Anti-spoofing method of optical fingerprint collector
Abstract: Fingerprint collector is an input device for fingerprint recognition system. It can be widely used in fingerprint lock, attendance system, e-commerce, bank savings, social insurance, PC and mobile phone. It can also be used for fingerprint matching operation in public security system. At present, there are three commonly used fingerprint collectors: optical, capacitive/inductive, and bio-RF. Among them, the optical fingerprint collector is the earliest fingerprint collector. It is also the most popular fingerprint collector at the current stage because of its long use time, reliable performance and low price. However, the optical imaging acquisition principle is vulnerable to deception. Its use in financial systems, especially in Treasury, is not safe. In order to overcome this shortcoming that is vulnerable to deception, this paper focuses on two new technical means for preventing fingerprint fraud by optical fingerprint collector.
I. Acquisition principle The optical fingerprint collection instrument collects the texture image of the fingerprint surface. The finger touches the surface of the prism. The light source illuminates the surface of the prism with the fingerprint to form the reflected light. The amount of reflected light is different due to the different 嵴 and 峪 bump of the fingerprint. The reflected light is then focused by the lens and captured by the photoelectric image sensor. According to the different collection principle, it can be divided into bright background fingerprint collector and dark background fingerprint collector: the dark background fingerprint collector adopts the principle of diffuse reflection, the ridge line of the fingerprint is in close contact with the collecting prism, and the beam incident on the ridge line occurs. Diffuse reflection, the lens group images the diffuse reflection of the fingerprint ridge contact portion onto the imaging device, which is displayed as a bright line, the valley line is not in contact, and the light is still transmitted out of the collection prism, so the valley line is displayed as a dark line on the imaging device, thereby The image of the dark background is obtained; the bright background fingerprint collector adopts the principle of total reflection, the ridge line of the fingerprint is in close contact with the collecting prism, destroying the total reflection condition, a part of the incident light is absorbed by the finger, and the remaining part of the light is diffusely reflected, and the lens group will The reflected beam is imaged onto the imaging device, and the ridge line absorbs part of the total reflected light, which is displayed as a dark line; the valley line is not in contact with the collecting surface, and the total reflection is still established, and is displayed as a bright line, thereby forming a fingerprint image with a bright background.
Second, the deception method Because the optical imaging acquisition method is to collect the difference in the degree of contact between the fingerprint ridge line and the optical prism surface, its anti-false fingerprint deception is weaker than other types (such as electricity, heat, pressure). . The deception method for the optical imaging acquisition method mainly comes from deceiving the residual fingerprint. The reason is that due to the sebum or contaminant on the finger, a residual image (fingerprint residue) is left on the fingerprint identification device, if the sensor is irradiated with external light; or, the developer residue may be detected by using the developer powder. . In criminal investigation work, this is an important means of extracting criminal fingerprints, but for fingerprint identification systems, it provides an opportunity to camouflage legitimate user fingerprints, especially in unattended applications, fingerprint residues left by legitimate users. If it is collected as a digital image, it is possible to pass the matching, which causes the illegal user to use the residual fingerprint to identify and authenticate, thereby posing a threat to the security of the fingerprint identification system.
Third, anti-spoofing technology can effectively distinguish fingerprint residue and fingers through live recognition and dual-mode acquisition.
The way of living body identification, through the capacitive proximity sensor, using the principle of capacitive sensing, the sensing plate is a plate-shaped electrode made of conductive material, which can be regarded as one or a group of capacitors, and the conductors and dielectric change poles appearing or passing nearby. The dielectric constant in the board system affects the electrostatic field distribution and thus the capacitance of the sensing plate. When the finger approaches or touches the sensing plate, the capacitance of the sensing plate increases; when the finger moves away from or away from the sensing plate, the capacitance of the sensing plate will decrease. The inductive measurement circuit is used to measure the change in capacitance of the sensing plate to determine whether there is a finger. The specific method is to add a capacitive proximity sensor to the optical fingerprint collection device, and place the capacitive proximity sensor sensing electrode under the collection prism. When the fingerprint collection module uses a surface light source and the surface light source is located below the collection prism, the sensing electrode is placed under the surface light source; when the fingerprint collection module uses the point light source, the sensing electrode can be placed under the collection prism.
The advantage of this method is as follows: Firstly, because the collecting electrode of the collecting instrument has strong penetrating ability and anti-interference ability, it can not directly contact with the finger, avoiding the exposure of the sensing electrode, being easily broken down, causing damage and being vulnerable to external non-finger. The shortcoming of interference from other objects, even if the glass media panel is splashed by water, it will not affect the identification finger proximity or contact signal; secondly, because the glass material of the collecting prism is a strong dielectric, the sensing electrode is separated from the human finger by a certain thickness of glass. The media still accurately recognizes the finger's proximity or contact signal.
The other is the dual mode acquisition mode, which sets the acquisition mode of dark background (diffuse reflection) and bright background (total reflection) simultaneously in the same collector. The collecting prism comprises two incident surfaces, and the light source comprises a bright background light source and a dark background light source, and the two are separated by a baffle to prevent the light from interfering with each other. The reflected light absorption rate due to fingerprint residual is much smaller than the fingerprint contact, so the residual image passes through the dark The background collection method can collect relatively clear images, but the bright background acquisition mode can not get the image or the obtained image is very blurred. Therefore, by comparing the clarity of the acquired images in the two modes, it is easy to determine whether it is Fingerprint residue.
IV. Summary The optical fingerprint collector is the earliest fingerprint collector. It is also the most popular fingerprint collector at this stage because of its long use time and low price. However, due to the shortcomings of the optical imaging acquisition principle, there are certain insecurities in its use. Although this shortcoming can be compensated by means of algorithm software, the diversity of the fingerprint collector will inevitably lead to confusion of the algorithm. Therefore, only by constantly improving and improving the performance of the acquisition device itself, from the beginning of image acquisition, the disadvantages of being vulnerable to fraud are overcome, and the standardized application and long-term development of the fingerprint identification system can be realized.
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