How Fingerprinting Works


Fingerprinting has been used by law enforcement agencies for more than a century. See more forensics pictures.
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A woman has been murdered. When the detectives arrive on the scene, the house is in shambles. Clothes are strewn about the floor, lamps are ov­erturned and there's no sign of the assailant. Then, one of the detectives picks up a glass. On its side is a smudged, bloody thumbprint. He takes it down to the lab, where it's analyzed and matched to a recorded set of prints. The detectives catch their killer.

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­This scen­e has been replayed in one crime drama after another. Ever since scientists discovered that every person's fingerprints are unique, and police officers realized this singularity could help them catch criminals, fingerprints have been an integral part of the law enforcement process. Today, fingerprints are also used to prevent forged signatures, identify ­accident victims, verify job applicants and provide personalized access to everything from ATMs to computer networks.

But fingerprinting has come a long way from the days when police officers lifted prints from a crime scene and checked them manually against their files. Modern fingerprinting techniques can not only check millions of criminal records simultaneously, but can also match faces, backgrounds and other identifiable characteristics to each perpetrator.

What are the basic characteristics of a fingerprint? How long have people been using prints as a form of identification? Find out in the next section.

What are fingerprints?

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Fingerprints are the tiny ridges, whorls and valley patterns on the tip of each finger. They form from pressure on a baby's tiny, developing fingers in the womb. No two people have been found to have the same fingerprints -- they are totally unique. There's a one in 64 billion chance that your fingerprint will match up exactly with someone else's.

Fingerprints are even more unique than DNA, the genetic material in each of our cells. Although identical twins can share the same DNA -- or at least most of it -- they can't have the same fingerprints.

Fingerprinting is one form of biometrics, a science that uses people's physical characteristics to identify them. Fingerprints are ideal for this purpose because they're inexpensive to collect and analyze, and they never change, even as people age.

­Although hands and feet have many ridged areas ­that could be used for identification, fingerprints became a popular form of biometrics because they are easy to classify and sort. They're also accessible.

Fingerprints are made of an arrangement of ridges, called friction ridges. Each ridge contains pores, which are attached to sweat glands under the skin. You leave fingerprints on glasses, tables and just about anything else you touch because of this sweat.

All of the ridges of fingerprints form patterns called loops, whorls or arches:

  • Loops begin on one side of the finger, curve around or upward, and exit the other side. There are two types of loops: Radial loops slope toward the thumb, while ulnar loops slope toward the little finger.
  • Whorls form a circular or spiral pattern.
  • Arches slope upward and then down, like very narrow mountains.

Scientists look at the arrangement, shape, size and number of lines in these fingerprint patterns to distinguish one from another. They also analyze very tiny characteristics called minutiae, which can't be seen with the naked eye.

If fingerprints are so unique and subtle, how are they recorded accurately? In the next section, we'll learn about dactyloscopy, or the art of fingerprinting.

The Fingerprinting Process

Traditional ink fingerprints typically contain rolled and flat prints.
Traditional ink fingerprints typically contain rolled and flat prints.
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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.

History of Fingerprinting

Traditional fingerprinting required careful analysis to match prints.
Traditional fingerprinting required careful analysis to match prints.
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There are records of fingerprints being taken many centuries ago, although they weren't nearly as sophisticated as they are today. The ancient Babylonians pressed the tips of their fingertips into clay to record business transactions. The Chinese used ink-on-paper finger impressions for business and to help identify their children.

However, fingerprints weren't used as a method for identifying criminals until the 19th century. In 1858, an Englishman named Sir William Herschel was working as the Chief Magistrate of the Hooghly district in Jungipoor, India. In order to reduce fraud, he had the residents record their fingerprints when signing business documents.

A few years later, Scottish doctor Henry Faulds was working in Japan when he discovered fingerprints left by artists on ancient pieces of clay. This finding inspired him to begin investigating fingerprints. In 1880, Faulds wrote to his cousin, the famed naturalist Charles Darwin, and asked for help with developing a fingerprint classification system. Darwin declined, but forwarded the letter to his cousin, Sir Francis Galton.

Galton was a eugenicist who collected measurements on people around the world to determine how traits were inherited from one generation to the next. He began collecting fingerprints and eventually gathered some 8,000 different samples to analyze. In 1892, he published a book called "Fingerprints," in which he outlined a fingerprint classification system -- the first in existence. The system was based on patterns of arches, loops and whorls.

Meanwhile, a French law enforcement official named Alphonse Bertillon was developing his own system for identifying criminals. Bertillonage (or anthropometry) was a method of measuring heads, feet and other distinguishing body parts. These "spoken portraits" enabled police in different locations to apprehend suspects based on specific physical characteristics. The British Indian police adopted this system in the 1890s.

Around the same time, Juan Vucetich, a police officer in Buenos Aires, Argentina, was developing his own variation of a fingerprinting system. In 1892, Vucetich was called in to assist with the investigation of two boys murdered in Necochea, a village near Buenos Aires. Suspicion had fallen initially on a man named Velasquez, a love interest of the boys' mother, Francisca Rojas. But when Vucetich compared fingerprints found at the murder scene to those of both Velasquez and Rojas, they matched Rojas' exactly. She confessed to the crime. This was the first time fingerprints had been used in a criminal investigation. Vucetich called his system comparative dactyloscopy. It's still used in many Spanish-speaking countries.

