this article is for you who want to know small thing about fingerprint scanners. (for further reading : www.howstuffworks.com)
- we are all happen to have built-in, easily accessible identity cards with unique design, which represents you alone at your fingertips. people have tiny ridges of skin which form through a combination of genetic and environmental factors. it forms from a result of random events, with innumerable factors influencing the formation of the fingers, that ensure there is virtually no chance of the same exact pattern forming twice, even an identical twin, in the entire course of human history.
- a fingerprint scanner system has two basic jobs - to get an image of your finger, and to determine whether the pattern of ridges and valleys in that image matches the other pattern in pre-scanned image.
- the most common methods to get an image of your finger are optical scanning and capacitance scanning. these methods will have the same result (image) but they go about it in completly different ways.
- optical scanner use a charge coupled device (CCD), same light sensor system used in digital cameras and camcoders, which will convert light to digital image. the scanner has its own light source to illuminate the ridges of the finger.
- capacitance scanner sense the fingerprint using electrical current. the sensor is made up of one or more semiconductor chips containing an array of tiny cells, each cell is smaller than the width of one ridge on a finger. the cell has two conductor plates and surface of the finger acts as a third capacitor plate. the capacitor in a cell under a ridge will have a greater capacitance than the capacitor in a cell under a valley. the capacitance from each cell will result a different voltage output, then after reading every cell, the processor can put together an overall picture of the fingerprint (similar result to optical image). 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 impressin of a fingerprint (makes the system harder to trick).
- in movie, automated fingerprint analyzers typically overlay various fingerprint image to find a match. in real life, this isn't a particularly practical way to compare fingerprints, because you're rarely going to get a perfect image overlay. and it will need a lot of processing power. instead, most scanner systems compare specific features of the fingerprint, generally known as munutiae. they only concentrate on points where ridge lines end or where one ridge splits into two (bifurcations). collectively, these and other distinctive features are sometimes called typica.
- the basic idea is to measure the relative positions of minutiae, in the same sort of way you might recognize a part of the sky by the relative positions of stars. a simple way to think of it is to consider the shapes that various minutiae form when you draw straight lines between them. if two prints have three ridge endings and two bifurcations, forming the same shape with the same dimensions, there's a high likelihood they're from the same print. (actually the scanner system doesn't have to find the entire pattern of minutiae both in the sample and in the print on record, it simply has to find sufficient number of minutiae patterns _varies according to the scanner programming_ that the two prints have in common)
- pros : harder to fake than identity cards; you can't guess a fingerprint pattern like you can guess a password; you can't misplace your fingerprint; you can't forget your fingerprints like you can forget a password
- cons : optical scanner can't always distinguish between a pic of a finger and the finger itself; capacitive scanner can sometimes be fooled by a mold of a person's finger (in a worse scenario, a criminal could even cut off somebody's finger to get past a scanner security system); some scanners have additional pulse and heat sensors to verify that the finger is alive, rather than a mold or dismembered digit, but even these systems can be fooled by a gelatin print mold over a real finger; if you accidentally tell secret password or lose your credit card, you can always get a new card or change your code, imagine if somebody steals your fingerprint (you can't change that! there's no way to get new prints!)
