How does the fingerprint sensor work?

For decades, computerized fingerprint scanners have been the stuff of weird technology in spy thrillers, and until recently they weren't used as real devices. Over the past few years, these scanners have found their way anywhere from police stations and high-security buildings to laptop or phone keyboards. Therefore, in this article, we will explore the secrets behind this exciting development in better security and identity protection and talk about how does a fingerprint sensor work? in simple language.

Even the fingerprints of identical twins are different!

Fingerprints are one of nature's intricate and strange designs, which means that we humans have internal identification cards that Convenience is available. Fingerprints are known as small ridges of skin that are located on the fingers and during the process of evolution, they have never been removed from the structure of our body, because the pattern of ridges and valleys on the fingers makes it easier to use objects and hold them in the hand. In exactly the same way that the tread pattern of the tire tread helps the tire stick better to the road surface.

Like everything else in the human body, these protrusions are formed randomly in the fetus through a combination of genetic and environmental factors. In general, it can be acknowledged that although the genetic code in DNA gives the cells general instructions on How to form the skin while the embryo is developing, the specific way these effects are formed is the result of a series of random events. This means that the exact position of the fetus in the uterus at a particular moment and the exact composition and density of the amniotic fluid around it determine each individual bump and depression in the fingerprint formation. In addition to countless cases that primarily and from a genetic point of view play a role in the formation of a unique fingerprint, many environmental factors also affect its formation. Just like certain weather conditions that shape clouds into different structures, the entire process of fingerprint formation and development is so chaotic that there is virtually no chance of an exact pattern forming twice in the entire course of human history.

As a result, fingerprints are an element unique to each of us, so that even identical twins do not have identical fingerprints. While two ink fingerprints may look essentially the same at a glance, a trained researcher or sophisticated software can detect clear and distinct differences. The same fingerprint analysis is the main idea in the field of crime investigation and information security. In fact, the task of a fingerprint scanner is to take the place of a human analyst by collecting printed samples and comparing them with other recorded samples and be able to distinguish between different fingerprints.

Optical or optical fingerprint sensors How do they work?

Fingerprint scanning systems must perform two important tasks. First, they take an image of your finger, and then they must be able to determine whether the pattern of ridges and valleys in the image matches the fingerprint pattern in the pre-scanned images. In this regard, according to the advancement of technology, there are various ways to take a fingerprint image, the most common procedures include optical scanning and capacitance scanning. Of course, both of these scanners provide the same result, but the methods of doing this are completely different in each.

The heart of an optical scanner is a "charge coupled device" or (CCD). In general, a CCD can be considered an array of light-sensitive diodes called photosites, which produce an electrical signal in response to photons of light. During this process, each photocell in the CCD records a pixel (a small dot that represents the light that hit that point), and together, the light and dark pixels form an image of the scanned area (for example, a fingerprint). Typically, an analog-to-digital converter in the scanner system processes the analog electrical signal to create a digital representation of the image.

The scanning process begins when you place your finger on a special glass screen, after which a CCD camera begins to capture the fingerprint. In general, scanners have their own light source, which in most cases is a set of light-emitting diodes that illuminate the bumps on the finger. The CCD system actually takes an inverted image of the finger, meaning that Darker areas indicate more reflected light (finger bumps) and lighter areas indicate less reflected light (finger dips). However, before comparing the scanned image with the previously stored data, the scanner's processor makes sure that the CCD has captured the image clearly. In this way, the processor checks the average of dark pixels or the overall values of this group of points in a small part of the image and rejects it if the overall image is too dark or too bright. By passing the scanned image, the device adjusts the exposure time so that the light enters more or less in a proportional way. Then everything mentioned is repeated again until the image is approved by the processor.

A demonstration of the operation of the optical sensors when we place the finger on the glass screen of the scanner.

If the dark areas of the scanned image are sufficient, the scanner system checks the image resolution limit and The processor traces multiple straight lines horizontally and vertically across the image. If the fingerprint image has good resolution, the line perpendicular to the fingerprint ridges will contain alternating sections of dark and light pixels. All this mentioned process is done in less than a minute in front of us, and if the fingerprint is approved (in the entry and exit system or on the phone), everything goes according to the user's wishes.