Sir Edward Henry, commissioner of the Metropolitan Police of London, soon became interested in using fingerprints to nab criminals. In 1896, he added to Galton's technique, creating his own classification system based on the direction, flow, pattern and other characteristics of the friction ridges in fingerprints. Examiners would turn these characteristics into equations and classifications that could distinguish one person's print from another's. The Henry Classification System replaced the Bertillonage system as the primary method of fingerprint classification throughout most of the world.

In 1901, Scotland Yard established its first Fingerprint Bureau. The following year, fingerprints were presented as evidence for the first time in English courts. In 1903, the New York state prisons adopted the use of fingerprints, followed later by the FBI.

But how has fingerprinting changed since the 19th century? In the next section, we'll find out about modern fingerprinting techniques.

Modern Fingerprinting Techniques

A background and identity check fingerprint capture machine is displayed in London. Modern technologies have made fingerprinting a much more effective means of identification.
A background and identity check fingerprint capture machine is displayed in London. Modern technologies have made fingerprinting a much more effective means of identification.
Daniel Berehulak/Getty Images

­The Henry system finally enabled law enforcement officials to classify and identify individual fing­erprints. Unfortunately, the system was very cumbersome. When fingerprints came in, detectives would have to compare them manually with the fingerprints on file for a specific criminal (that's if the person even had a record). The process would take hours or even days and didn't always produce a match. By the 1970s, computers were in existence, and the FBI knew it had to automate the process of classifying, searching for and matching fingerprints. The Japanese National Police Agency paved the way for this automation, establishing the first electronic fingerprint matching system in the 1980s. Their Automated Fingerprint Identification Systems (AFIS), eventually enabled law enforcement officials around the world to cross-check a print with millions of fingerprint records almost instantaneously.

­AFIS collects digital fingerprints with sensors. Computer software then looks for patterns and minutiae points (based on Sir Edward Henry's system) to find the best match in its database.­

The first AFIS system in the U.S. was

speedier than previous manual systems. However, there was no coordination between different agencies. Because many local, state and federal law enforcement departments weren't connected to the same AFIS system, they couldn't share information. That meant that if a man was arrested in Phoenix, Ariz. and his prints were on file at a police station in Duluth, Minn., there might have been no way for the Arizona police officers to find the fingerprint record.

That changed in 1999, with the introduction of Integrated AFIS (IAFIS). This system is maintained by the FBI's Criminal Justice Information Services Division. It can categorize, search and retrieve fingerprints from virtually anywhere in the country in as little as 30 minutes. It also includes mug shots and criminal histories on some 47 million people. IAFIS allows local, state and federal law enforcement agencies to have access to the same huge database of information. The IAFIS system operates 24 hours a day, 365 days a year.

But IAFIS isn't just used for criminal checks. It also collects fingerprints for employment, licenses and social services programs (such as homeless shelters). When all of these uses are taken together, about one out of every six people in this country has a fingerprint record on IAFIS.

Despite the modern technologies, fingerprinting is still an old detective's trick. What are some other ways to catch a thief? Find out in the next section.

Other Biometrics

U.S. Marines take the retinal scans and fingerprints of Iraqi residents of Fallujah for their required biometric cards.
U.S. Marines take the retinal scans and fingerprints of Iraqi residents of Fallujah for their required biometric cards.
Scott Peterson/Getty Images

Fingerprinting isn't the only way to catch a criminal, or perform one of the many other biometrics-driven technologies now available. Eye scans, voice fingerprints and even DNA are now providing means of identification, as well as access to everything from ATMs to cars.

Here are just a few of the biometrics you might be using in the near future:

  • Eye scans: Both the retina (the layer of tissue in the back of the eye that converts light into nerve signals) and the iris (the colored part of the eye) have unique characteristics that make them highly accurate biometrics. For a retinal scan, a person holds his or her eye close to the scanning device for 10 to 15 seconds while a low-intensity light and sensor analyze distinct patterns. Although retinal scans are used in very high-security institutions like power plants and the military areas, they are currently too expensive to be practical for widespread use. Irises have more than 200 different unique identifying characteristics (about six times more than fingerprints) ranging from rings to freckles. Iris identification systems take only about two seconds to scan the iris and look for patterns. They're used in some prisons and a few airports.
  • Ear scans: Ears are unique in size, shape and structure. Scientists use these traits to develop biometric scans of the ear. In ear scans, a camera creates an image of the ear that is analyzed for identifying characteristics.
  • Voice fingerprints: Every time a new Osama bin Laden tape comes out, the FBI Audio Lab in Quantico,Va. runs it through a voice analyzer, which captures the frequency, intensity and other measurements to determine whether the tape is authentic. These so-called "voice fingerprints" aren't as definitive as fingerprints or DNA, but they can help distinguish one person from another.
  • DNA fingerprints: Every individual has unique DNA. While you can change your appearance, you can't change your DNA. Because of this, scientists are starting to use DNA analysis to link suspects to blood, hair, skin and other evidence left at crime scenes. DNA fingerprinting is done by isolating the DNA from human tissues. The DNA is cut using special enzymes, sorted and passed through a gel. It's then transferred to a nylon sheet, where radioactive probes are added to produce a pattern -- the DNA fingerprint.

Some of these technologies are still in development, so it isn't yet known which is the most effective form of identification. And of course, some types of biometrics are better suited for specific tasks than others. For example, voice fingerprints are most appropriate for phone financial transactions.

To find out more about fingerprinting, biometrics and other tales of true crime, look over the links on the next page.

Related HowStuffWorks Articles

More Great Links

Sources

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  • Biometrics.gov. "Introduction to Biometrics." http://www.biometrics.gov/ReferenceRoom/Introduction.aspx.
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