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How Fingerprinting Works


The Fingerprinting Process
Traditional ink fingerprints typically contain rolled and flat prints.
Traditional ink fingerprints typically contain rolled and flat prints.
Robert Clare/Taxi/Getty Images

The technique of fingerprinting is known as dactyloscopy. Until the advent of digital scanning technologies, fingerprinting was done using ink and a card.

To create an ink fingerprint, the person's finger is first cleaned with alcohol to remove any sweat and dried thoroughly. The person rolls his or her fingertips in ink to cover the entire fingerprint area. Then, each finger is rolled onto prepared cards from one side of the fingernail to the other. These are called rolled fingerprints. Finally, all fingers of each hand are placed down on the bottom of the card at a 45-degree angle to produce a set of plain (or flat) impressions. These are used to verify the accuracy of the rolled impressions.

Today, digital scanners capture an image of the fingerprint. To create a digital fingerprint, a person places his or her finger on an optical or silicon reader surface and holds it there for a few seconds. The reader converts the information from the scan into digital data patterns. The computer then maps points on the fingerprints and uses those points to search for similar patterns in the database.

Law enforcement agents can analyze fingerprints they find at the scene of a crime. There are two different types of prints:

  • Visible prints are made on a type of surface that creates an impression, like blood, dirt or clay.
  • Latent prints are made when sweat, oil and other substances on the skin reproduce the ridge structure of the fingerprints on a glass, murder weapon or any other surface the perpetrator has touched. These prints can't be seen with the naked eye, but they can be made visible using dark powder, lasers or other light sources. Police officers can "lift" these prints with tape or take special photographs of them.

When did this basic form of identification become a law enforcement staple? How did Babylonians and the ancient Chinese use fingerprints? Go to the next section to find out.


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