Fingerprint scanning is a
biometric identification process using the user's
fingerprint. Fingerprints have been used since the development of
forensics to identify a person, and a fingerprint
scanner takes this process from after-the-fact ID to real time.
There are two primary methods to scan a fingerprint; one using the image of the fingerprint and one using the electrical properties of the surface of the fingerprint.
An optical system literally "looks" at the fingertip with a camera or other light-based image-capture device and then converts the picture of the print into a standard plot of whorls and line intersections for comparison to the prints in the database (that the file created or the database itself may be tampered with is a different matter entirely.) The drawback to an optical system is that it can be easily fooled with a rubber (or cut-off) finger, and in some cases even a photocopy will do.
A "touch" system actually measures the electrical differences between the "live" skin and "dead" ridges of the fingerprint. This can be accomplished with passive capacitance measurement or by injecting an electric field (the active method) into the skin via electrodes flanking the sensor surface and measuring the field fluctuations caused by the different densities in the skin of the fingertip surface. The resulting information is converted into the same whirls-and-intersections plot for comparison. Touch technology doesn't have the shortcomings of an optical system, since the fingertip must be living tissue for the sensor to function. The drawback here is that an excessively dry fingertip will not operate a passive system, although this is significantly reduced in an active system. An additional advantage is that a capacitive fingerprint touchpad can also be used as a touch controller for cursor control, since the chip can also detect position and movement.