How does the fingerprint reader in the phone work?

More and more smartphones today have a built-in fingerprint sensor, or "Fingerprint". Whether placed forward, backward, or sideways, the way they function is the same. This article simply explains how this sensor works.

The fingerprint works in much the same way as, say, that trackpad on a laptop, or any newer touch screen. These devices use capacitance to register a touch.

Capacitive plates

You don't have to understand exactly how this technology works, you just need to know that these touch-sensitive surfaces detect static electricity of your finger. At the point where the voltage changes, the processor calculates the X and Y coordinates and sends this information for further processing.

Now, the classic fingerprint sensor works the same way, except that instead of one larger capacitive surface, there are many smaller ones. These small touch-sensitive areas are located just below the layer of protection on which you rest your finger.

These tiny tiles are arranged next to each other in a "housing" and all together act as one plate. The tiles themselves, and the distance between them, are smaller than those lines that make up a fingerprint. As you know, a fingerprint is nothing but a unique pattern on the fingers that consists of indentations and protrusions.

When you rest your finger on the sensor, those bulging lines touch those tiny capacitive plates, while the indentations do not. The sensor thus "knows" which tiny tiles are touched and which are not. Based on this, a "picture" of the fingerprint is created.

So, here we do not have the X and Y axes, but simply the state "YES" or "NO", or "Touches" or "Does not touch".

Precision

This is the basic principle of operation of these standard sensors, but the software also plays an important role in the whole process. When you add your fingerprints to the database, the phone uses information from the sensor, but also additional functions that create a clear and precise image.

If you remember when you added your fingerprint you had to put your finger on the sensor several times, each time from a different angle. This is so that the phone can take a picture of the entire fingerprint (since the sensor is smaller than the fingerprint), and also create a more accurate image. This increases accuracy but also reduces safety.

Ultrasonic sensors

So far, I've written "standard" sensors because I mean these capacities that are used in all today's smartphones. However, these sensors have limitations, so we are working on alternative solutions.

The limitation is that they take up a lot of space because they have to have direct contact with the finger. They cannot be placed under the phone's screen because the screen itself is touch-sensitive and uses the same technology. The solution is ultrasonic sensors.

These sensors work by emitting ultrasonic waves that bounce off the surface of the finger and return back to the sensor. It then calculates exactly where the signal "hit" and how long it took it to return to the sensor. When all this is taken into account, a fingerprint image can be created.

They can also be placed below the phone screen because ultrasonic sensors can be emitted through the screen. They also have an advantage in terms of reliability because they can read both wet and dirty fingers, and are additionally protected by glass.