aren't the most foolproof way to control access to something, as they can often be fooled by good patterns, anything from a 2D image to a 3D prosthetic. They're getting better, and the security protocols for handling fingerprint data are pretty strong (as long as they're followed), but if you're really worried about someone physically gaining access to your phone, fingerprints aren't not the strongest lock. As ultrasonic sensors improve, they will likely become faster and more secure, and integrating them under the screen is a very nice advantage.

Image credits:Fingerprint (PSF), Crime Scene Fingerprint, Capacitive Fingerprint Sensor, Xiaomi Redmi Note 3, Fingerprint Reader, Qualcomm