AFIS

Automated Fingerprint Identification System is one of the many biometric tools along with CODIS used by both law enforcement and government agencies around the world for identifying individuals. AFIS is by far one of the most prevalent ID databases out there. Each person's prints are unique, in the same way that your own DNA is unique to you.

The way AFIS works is this:
1. Prints are obtained by use of a standardized ten-print card, which a technician will obtain using a black ink pad pressing each of your fingers onto the ink, and then rolling the finger across the appropriate space on the card. Five prints per hand, one per finger and thumb and then 2 separate prints for all 4 fingers to the second knuckle on each hand excluding thumbs.

2. The ten-print card is then scanned into a special AFIS program which runs an algorithm and analyzes the combination of ridges and valleys on each print and measures the type and distances between them. Each time there are intersections in the ridges and valleys (or minutae), a marker is registered in the scanned print showing the relative position and direction of the marker.

3. The constellation of markers is recorded in AFIS, instead of the image of the fingerprint (which by the FBI's standard is scanned at 500dpi). This way, the database size can be comparatively small and fast instead of requiring petabytes of image data for the millions of 10-print cards that are currently on file.

The identifying markers in the print which are typically logged are:

1. Ridge endings - a ridge that ends abruptly......inverted to:
2. bifurcation - a single ridge that splits into two ridges
3. dot - an independant ridge with approximately equal length and width......inverted to:
4. lake - a single ridge that bifurcates and reunites shortly afterwards to continue as a single ridge
5. spur - a bifurcation with a short ridge branching off a longer ridge
6. crossover - a short ridge that runs between two or more parallel ridges

Fingerprinting is by no means an exact science, since there are many factors that can alter prints including but not limited to, friction, wear, damage, disease, smudging, elasticity or contaminants on the fingers, etc. Prints are scanned for identification through active biometrics scanners in secured facilites, through consumer grade scanners (Microsoft and Logitech produce several to eliminate the need for passwords on your own PC) or through latent prints obtained through forensic science which get left behind when anything gets touched as evidence of your prior presence at any location, popularized by such prime-time-crime TV shows like CSI.

Sources:
howstuffworks.com
Sagem-Morpho
United States Federal Bureau of Investigation (FBI)