Fingerprint+Scanner

A fingerprint scanner has two functions and they are to: -scan the image of your fingerprint -match the image with a previous scan of that finger There are two main ways of scanning a finger print and they are an optical scanner and a capacitance scanner

Optical Scanner
The main part of an optical fingerprint scanner is a charge coupled device (CCD), this is an array of light sensitive diodes calle photosites which generates an electrical signal in response to light photons. A photosite records a pixel and shows a light to record that pixel, an image is formed by the light and dark pixels, the image is of your fingerprint.

The scanner shines a light onto the fingerprint when you place your finger on the glass then the CCD takes a picture of your finger. The image that comes out is a inverted image. The darker parts of the image shows where most light has been reflected back (ridges of the finger) and the light part indicating less light being reflected (the valleys between ridges).

Capacitance Scanner
A capacitance scanner, like the optical scanner, uses the ridges and valleys of the finger to create an image. So instead of using light the use electrical current to sense the print.

The sensor is made up of one or more semiconductor chips containing an array of cells. Each cell has two conductor plates, that are smaller than the width of a ridge on your finger, covered by an insulating layer. The sensor is connected to an integrator, a circuit built around an inverting operational amplifier To scan the finger all the reset switch is closed fro each cell. When it is opened again the capacitors are charge up. The capacitance of the feedback loop's capacitor affect the voltage at the amplifier's input, which affects its output. Since the distance to the finger alters capacitance, a finger ridge will result in a different voltage output than a finger valley. The scanner reads this voltage output and figures out whether it is a ridge or a valley. By reading every cell the processor can form an overall picture of the fingerprint.

The main advantage of a capacitive scanner is that it requires a real fingerprint-type shape, rather than the pattern of light and dark that makes up the visual impression of a fingerprint. This makes the system harder to trick. Additionally, since they use a semiconductor chip rather than a CCD unit, capacitive scanners tend to be more compact that optical devices.

To match a fingerprint to one that has been saved on records the system uses specific feature (where a ridge ends in a finger or where one ridge splits into two) of the fingerprint rather than the whole print. To get a match the system doesn't have to find the entire pattern of key features both from the sample and the print, it only has to find a sufficient number.