Dealing with the minutiae of fingerprint analysis

Wen Li and Srik Gurrapu, Texas Instruments

April 04, 2011

Wen Li and Srik Gurrapu, Texas InstrumentsApril 04, 2011

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Fingerprint analysis involves using fingerprint sensors and sophisticated DSP-based image-processing algorithms. Here’s a description of the minutiae-based system for fingerprint analysis.

Many biometric characteristics are used to verify identity, including hand and face geometry, retinal scans, voice recognition, and fingerprint analysis. Because no two fingerprints have ever been proved identical, fingerprint matching is among the most popular and accurate biometric techniques used for identification purposes.

A popular and reliable way to compare fingerprints is to analyze the minutiae of the fingerprint. To do so, a system must first capture the image of a fingerprint and process the image to make it easy for image analysis. After the analysis, the minutiae will be extracted and saved in a template format. The system must then store the data of the minutiae to be used for future comparison. Figure 1 illustrates a typical fingerprint authentication processing flow.

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Fingerprint sensors
Based on the fingerprint processing diagram, a sensor is the “front-end” of the system, playing an import role: it captures the image of the fingerprint. The two types of popular sensors used in fingerprint analysis are the optical and swipe sensors. Here are basic descriptions of how they operate.

Optical sensor. An optical fingerprint sensor captures a digital image of the fingerprint by using visible light. The following describes the operation principle of an optical sensor (see Figure 2 for an illustration):

  • First, the light source inside of a sensor will emit light to illuminate the surface of the finger.
  • The light reflected from the finger passes to a solid-state pixels sensor (either a charge-coupled device [CCD] or complementary metal oxide semiconductor [CMOS] image sensor), which captures a visual image of the fingerprint. On the path of the light, a specially designed lens will be used.

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Swipe sensor. A swipe sensor is a type of active capacitance sensor. Here is how an active capacitance sensor works (see Figure 3 for an illustration):

  • First, the active capacitance sensors will apply a voltage to the skin. This will generate an electrical field in the space between the skin and sensor.
  • Since the electrical field between the finger-skin and sensor follows the pattern of the ridges in the dermal skin layer, the effective capacitance will be measured across this field.
  • The distances between skin and sensors can then be calculated mathematically by using this equation: C = ε0*εr*(A/d); here “d” represents the distance.
  • Based on the calculated distances across the field, the fingerprint image can be mapped.

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