How does the capacitive fingerprint scanner work? ?

Like optical scanners, capacitive fingerprint scanners produce an image of the bumps and depressions of the finger. But the approach of this group of sensors is that instead of using light, they use electric current. The diagram below shows a simple capacitive sensor consisting of one or more semiconductor chips made from an array of small units. In general, each unit in semiconductor chips consists of two conductor plates covered with an insulating layer. It is interesting to know that each of these units has a width equal to a bump on a finger.

The capacitive sensor is connected to an RC integrator circuit, such an electrical circuit built around an inverting operational amplifier. it's gonna be built. The inverse operational amplifier is a complex semiconductor device that consists of a number of transistors, resistors and capacitors, the details of the operation of this device are beyond the scope of this article; But in order to create a proper understanding of capacitive fingerprint sensors, we try to simplify the explanation as much as possible.

Like any amplifier, an inverting operational amplifier changes electrical current based on other current fluctuations. More precisely, this amplifier changes the supply voltage of the device. This change is based on the relative voltage of the two inputs of the device called "inverting terminal" and "non-inverting terminal". In this case, the non-inverting terminal is connected to the ground and the inverting terminal is connected to the main voltage source and the feedback circuit. It should be noted that the feedback circuit itself is connected to the output of the amplifier, which includes two conductor plates.

Simple Capacitive Sensor

As you may have noticed, two conductive plates form a basic capacitor, which is responsible for storing the baroelectricity. The finger surface acts as a third plate capacitor separated by insulating layers from the cell structure for the bumps and by air voids for the depressions. Changing the distance between the plates of the capacitor (by moving the finger closer or further away from the conducting plates) changes the total capacitance (the ability to store charge or charge) of the capacitor. For this reason, the capacitor under the protrusions of the finger has a greater capacity than the capacitor under the depressions.

For fingerprint scanning, the device's processor first closes the reset switch for each unit, to short the input and output of each amplifier and balance the RC integrator circuit. When the switch is opened again, the processor applies a constant amount of charge to the integrator circuit, the capacitors are charged. The capacitor capacity of the feedback circuit affects the input voltage of the amplifier, which naturally affects the output of the amplifier. Since the distance between the sensor plate and the finger is a factor in changing the capacitance of the capacitor, the ridges on the finger lead to a different voltage output compared to its depressions. In such a case, the processor of the capacitor scanner reads this voltage output and determines the peak or trough voltage. By reading all the voltages in an array of sensor units, the processor can piece together an overall picture of the fingerprint, similar to the image captured by an optical scanner.

It is interesting to know that the main advantage of the capacitive fingerprint sensor is that instead of the dark and light pattern that visually creates an image of the fingerprint, the capacitive system captures the fingerprint in a real way. Naturally, this makes it harder for people who want to hack into a particular system to trick the system. In addition, since these types of sensors use a semiconductor chip instead of a CCD unit, capacitive scanners are more compact and smaller than optical fingerprint sensors.

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How do they check and analyze fingerprints?

You must have seen in movies and TV shows that automated fingerprint analyzers usually scan different images of fingerprints to find a match. They put a special fingerprint on each other. In this regard, it is necessary to know that such a method is not a practical approach for comparing fingerprints with other data, because a small stain can make two similar images look the same. In addition, using the entire fingerprint image in comparative analyzes requires a lot of processing power.

Instead, in new ways, most fingerprint scanner systems capture specific features of fingerprints that are usually Known as details, they compare with each other. In general, researchers or computer analysis systems usually focus on the points of the fingerprint where the lines of ridges end or a ridge (branching ridges) is divided into two parts. In this regard, scanner software uses very complex algorithms to identify and analyze these details. In fact, the main idea in this category of algorithms is based on examining the relative positions of the effect with details inside it, which helps to better analyze and analyze the data.

Source: howstuffworks