There are four main types of fingerprint image acquisition technologies: optical fingerprint scanning devices (such as micro prism arrays), temperature difference sensing fingerprint sensors, semiconductor fingerprint sensors, and ultrasonic fingerprint scanning.
First, optical fingerprint recognition technology
The use of optical technology to capture fingerprints is the oldest and most widely used technology. Place the finger on the optical lens, and use the prism to project it on the charge-coupled device (CCD) under the illumination of the built-in light source, and then form the ridgeline (the line with a certain width and direction in the fingerprint image) is black and valley. The lines (recessed portions between the lines) are white, digitized, multi-gray fingerprint images that can be processed by the fingerprint device algorithm.
Optical fingerprint acquisition technology has obvious advantages: it has been tested for a long time, to some extent adapt to temperature variations, can reach a higher resolution of 500 DPI, etc., the most important is the low price. There are also significant disadvantages: due to the requirement for a sufficiently long optical path, a sufficiently large size is required, and excessively dry and excessively greasy fingers will also deteriorate the effect of the optical fingerprint product.
The limitations of optical fingerprint sensing are reflected in the potential fingerprints (potential fingerprints are left after the finger is pressed on the platen), which not only reduces the quality of the fingerprint image but also may cause two fingerprints to overlap, which is difficult to meet. Practical application needs. Also, platen coatings and CCD arrays can cause losses over time, which may result in reduced quality of captured fingerprint images. However, it has disadvantages such as the inability to perform living fingerprint identification and poor applicability to wet and dry fingers.
The optical fingerprint recognition system can only scan the surface of the finger skin or scan the surface of the dead skin because the light cannot penetrate the surface of the skin (dead skin layer), but it cannot penetrate the dermis layer. In this case, the cleanness of the finger surface directly affects the recognition effect. If there is more dust on the user's finger, an identification error may occur. Moreover, if people follow a finger and make a fingerprint model, it is also possible to identify the system, which is not very safe and stable for the user.
Second, temperature difference sensing identification technology
The temperature difference sensing technology is based on the principle of temperature sensing. Each pixel is equivalent to a miniaturized charge sensor, which is used to sense the temperature difference between a finger and the chip image area to generate a representative image information. Electrical signal.
Its advantage is that the fingerprint image can be obtained within 0.1s, and the sensor has the smallest volume and area, which is the so-called sliding fingerprint reader. The disadvantage is: subject to temperature limitations, the length of time, the finger and the chip are at the same temperature.
Third, semiconductor silicon technology (capacitive technology)
In the late 1990s, technology based on semiconductor silicon capacitance effects matured. The silicon sensor becomes one of the plates of the capacitor, and the finger is the other plate, which uses the capacitance difference between the and of the fingerprint line of the hand relative to the smooth silicon sensor to form an 8-bit grayscale image. The capacitive sensor emits an electrical signal that will pass through the surface of the finger and the layer of dead skin, directly to the living layer (dermis layer) of the finger skin, and directly read the fingerprint pattern. Due to the depth of the dermis, the sensor can capture more real data, is not susceptible to dust on the surface of the finger, improve the recognition accuracy, and effectively prevent identification errors.
Semiconductor fingerprint sensors include semiconductor pressure-sensitive sensors, semiconductor temperature sensing sensors, etc. Among them, semiconductor capacitive fingerprint sensors are the most widely used.
The semiconductor capacitive sensor determines the position of the position based on the magnitude of the capacitance formed by the and of the fingerprint and the semiconductor capacitive sensing particle. The working process is to pre-charge the capacitive sensing particles on each pixel to a certain reference voltage. When the finger touches the fingerprint representation of the semiconductor capacitor, since the is convex and the is concave, according to the relationship between the capacitance value and the distance, different capacitance values are formed at the and. The discharge is then performed using a discharge current. Since the capacitance values corresponding to and are different, the speed of discharge is also different. The pixels under the arm (high capacitance) discharge slowly, while the pixels under the arm (low capacitance) discharge faster. Depending on the discharge rate, the positions can be detected to form fingerprint image data.
Unlike optical devices, which use manual adjustment to improve image quality, the capacitive sensor uses automatic control technology to adjust the fingerprint image pixels and the sensitivity of the local range of the fingerprints and combines the feedback information to generate high-quality images in different environments. Due to the local adjustment capability, even images with poor contrast (such as areas where the finger is pressed lightly) can be effectively detected, and the sensitivity is increased for these pixels at the captured instant to generate a high-quality fingerprint image.
Semiconductor capacitive fingerprint sensors have the advantages of good image quality, generally no distortion, small size, and easy integration in various devices. The electronic signal emitted by it will pass through the surface of the finger and the layer of dead skin, reaching the living layer (dermis layer) of the finger skin, and directly reading the fingerprint pattern, thereby greatly improving the safety of the system.
The most important advantage of semiconductor silicon technology is the ability to achieve live fingerprint recognition. Better image quality than optical technology can be achieved on smaller surfaces, with resolutions of 200-300 lines on a 1 cm x 1.5 cm surface (smaller surfaces also result in cost reductions and can be integrated into more Small devices). Small size, low cost, high imaging accuracy, and low power consumption make it ideal for use in security and high-end consumer electronics. It is called the second generation fingerprint recognition technology after optics.
Fourth, ultrasonic technology
Ultrasonic fingerprint acquisition is a new type of technology. Its principle is to use ultrasonic waves to have the ability to penetrate materials, and to produce echoes of different sizes depending on the material (the ultrasonic waves are absorbed, penetrated and reflected differently when they reach different material surfaces). ). Therefore, by using the difference in skin impedance between the skin and the air, it is possible to distinguish the position where the fingerprint is located.
Ultrasonic technology uses an ultrasonic frequency of 1 x 104 Hz - 1 x 109 Hz, and the energy is controlled to the extent that it is not destructive to the human body (same intensity as medical diagnosis). Ultrasonic technology products can achieve the best precision. It requires less cleaning of fingers and planes, but its acquisition time will be significantly longer than the above two types of products, and it is expensive and cannot achieve live fingerprint recognition. Use is rare.
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