Fingerprints

Fingerprints often are how police identify criminals and solve crimes, even crimes that are decades old. Narrow valleys are called grooves. Hills are known as friction ridges. When looking at fingerprints you’re looking at the friction ridges. Friction ridges give your fingers traction.

Alphonse Bertillon The first person to try and identify people – anthropometry. Used key measurements to differentiate people. He eventually added fingerprints, but only the right hand.

Using Ridge Patterns The usefulness of fingerprints depends upon 3 principles: A fingerprint is individual and not shared by any 2 people. (includes identical twins)

Using Ridge Patterns 2. A fingerprint remains unchanged throughout life. Even if injured – disappear for a while, but as skin heals itself  reappear. However, more severe damage that involves deeper layers of skin may leave a permanent scar and prevent prints from reemerging. Nonetheless, completely obliterating a print is difficult, and any scars left behind by attempts to do so create new individual characteristics that an examiner ca use for making a match.

Using Ridge Patterns 3. Fingerprints exhibit general patterns that provide a basis for classification. General patterns exist within every person’s prints, and all people share these patterns to varying degrees. Prints therefore can be systematically classified. Thus, reducing the # of records that must be searched when looking for a match.

Classifying prints The FBI has more than 200 million fingerprint files. Organizing the prints into groups makes it easier. In 1685, Marcello Malphigi recognized patterns in fingerprints and named them loops and whorls. The arch pattern followed more than 200 years later, when Sir Francis Galton identified it in 1892.

Classifying prints Whorls, loops, and arches are still the basis for fingerprint matching and identification, b/c although everyone has them, how they have them is unique. Differ  dif. # of the types and patterns vary

Classifying prints Arches – are ridge lines that rise in the center to create a wavelike pattern. Arches are sub-grouped into plain and tented varieties. Tented arches have a sharper central rise than do plain arches. Only 5% of all pattern types are arches.

Classifying prints Loops – loops are comprised of one or more ridges that double back on themselves. About 60% of patterns in human fingerprints are loops. Radial loops – flow downward and toward the radius or the thumb side. Ulnar loops – flow toward the ulna or little finger side.

Classifying prints Whorls – look like little whirlpools of ridgelines. They make up 35% of patterns seen in human fingerprints. Plain whorls – concentric circles like a bull’s eye or spirals like a spring. Central pocket loops whorls – resemble a loop with a whorl at its end. Double loop whorls – includes 2 loops that collide to produce a S shaped pattern. Accidental loop whorls – slightly different/irregular.

The Henry System Sir Edward Henry – an inspector general of the British police in India’s Bengal province. He completed his fingerprinting system in 1899. This system is still used today – with a few modifications. Separates fingerprints into 1,024 groups, thus narrowing the focus of fingerprint searches. The actual matching is done by hand, the system doesn’t make the match, it simply narrows it down.

AFIS Automated Fingerprint Identification System AFIS came about during the 1960’s as part of a collaboration b/n the FBI and the national Bureau of Standards (no the national Institute of Standards and Technologies or NIST)

AFIS The AFIS computer scans and digitally encodes fingerprints, storing that information in massive databases. It can search thousands of files every second while attempting to match them to an unknown ten-print set or even a single or partial print. Current AFIS computers search through a batch of 500,000 prints in less than 1 second.

AFIS After the computer establishes a match, an agent trained in fingerprint evaluation then hand-checks the file or files. Even in the computer age, the final match is made by the trained eye of a fingerprint expert.

AFIS Not only can the computer match prints with dizzying speeds, it can improve the quality of the print through minor digital manipulations. Brightness and contrast can be enhanced, and fuzzy images can be made sharper, all of which results in a clearer print that is more easily used in matching.

Tracking Down Fingerprints Fingerprints come in 3 general types that depend on how and where they were left – a print left on a wall is easier to find than one left on a garbage bag without any visible substance.

Breakdown of prints Patent prints – occur when a substance such as blood, ink, paint, dirt, or grease on the fingers of the perp leaves behind a readily visible print.

Breakdown of prints Plastic Prints – have a three-dimensional quality and occur when the perpetrator impresses a print into a soft substance such as wax, putty, caulk, soap, cold butter, or even dust. Can be photographed and photo can be used for matching.

Breakdown of prints Latent prints – are invisible and can’t be seen without special lighting or processing. Black powder, chemical reaction, lasers, superglue Which method depends upon the surface beneath the print.

Powdering the print Fingerprint powders adhere to moisture and oils of the residue in a latent print and thereby expose the pattern of the friction ridges. The powders come in a variety of colors and types. Criminalists use the color that gives the greatest degree of contrast with the background surface. Black powder, which is made from carbon black or charcoal, and gray powder, made from aluminum or titanium powder, are used most often.

Powdering the print Other specialized powders are fluorescent. After they are applied, the print fluoresces (glows) under a laser light. After the powdering process is complete, the print is either photographed or lifted. Lifting is done by gently laying the sticky surface of a strip of transparent tape over the print. As the tape is peeled off, the print pattern sticks to the tape, which is then placed on a card for later examination and matching.

Chemicals – expose prints Latent prints found on more porous surfaces are treated with chemicals that reveal print patterns by reacting with some component of the print residue. The reaction creates another compound that is more clearly visible. Common chemical used for exposing prints include: cyanoacrylate vapor, iodine fuming, ninhydrin, and silver nitrate.

Chemicals – expose prints Cyanoacrylate vapor – Super Glue – when heated or mixed with sodium hydroxide, cyanoacrylate releases vapors that bind to amino acids that are present in print residues, thus forming a white latent print. Iodine fuming – When heated, solid crystal iodine releases iodine vapors into a fuming chamber, where iodine fumes combine with oils in the latent print to produce a brownish print. This kind of print fades quickly, so it must be photographed right away or fixed by spraying it with a solution of starch in water.

Chemicals – expose prints Ninhydrin – A staple of law enforcement. The object is sprayed or dipped in a ninhydrin solution. The reaction is extremely slow and may take several hours to appear as a purple-blue print. Silver nitrate – a component of black-and-white photographic film. When investigators expose a latent print to silver nitrate, the chloride in salt molecules present in the print residue reacts with the silver nitrate and forms silver chloride. This colorless compound develops, or becomes visible, when it’s exposed to UV light.

Cleaning up the print: digital techniques More often than not a print or partial print is unclear. Its minute details may by fuzzy or difficult to see. Digital technology has stepped up and helped remedy this problem. Prints are scanned into a computer and then subjected to one of many programs that can enhance, improve, and clean up the computer-generated image of the print. Changing t he light, contrast, clarity, and background patterns can make a previously obscured print jump into clear view, speeding up the matching process and making it more accurate to boot.