Fingerprint image acquisition technology there are four main types: optical scanning device (such as micro prism matrix), ultrasonic fingerprints scanned, temperature induction fingerprint sensor, semiconductor fingerprint sensor. To explain some of the types below:
A, optical recognition technology
Using optical technology of fingerprint is the oldest, the most widely used technology. Put the finger on the optical lens, finger under the built-in light irradiation, the projection with a prism on the charge-coupled device (CCD), and ridges (in the fingerprint image has a certain width and direction of the grain line) in black, valley lines (grain line between the concave part) with a white, equipment can be fingerprint algorithm of digital processing of gray-scale fingerprint image.
Optical fingerprint technology has obvious advantages: it has been a long time to the application of the test, to some extent, adapt to the temperature variation, can achieve higher resolution of 500 dpi, the most main is cheap. Also have obvious disadvantages: as for enough long optical path, so the size of the demand is large enough, and too dry and too greasy fingers also will make the effect of the optical fingerprint products go bad.
Optical fingerprint sensor limitations embodied in latent fingerprints aspects (potential fingerprint is finger press after left on the counter), not only can reduce the quality of fingerprint image, serious when still can lead to two fingerprints overlap, obviously, is difficult to meet the needs of the practical application. In addition, bedplate coating and CCD array can produce loss over time, may be led to the decrease of the acquisition of fingerprint image quality. But has not in live fingerprint identification, poor applicability of dry wet finger defect.
Optical fingerprint recognition system consists of light cannot penetrate the skin surface (fatal skin layer), so only able to scan the surface of the skin, finger or scanning to the deadly skin layer, but not deep into the dermis. In this case, the clean degree on the surface of the finger, directly affect the recognition results. If the user finger stick more dust, identify the error may happen. And, if people in accordance with the finger, make a fingerprint, fingerprint may also by identifying system, for the user to use is not very safe and stable.
To happen at the end of last year as the prison and the case of Inner Mongolia, prison inmates is to cut down the c.o. finger optical fingerprint verification machine to open the prison door. And each big media reported recently, spend 100 yuan on taobao can buy silicone order refers to the mode, can be easily verified by optical fingerprint machine, workers use it to substitute the fingerprint attendance.
In addition, the optical sensor in a prism, the volume is larger, generally for several times and even 10 times the size of semiconductor, so limits its application in small devices. Use on similar attendance machines, access control and other large equipment, there is no limit to the volume of the problem, but in the U disk, mobile hard disk, handheld devices, become the biggest obstacle to volume. Low cost has been considered to be the biggest advantage of optical sensor, but because of its manufacturing process more difficult to ensure consistency, as represented by the sensor of the large-scale development of the semiconductor sensor, optical sensor cost advantage also is no longer apparent. Although most companies still use optical sensors, its development trend is innovative, high quality semiconductor capacitive fingerprint sensor.
Second, the ultrasonic technology
Ultrasonic fingerprint is a new kind of technology, its principle is to use ultrasonic has the ability to penetrate materials and different materials produce different size of echo (ultrasonic wave arrived at different material surface, are at different levels of absorption, transmission and reflection). Therefore, the use of the skin with air for acoustic impedance difference, can distinguish between fingerprint ridge and valley is located.
Ultrasonic technology used in the ultrasonic frequency is 1 x 104 x 109 hz - 1 hz, energy is the degree of control on human body condition (the same as the strength of a medical diagnosis). Ultrasound technology products can reach the best accuracy, lower it to finger peace cleanliness requirements below, but the acquisition time is significantly longer than the two kinds of product, and expensive, also can't do living fingerprint identification, so use scarce at present.