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How Do They Work? Fingerprint Scanners

Updated: Nov 30, 2023

Nothing is completely secure. Locks can be picked, safes can be broken into, and online passwords can be guessed sooner or later.

How, then, can we protect the things that we value?

One way is to use biometrics—fingerprints, iris scans, retinal scans, face scans, and other personal information that is more difficult to forge.

Not so long ago, if you’d had your fingerprints taken, chances are you were being accused of a crime; now, it’s innocent people who are turning to fingerprints to protect themselves.

And you can find fingerprint scanners on everything from high-security buildings to ATM machines and even laptop computers. Let’s take a closer look at how they work!

Fingerprint scanner are basically of three types:

1) Optical.

2) Capacitive.

3) Ultrasonic.

1) Now First Is Optical Sensor:

Optical fingerprint scanners are the oldest method of capturing and comparing fingerprints.

Their technique relies on capturing an optical image photograph, and using ways and set of rules to detect unique patterns on the surface, such as lines or unique marks by analyzing the lightest and darkest areas of the image.


Just like smartphone cameras, these sensors can have a finite resolution, and the higher the resolution, the finer details the sensor can discern about your finger, increasing the level of security.

These sensors capture much higher contrast images than a regular camera. These scanners typically have a very high number of diodes per inch to capture these details up close.

Of course, it’s very dark when your finger is placed over the scanner, so optical scanners also incorporate arrays of LEDs as a flash to light up the picture come scan time. Such a design is a bit bulky for a smartphone though, where slim form factors are important.


The major drawback with optical scanners is that they aren’t difficult to fool. As the technology is only capturing a 2D picture, prosthetics and even other pictures of good enough quality can be used to fool this particular design. This type of scanner really isn’t secure enough to trust your most sensitive details.

2) Now Capacitive Sensor:

The most commonly found type of fingerprint scanner used today is the capacitive scanner. Again the name gives away the core component, providing you’re familiar with a little electronics, the capacitor.


Instead of creating a traditional image of a fingerprint, capacitive fingerprint scanners use arrays tiny capacitor circuits to collect data about a fingerprint. As capacitors can store electrical charge, connecting them up to conductive plates on the surface of the scanner allows them to be used to track the details of a fingerprint.

The charge stored in the capacitor will be changed slightly when a finger’s ridge is placed over the conductive plates, while an air gap will leave the charge at the capacitor relatively unchanged. An op-amp integrator circuit is used to track these changes, which can then be recorded by an analog-to-digital converter.

Once captured, this digital data can be analyzed to look for distinctive and unique fingerprint attributes, which can be saved for comparison at a later date.

What is particularly smart about this design is that it is much tougher to fool than an optical scanner. The results can’t be replicated with an image and is incredibly tough to fool with some sort of prosthetic, as different materials will record slightly different changes in charge at the capacitor. The only real security risks come from either hardware or software hacking.

Creating a large enough array of these capacitors, typically hundreds if not thousands in a single scanner, allows for a highly detailed image of the ridges and valleys of a fingerprint to be created from nothing more than electrical signals. Just like the optical scanner, more capacitors results in a higher resolution scanner, increasing the level of security, up to a certain point.

3) Now Ultrasonic sensor:

The latest fingerprint scanning technology to enter the smartphone space is an ultrasonic sensor, which was first announced to be inside the Le Max Pro smartphone. Qualcomm and its Sense ID technology are also a major part of the design in this particular phone.


To actually capture the details of a fingerprint, the hardware consists of both an ultrasonic transmitter and a receiver. An ultrasonic pulse is transmitted against the finger that is placed over the scanner. Some of this pulse is absorbed and some of it is bounced back to the sensor, depending upon the ridges, pores and other details that are unique to each fingerprint.

There isn’t a microphone listening out for these returning signals, instead a sensor that can detect mechanical stress is used to calculate the intensity of the returning ultrasonic pulse at different points on the scanner.

Scanning for longer periods of time allows for additional depth data to be captured, resulting in a highly detailed 3D reproduction of the scanned fingerprint. The 3D nature of this capture technique makes it an even more secure alternative to capacitive scanners.

4) Now working of fingerprint scanner on smartphone:

When You put Your finger on phone’s fingerprint scanner it is first sensed and readed and high quality image or fully detailed sensed data with all possible details ridges, pores and pattern and then formed highly detailed sensed image or sensed data and this image or sensed data stored by mobile for future purpose then when u put finger on Your phones fingerprint scanner sensor again then it matches from sensed data or image.

If it matches it give access to mobile and unlock the smartphone.

5) Why are fingerprints unique?

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Your fingerprints are like unique keys you carry everywhere. In theory, no-one else has the same prints as you.

The word “fingerprint” is a bit misleading: these ridges on the ends of your fingers and toes aren’t there to help people identify you, but to give you better grip on things you pick up or walk on.

It’s pretty obvious why we have fingerprints—the tiny friction ridges on the ends of our fingers and thumbs make it easier to grip things.

By making our fingers rougher, these ridges increase the force of friction between our hands and the objects we hold, making it harder to drop things.

