How Fingerprinting Works

By Stephanie Watson

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.

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 had 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 simultaneously check millions of criminal records and match faces, backgrounds, and other perpetrators’ identifiable characteristics.

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.

 

CAN YOUR FINGERPRINTS BE CHANGED — OR STOLEN?

A minor scrape, scratch or even burn won’t affect the ridges’ structure in your fingerprints — new skin reforms in its original pattern as it grows over the wound. But each ridge is also connected to the inner skin by small projections called papillae. If these papillae are damaged, the ridges are wiped out, and the fingerprint destroyed.

Some criminals have tried to evade capture by tampering with their own fingerprints. Chicago bank robber John Dillinger reportedly burned his fingertips with acid in the 1930s. Recently, a man in Lawrence, Mass., tried to hide his identity by cutting and stitching up all ten of his fingertips (fortunately, a police officer recognized his face).

But as fingerprint technology becomes a common form of authentication from bank vaults to luxury cars, law enforcement officials worry that would-be criminals might try to steal entire fingers for the prints. In one case, robbers in Malaysia cut off a man’s fingers so they could steal his Mercedes. Companies that make biometrics security equipment realize the potential dangers of this system and are now creating scanners that detect blood flow to ensure the finger is still alive.

What are fingerprints?

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 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 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 lines in these fingerprint patterns to distinguish one another. They also analyze very tiny characteristics called minutiae, which can’t be seen with the naked eye.

 

The Fingerprinting Process

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

The person’s finger is first cleaned with alcohol to remove any sweat and dried thoroughly to create an ink fingerprint. 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, each hand’s fingers 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 their 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 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 fingerprints’ ridge structure 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 visible using dark powder, lasers, or other light sources. Police officers can “lift” these prints with tape or take special photographs of them.

 

History of Fingerprinting

There are records of fingerprints being taken many centuries ago, although they weren’t nearly as sophisticated as 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 worked as the Chief Magistrate of the Hooghly district in Jungipoor, India. To reduce fraud, he had the residents record their fingerprints when signing business documents.

A few years later, Scottish doctor Henry Faulds worked in Japan when he discovered fingerprints left by artists on ancient clay pieces. This finding inspired him to begin investigating fingerprints. In 1880, Faulds wrote to his cousin, the famed naturalist Charles Darwin, and asked to develop 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 worldwide 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) measured 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, developed his own fingerprinting system variation. In 1892, Vucetich was called in to investigate two boys murdered in Necochea, a village near Buenos Aires. Suspicion had initially fallen 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 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, the Metropolitan Police of London’s commissioner, soon became interested in using fingerprints to nab criminals. In 1896, he added Galton’s technique, creating his own classification system based on the direction, flow, pattern, and other characteristics of the friction ridges. 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 countries.

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 fingerprints, followed later by the FBI.

Modern Fingerprinting Techniques

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 worldwide 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 access the same huge information database. 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.

Other Biometrics

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

Here are just a few of the biometrics you might be using shortly:

  • 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. A person holds his or her eye close to the scanning device for 10 to 15 seconds for a retinal scan while a low-intensity light and sensor analyze distinct patterns. Although retinal scans are used in very high-security institutions like power plants and 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, capturing 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.

Sources

  • American Psychological Association. “To Catch a Thief: The Psychology of Fingerprints.” http://www.psychologymatters.org/galton.html.
  • Biometrics.gov. “Introduction to Biometrics.” http://www.biometrics.gov/ReferenceRoom/Introduction.aspx.
  • Encyclopedia Britannica. “Fingerprinting.” http://search.eb.com/eb/article-9034291.
  • Encyclopedia Britannica. “Police.” http://search.eb.com/eb/article-260948.
  • FBI. “Integrated Automated Fingerprint Identification System.” http://www.fbi.gov/hq/cjisd/iafis.htm
  • FBI. “Taking Legible Fingerprints.” http://www.fbi.gov/hq/cjisd/takingfps.html
  • FBI. “What we Do.” http://www.fbi.gov/kids/k5th/whatwedo2.htm.
  • Hillsborough County Sheriff’s Office. “Fingerprinting Identification.” http://www.hcso.tampa.fl.us/SOD/ffingerprintid.htm
  • International Biometric Group. “Henry Classification System,” 2003.
  • Iowa State University. “DNA Fingerprinting in Human Health and Society.” http://www.biotech.iastate.edu/biotech_info_series/bio6.html.
  • Jackall, Robert. “Tales Told by Loops, Whorls, and Ridges.” Science, September 7, 2001, Vol. 293, pgs. 1771-1772.
  • Komarinski, Peter. “Automated Fingerprint Identification Systems.” Elsevier, Academic Press, 2004.
  • National Center for State Courts. “Fingerprint.” http://ctl.ncsc.dni.us/biomet%20web/BMFingerprint.html.
  • National Center for State Courts. “Retinal Scan.” http://ctl.ncsc.dni.us/biomet%20web/BMRetinal.html
  • National Institutes of Health. “Cases.” http://www.nlm.nih.gov/visibleproofs/galleries/cases/vucetich.html.
  • New York State Division of Criminal Justice Services. “Fingerprint Classification Systems Compared.” http://criminaljustice.state.ny.us/ojis/history/ph_am_hn.htm.
  • Scanlon, Lisa. “Fingerprinting’s Finger-Pointing Past.” Technology Review, June 2003, Vol. 106, pg. 80.
  • The Boston Channel. “Criminals Cutting Off Fingerprints To Hide IDs.” http://www.thebostonchannel.com/news/15478914/detail.html

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