Device for Ultrasound Imaging of Fingerprints

Biometrics – the measurement of physical and biological characteristics unique to each individual for the purpose of personal identification – is critical in many security-related situations. These include restricted access to areas, data, objects, and in some cases, medical emergencies. Direct measurement can be more convenient, user-friendly and secure compared to some form of tag (ID, key or password) assigned to the person.

Schematics

The objective of this research is to develop a new robust fingerprint identification technology based upon forming surface-subsurface (under skin) ultrasonic 3D images of the finger pads. The work presented below aims to create specialized ultrasonic scanning methods for biometric purposes. Preliminary research has demonstrated the applicability of acoustic microscopy for fingerprint scanning. The additional information from internal skin layers and dermis structures contained in the scan improves confidence and reliability in the identification. Advantages of the proposed technology are the following:

  • Spatial resolution up to 0.05mm when operating in pulse-echo mode
  • Full-range “nail-to-nail” scanning of the fingerprint area (2.5 x 2.5 cm) is done in seconds with a resolution of up to 1000 dpi.
  • In addition to fingerprints, sweat pores can be identified at the surface and under the skin
  • No sensitivity to finger surface conditions (ex. oily or dry skin, cuts, etc.), unlike traditional surface-imaging techniques
  • Integrated liveness check via Doppler-based blood-flow measurement
  • Utilization as a polygraph device, monitoring blood-flow and pore dilation
  • Compatibility with fingerprint databases obtained with other techniques

A laboratory prototype of the system based on these principles has been designed, built and tested. It is the first step toward a commercial implementation of this technology. Next, the scanning time will be reduced by parallel data acquisition – n (likely four) simultaneously operating focused transducers.

Accounting for the cylindrical shape of the scanned object (finger), we proposed the following design: a set of n spherically-focused acoustic lenses are mounted on a cylindrical rotor; its rotation provides fast scanning along one direction (X-direction); and linear axial motion of the rotor gives a second direction (Y-direction), as shown below. We excluded classical mechanical scanners with back-and-forth transducer motion due to the time wasted for acceleration and deceleration.


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The coupling between the transducers and finger is maintained with a stationary acoustically-transparent finger holder and liquid layer between the rotor and holder. An electronic switch connects each lens to the pulser-receiver circuitry as it passes under the finger. An image of this can be seen below.

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Acquired data is processed by a host computer to construct high-quality fingerprint images that are compatible with other fingerprint databases.


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