The user's biometric information already came into general use to perform individual authentication using information devices because of its highly invariance and uniqueness. Among other things, fingerprint recognition becomes a certification means that is receiving most attention and generalized for a simple structure and a superior performance in comparison to other means.
An individual authentication is mainly used in the field that requires the importance of security such as access control, e-commerce, financial transaction, security of a personal computer (PC), office settlement system, etc., and thus it is more important than anything to effectively identify a fingerprint that is made artificially (hereinafter referred to as a ‘fake fingerprint’).
Therefore, a technique for identifying fake fingerprint is proposed in which physiological material such as hemoglobin is analyzed through spectral distribution characteristic of light that is reflected or scattered from a finger after providing the light to the finger. Another technique senses the variation in capacitance caused by a finger when fingerprint is touched on a metal plate of a fingerprint sensor prism in a state where a small amount of charges is formed on the metal plate, thereby identifying the fake fingerprint.
In addition, another technique of identifying fake fingerprint throws light on a finger but irradiates from the outside of the finger so that the light passes through along an inner surface adjacent to the surface of the finger and obtains a surface image from the light that has passed through the finger and exited to the outside. The surface image is then analyzed to determine whether it is derived from the fake fingerprint.
In general, because a living body is subjected to a high extent of light attenuation, the light is rapidly attenuated and is vanished while progressing in the living body. On the contrary, for a fake fingerprint, most of the light passes through the fake fingerprint to yield a small difference between an incident light amount and a transmitted light amount due to a low light attenuation of the fake fingerprint. Based on this phenomenon, the aforementioned techniques can identify the fake fingerprint.
However, the following problems occur in applying these conventional techniques to a diffusive fingerprint sensor that is widely used in the field of fingerprint recognition. In the characteristic of a total reflection fingerprint sensor, the problem is that there may be the difference in the transmitted light amount that is detected depending on the degree of contact between the fingerprint and the sensor. In other words, if the contact between the fingerprint and the sensing surface of the sensor is not good due to a dried surface of the fingerprint even though the finger has the same transmission characteristic, the transmitted light amount is substantially less. On the contrary, if the contact is good, the transmitted light amount increases. Consequently, the accuracy of fake fingerprint identification using the gray level distribution of the surface image decreases significantly.
That is, in the diffusive fingerprint sensor, the fake fingerprint identification technique using an external light source may occur the judgment error caused by the difference in the contact degree depending on the state of the fingerprint since the amount of the light that passes through the inside of the prism varies according to the degree of contact between the finger and the surface of the prism.