How are fingerprints identified?

How to identify fingerprints? Fingerprints are naturally formed in the process of human evolution. On the finger, sweat holes are distributed on the convex mastoid line, which are very dense. Small furrows have no sweat holes and will not secrete any substance. How to identify fingerprints?

How to identify fingerprints 1 The fingerprints of our fingers are unique at least at present, and no one with the same fingerprints has been found. So as long as the fingerprint is identified, the identity will be verified naturally.

For example, the first time we use a mobile phone or a punch card machine, we have to hire our fingerprints first. When inputting fingerprints, our equipment must first identify our fingerprints.

So far, there are three fingerprint identification technologies. The first one is a punch card machine and some mobile phones that support off-screen recognition. Optical fingerprint identification, optical fingerprint identification, is to illuminate our fingerprints by light, because our fingerprints are uneven, and then the light is reflected to the receiver, and the lines of our fingerprints can be obtained. On the left side of the picture, this is the structure of optical fingerprint recognition on the card punching machine. At present, the off-screen fingerprint identification on our mobile phone is based on the organic light-emitting diode screen. The fingerprint identification module that receives the reflected light is placed under the organic light-emitting diode, which is illuminated by the light emitted by the pixels on the screen of the organic light-emitting diode and reflected to the fingerprint identification module to identify our fingerprints (right).

The second is the capacitive fingerprint identification that we use every day now. Capacitive fingerprint identification is that there is a capacitive plate below. Because the fingerprints of our fingers are uneven, the capacitance of each capacitor plate will be different because of the distance from the plate when fingerprint identification is carried out. The place with large capacitance is a raised line, and the place with small capacitance is a sunken place, that is, the fingerprint line can be identified by capacitance.

Then the third is the ultrasonic fingerprint identification used by some mobile phones. This is similar to the familiar sonar system used in bats and ships. Generate ultrasonic waves and then receive the reflected sound waves to identify our fingerprints.

When the fingerprint identification system above identifies our fingerprints, the fingerprints obtained are similar to those above, but the identified fingerprint lines may not be easy to identify, so corresponding processing is needed to make the lines easier to identify. After the processing is completed, the fingerprint identification system will start to look for some features of our fingerprints, such as the beginning of asking for directions, the place where the lines diverge or a short section of such lines, as identification feature points, and the fingerprint identification system will complete the entry at this place.

When we brush our fingerprints again in the future, we will repeat the previous steps. Fingerprint identification will compare it with the fingerprint feature points we entered for the first time after identifying the fingerprint feature points again. As long as most of them can match the fingerprint feature points entered for the first time, they will pass the fingerprint verification.

How are fingerprints identified? The reason why two people left fingerprints:

Fingerprints are naturally formed in the process of human evolution. The first is obvious lines, which are visible to the naked eye. Such as paint, blood, ink and other items stained by hands, are usually printed on fingerprint cards as basic information.

The second type is molded patterns, which refer to fingerprints found on soft substances, such as candles and clay.

The third category is latent fingerprints, which are fingerprints formed by the transfer of human natural secretions such as sweat, which are not easy to be found by naked eyes and are the most common fingerprints at the crime scene.

Fingers first touch grease, sweat or dust, and then touch clean surfaces, often leaving potential fingerprints. Although these fingerprints are invisible to the naked eye, they can be revealed by special methods and special chemical reagents. The most common fingerprint that forensic doctors come into contact with is latent fingerprint.

If fingerprints are left on the surface of non-absorbent objects such as metal, plastic, glass and ceramic tiles, the inspection method will be easier. Usually, powder method can be used to select powder with large color contrast and scatter it on the surface of the object to extract a complete fingerprint; Another method is magnetic powder method, which uses fine iron powder particles as brushes and brushes them back and forth to show fingerprints.

There are usually four ways to display fingerprints:

First, iodine vapor method: smoke with iodine vapor, because iodine can be dissolved in the oil printed on the fingerprint, and the fingerprint can be revealed. This method can detect fingerprints from several months ago.

Second, silver nitrate solution method: spray silver nitrate solution on the fingerprint, and the sodium chloride printed on the fingerprint will be converted into insoluble silver chloride. After sunlight, silver chloride is decomposed into tiny silver particles, which will show brown-black fingerprints like photo studio photos. This is a common method in criminal investigation. This method can detect fingerprints earlier.

3. Organic chromogenic method: Because fingerprints contain a variety of amino acids, a reagent called dihydrostilbene trione reacts with amino acids to generate purple substances, so fingerprints can be detected. This method can detect fingerprints from a year or two ago.

Fourth, laser detection method: the fingerprint is revealed by laser irradiation. This method can detect fingerprints as early as five years ago.

How to identify fingerprints 3 fingerprints will not disappear as long as they are not wiped by water or cloth. Even if there is dust, there will still be fingerprints brushed off with a small brush. Fingerprints are traces of human skin oil on smooth objects. This is related to the decomposition speed of oil under natural conditions. It doesn't usually disappear in a few days.

Fingerprint heritage can be divided into positive fingerprint and negative fingerprint.

In most cases, residues belong to additional layers, such as sweat attached to or adsorbed on objects. Delamination is mainly caused by sweat sticking away dust.

From the time point of view, because the main components of sweat are water molecules and amino acids, even if water molecules evaporate, amino acids and other substances will remain, which is mainly reflected in fingerprints. If it is a dustfall fingerprint, it will take too long and be covered with new dust, so it is difficult to identify the texture of the fingerprint. If there is no new dust cover, there is no time limit.

So there is no time limit in theory, but it is also related to the environment.

When the fetus develops in the mother's body for three to four months, the fingerprint has been formed, but the child's fingerprint will change slightly during the growth period, and it will not be finalized until puberty is about 14 years old.

In the process of skin development, although the epidermis, dermis and matrix layer all grow together, the soft subcutaneous tissue grows faster than the relatively hard epidermis, so it will generate a continuous jacking pressure on the epidermis, forcing the slow-growing epidermis to contract and collapse towards the internal tissue, and gradually bend and wrinkle, so as to reduce the pressure exerted by the subcutaneous tissue.

On the one hand, it is hard to attack, on the other hand, it is forced to withdraw, which leads to uneven skin and lines. This process of bending and wrinkling fluctuates with the change of the upward pressure generated by the internal tissue, forming uneven ridges or wrinkles until the development process is terminated, and finally it is shaped into a fingerprint that will remain unchanged until death.

Age can be judged by fingerprints. When people are young, the palm area is also small and the fingerprint line is thin. We can judge whether this person is a child or an adult by the thickness and density of the lines. There are sweat glands in the skin of fingers, and sweat transports the waste of human metabolism to sweat pores through sweat gland ducts, and then excretes it.

On the finger, sweat holes are distributed on the convex mastoid line, which are very dense. But the small furrow has no sweat holes and will not secrete any substance. The secretions are all on this line. When touching something, sweat will be left at the place where it touches the mastoid line, and then fingerprint patterns will be left.

The so-called fingerprint reveals lifestyle, in fact, because sweat reveals lifestyle. For example, salty food is often eaten during this period, and the content of sodium chloride in sweat is high. This can reflect a person's eating habits.