Image analysis – Applications – Personnel identification
Reexamination Certificate
2000-06-16
2003-10-14
Mehta, Bhavesh M. (Department: 2625)
Image analysis
Applications
Personnel identification
C382S125000, C340S005530
Reexamination Certificate
active
06633656
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a microthermistor based fingerprint sensor.
DESCRIPTION OF THE PRIOR ART
Finger tips of human beings exhibit a pattern of ridges and valleys called a fingerprint. This structure is unique to every human being and has long been used for the identification of a person. Of particular interest are the ridge branching and ending points of the structure. These fingerprint features are called minutiae. The position and orientation of the minutiae can be used to characterise a fingerprint and thus minimise the information that has to be stored and processed for identification purposes, while still keeping an accurate fingerprint representation.
Fingerprint identification can be a time consuming activity when performed by human operators. This explains why automation of fingerprint identification has received considerable attention. Automated fingerprint identification requires the implementation of two main functions. First, the fingerprint pattern of ridges and valleys must be translated into an electronic format representative of the fingerprint pattern. This can be achieved through the use of various sensor types and arrangements. The fingerprint electronic representation can then be stored in a memory, if convenient, for further analysis. Second, the electronic data must be processed in order to achieve recognition. Various algorithms have been used for this purpose in the past. Some of them are based on a 2-dimensional representation of the fingerprint while others are based on a 1-dimensional fingerprint representation (see, for example, U.S. Pat. No. 5,745,046). Often, these algorithms make use of minutiae to characterise a fingerprint.
Advances in microelectronics and micro-machining have allowed substantial miniaturisation of fingerprint identification systems, which open a wide application range for these devices. They can be integrated to cars and houses as a replacement for keys and locks. They can be used to control access to high security areas. Fingerprint identification systems can replace Personal Identification Numbers (PINs) and ATM cards. They could also be placed in devices like cellular phones, computers or firearms to prevent unauthorised use of the same. Finally, law enforcement forces can bring them in the field as a very powerful tool for real-time identification of a person.
Historically, the first medium used for fingerprint recording was ink on paper. This process does not provide a good resolution and is not very convenient for users, as it involves the use of ink on a finger, which is then transferred to a piece of paper. After having placed the fingerprint on the piece of paper, the user must clean off the ink, which makes the process messy. Moreover, using these records for fingerprint identification is a very cumbersome and time consuming process.
Many optical systems have been implemented for fingerprint recording (see, for example, U.S. Pat. Nos. 5,548,394, 5,937,557, 5,892,599 and 5,920,384). Generally, an image of the fingerprint is projected on a 2D detector array or is scanned on a 1D detector array. Optical arrangements used to produce this image are very diverse and can make use, for example, of optical fiber bundles (U.S. Pat. No. 5,937,557), holographic optical elements (U.S. Pat. No. 5,892,599) or mini-prism arrays (U.S. Pat. No. 5,920,384). Some approaches take advantage of Frustrated Total Internal Reflection (FTIR) to enhance the contrast between the fingerprint ridges and valleys (U.S. Pat. No. 5,548,394). In these systems, the finger to be identified is typically pressed against a prism which provides the required FTIR. One operational drawback of this approach is that the optical surfaces in contact with the finger must be cleaned regularly to maintain the system performance. Moreover, such systems are typically large and the alignment of the various optical components part of the system must be kept within tight tolerances.
Other fingerprint identification systems sense the distribution of some electrical properties representative of the finger skin pattern. In some cases, the sensor is a micro-switch array (U.S. Pat. No. 3,781,855). The skin being an electrical conductor, direct contact of the fingerprint ridges with the micro-switches closes a circuit. Micro-switches of the array beneath fingerprint valleys remain opened. The pattern of closed and open switches in the array provides a representation of the fingerprint structure. Reading of this pattern is achieved through an integrated circuit, which is part of the micro-switch array chip. A variation of this approach involves measurements of the skin electrical resistance instead of simple switch state reading. It is therefore possible to evaluate to which extent the finger is in contact with various points of the underlying chip. This allows a better resolution of the transition zone between ridges and valleys and the fingerprint representation obtained is more accurate.
Other systems measure the capacitance between the finger skin and microelectrodes, as described for example in U.S. Pat. No. 5,325,442. In this approach, the finger skin is one of the capacitor plates and the microelectrode is the other capacitor plate. The value of this capacitance is a function of the distance between the finger skin and the electrode. When the finger is placed on a microelectrode array, the capacitance variation pattern measured from electrode to electrode gives a mapping of the distance between the finger skin and the various microelectrodes underneath. This corresponds to the ridge and valley structure on the finger tip. Here again, the parameter of importance, i.e. the capacitance, is read using a integrated circuit fabricated on the same substrate as the microelectrode array. More recent systems involve measurement of the finger skin equivalent complex impedance as part of a read-out circuit (U.S. Pat. No. 5,953,441) instead of simply measuring the ohmic resistance or the capacitance as described above.
Finally, fingerprint recording devices, closely related to the capacitance measurement systems described above, make use of electric field sensor array in order to obtain the fingerprint representation (U.S. Pat. No. 5,940,526).
Another category of fingerprint reading systems relies on pressure sensor arrays. In this case, various mechanisms are used to measure the pressure applied by the finger tip at different points of the sensor array. For points where a fingerprint ridge is in contact with a sensor, the pressure is high, while it is null for sensors underneath fingerprint valleys. The sensing mechanisms used here are very diverse. Some systems are based on micro-switch arrays (as described in U.S. Pat. Nos. 4,577,345 and 5,400,662). The state of the individual micro-switches (on or off) depends on the amount of pressure applied on them. Implementation of this approach is often done using a thin membrane which is electrically conductive or has a conductive layer on its side facing the switch array. This membrane must be soft enough so it takes the fingerprint shape when pressed by the finger against the switch array. Membrane points corresponding to fingerprint ridges touch the sensor array and close the underlying switches. For the membrane points corresponding to fingerprint valleys, the micro-switches remain in the off state. The main disadvantage of this approach is that it is very demanding on the membrane properties, as it must be conductive (at least partially), very soft and yet capable of withstanding repeatedly the deformations induced by a user's finger. An improved version of this system, as described in U.S. Pat. No. 5,844,287, makes use of an array of micro-membranes instead of a single membrane. One micro-membrane is fabricated over each micro-switch in the array. When a fingerprint ridge touches a micro-membrane, it brings it into contact with the underlying circuit which closes the associated micro-switch. This approach is less demanding on the membrane material, as the micro-membrane deformation can be made
Bayat Ali
Institut National d'Optique
Mehta Bhavesh M.
Merchant & Gould P.C.
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