You have fingerprints even before you’re born. In fact, fingerprints are completely formed by the time you’re seven months old in the womb. Unless you have accidents with your hands, your fingerprints remain the same throughout your life.

What makes fingerprints such a brilliant way of telling people apart is that they are virtually unique: fingerprints develop through an essentially random process according to the code in your DNA (the genetic recipe that tells your body how to develop). Because the environment in the womb also has an effect, even the prints of identical twins are slightly different.

While it’s possible that two people could be found who had identical fingerprints, the chances of this happening are so small as to be virtually negligible. In a criminal case, there are usually other pieces of forensic evidence that can be used with fingerprints to prove a person’s guilt or innocence beyond reasonable doubt.

Where fingerprints are being used to control access to something like a computer system, the chances of a random person having just the right fingerprint to gain entry are, generally speaking, too small to worry about—and much less the chance of someone guessing the right password or being able to break through a physical lock.

6) Enrollment and verification:

Suppose you’re in charge of security for a large bank and you want to put a fingerprint scanning system on the main entry turnstile where your employees come in each morning. How exactly would it work?

There are two separate stages involved in using a system like this. First you have to go through a process called enrollment, where the system learns about all the people it will have to recognize each day. During enrollment, each person’s fingerprints are scanned, analyzed, and then stored in a coded form on a secure database.

Typically it takes less than a half second to store a person’s prints and the system works for over 99 percent of typical users (the failure rate is higher for manual workers than for office workers).

Once enrollment is complete, the system is ready to use—and this is the second stage, known as verification.

Anyone who wants to gain access has to put their finger on a scanner. The scanner takes their fingerprint, checks it against all the prints in the database stored during enrollment, and decides whether the person is entitled to gain access or not. Sophisticated fingerprint systems can verify and match up to 40,000 prints per second!

7) How fingerprints are stored and compared:

When fingerprints were first used systematically for criminal investigation in 1900, by Sir Edward Henry of the Metropolitan Police in London, England, they were compared slowly and laboriously by hand.

You took a fingerprint from a crime scene and another fingerprint from your suspect and simply compared them under a magnifying glass or microscope.

Unfortunately, fingerprints taken under different conditions can often look quite different—the one from the crime scene is much more likely to be incomplete or smudged—and comparing them to prove that they are identical (or different) sometimes takes great skill.

That’s why forensic scientists (people who study evidence collected from crime scenes) developed a reliable system for matching fingerprints where they looked for between eight and sixteen distinct features. In the UK, two fingerprints need to match in all sixteen respects for the prints to be judged the same; in the United States, only eight features need to match.

When a computer checks your fingerprints, there obviously isn’t a little person with a magnifying glass sitting inside, comparing your fingerprints with all the hundreds or thousands stored in the database! So how can a computer compare prints?

During enrollment or verification, each print is analyzed for very specific features called minutiae, where the lines in your fingerprint terminate or split in two.

The computer measures the distances and angles between these features—a bit like drawing lines between them—and then uses an algorithm (mathematical process) to turn this information into a unique numeric code. Comparing fingerprints is then simply a matter of comparing their unique codes. If the codes match, the prints match, and the person gains access.

8) What happens during a Fingerprint scan?

Unlike ordinary digital photos, scans have to capture exactly the right amount of detail—brightness and contrast—so that the individual ridges and other details in the fingerprint can be accurately matched to scans taken previously.

Remember that fingerprints might be used as evidence in criminal trials, where a conviction could result in a long jail sentence or even the death penalty.

That’s why “quality control” is such an important part of the fingerprint scanning process.

9) Here’s how the process works with a simple optical scanner:

A row of LEDs scans bright light onto the glass (or plastic) surface on which your finger is pressing (sometimes called the platen).

The quality of the image will vary according to how you’re pressing, how clean or greasy your fingers are, how clean the scanning surface is, the light level in the room, and so on.

Reflected light bounces back from your finger, through the glass, onto a CCD or CMOS image sensor.

The longer this image-capture process takes, the brighter the image formed on the image sensor.

If the image is too bright, areas of the fingerprint (including important details) may be washed out completely—like an indoor digital photo where the flash is too close or too bright. If it’s too dark, the whole image will look black and details will be invisible for the opposite reason.

An algorithm tests whether the image is too light or too dark; if so, an audible beep or LED indicator alerts the operator and we go back to step 1 to try again.

If the image is roughly acceptable, another algorithm tests the level of detail, typically by counting the number of ridges and making sure there are alternate light and dark areas (as you’d expect to find in a decent fingerprint image). If the image fails this test, we go back to step 1 and try again.

Providing the image passes these two tests, the scanner signals that the image is OK to the operator (again, either by beeping or with a different LED indicator).

The image is stored as an acceptable scan in flash memory, ready to be transmitted (by USB cable, wireless, Bluetooth, or some similar method) to a “host” computer where it can be processed further.

Typically, images captured this way are 512×512 pixels (the dimensions used by the FBI), and the standard image is 2.5cm (1 inch) square, 500 dots per inch, and 256 shades of gray.

The host computer can either store the image on a database (temporarily or indefinitely) or automatically compare it against one or many other fingerprints to find a match.

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