How Does Fingerprinting Work?

What Are Fingerprints?

Fingerprints are an impression left behind when the raised ridges on the skin on the bottom of the last joint of the fingers and thumb come into contact with a surface. As each person's finger prints are unique, the taking of fingerprints provides a means of identification. It should be noted that--while comprehensive scarring might do sufficient damage to eliminate the old ridge pattern on the fingertips--it replaces it with a new and equally unique pattern. Fingerprints can be deliberate, patent or latent.

Basic Fingerprinting

In the sort of deliberate fingerprinting used in the identification processing for a government job application or when placed under arrest, recording fingerprints is easy: the fingers are inked and then rolled onto a form to record the fingerprint image.

Patent fingerprints are left when a substance (such as blood, dye, ink, or paint) is transferred from the finger to a surface, making the fingerprint image clear and easy to photograph. Patent fingerprints should be thought of as being similar to the deliberate ink fingerprinting done when a person is placed under arrest in terms of clarity. While they may be smudged or partial, at least they stand out. Latent prints do not.

Lifting Latent Prints

Lifting fingerprints from a crime scene is more difficult. Fingerprints on surfaces can be either latent or patent. Latent fingerprints are the result of the normal secretions of the skin, such as oil, being let behind on a surface through direct fingertip contact. These are usually not readily visible. Making them visible requires the use a powder or liquid agent called a "developer," which intensifies the contrast between the ridges and valleys in the image. The use of powders to do this has given rise to the expression "dusting for fingerprints," which is often put to general use on crime shows, even though not all latent fingerprints are "dusted."

The Henry Classification System

The system for identifying and cataloging fingerprints used in the English-speaking world is the Henry Classification System. According to this system, there are three basic patterns in finger prints: the arch, the loop and the whorl. Loops can be radial or ulnar, depending on which side of the hand the loop's tail points towards. Whorls are divided into accidental whorls, double loop whorls, central pocket loop whorls, and plain whorls.

This system categorizes fingerprints into types. By categorizing them, the time and effort spent looking for a match is reduced. This was an important consideration in the days before computers and digital records, when up to hundreds of thousands of fingerprints could be on file at a major police department. All searching and matching would have to be done by hand. It still remains useful for computerized searches. Once the vast bulk of fingerprints on file are excluded for not being in the same category, a more detailed examination of individual fingerprints can begin. The Henry System is the basis for the Automated Fingerprint Identification System.

Resources

• FBI on Lifting Fingerprints
• Fingerprint History
• Fingerprinting.com