Currently, there are several physical traits which can be used as Biometric identifiers. These include a persons facial shape, fingerprints, iris patterns, retina patterns, hand geometry, speech, handwriting / keystroke order and finally even wrist vein patterns. Most of these identifiers are very difficult to compromise, and some like the keystroke order or speech recognition may be difficult to reproduce consistently.

It's the task of the biometric product to apply a mathematical algorithm to the identifier (retina, or fingerprint for example) to determine if a user is who they are pretending to be. In devices where a finger print is the means of identification, a small device will scan the fingerprint of the user and momentarily record the fine details. This may include features such as the finger prints' hills and valleys, the direction and branching points, line endings and dozens of other minutiae.

The Infineon sensor

The fingers features can be acquired optically, thermally based on temperature differences between the fingertip's valleys and ridges, by pressure sensor or via a capacitive sensor (essentially a small silicon chip with many thousands of sensing elements). Infineon's "FingerTIP sensor uses the capacity difference between the surface of the sensor and the surface of the finger. The capacity measured at a ridge differs from that at a valley. Thus, about 65.000 capacitors acquire the data in a field of 224 x 288 pixels and transform it into a digital signal."

Optical sensors, consisting of a lens or prism optical system with CCD or CMOS camera measure light deflections produced by a finger on a sensor surface. Thermal sensors rely on measuring temperature differences at the fingertip so they can be influenced by changes in temperature in the surrounding environment. Generally thermal sensors must operate in a O° C or above ambient temperature.