Fingerprints wHAT they ARE and how they are produced

A fingerprint can be defined as a replica of the friction ridges of the skin cast on a surface. Despite the name fingerprints, these friction ridges are also produced by the palms, the toes, and the soles of the feet. While there are a few exceptions among those who are physically impaired, everyone has the friction ridges that produce fingerprints. Friction ridge patterns are affected by genetics, but even twins with identical genes have different fingerprints. Since a print of one finger has never been known to duplicate the print of another finger, even in the case of identical twins, it is possible to identify an individual with just one impression. Despite factors such as the environment and aging, one's friction ridges do not change, thus making fingerprints a permanent record of an individual throughout life.

To understand the concepts of what kind of prints exist, it is necessary to be able to recognize what a print is composed of. An invisible print, or latent print, is created when the friction ridges deposit materials on a surface that are generally invisible to the naked eye. A print is mostly made up of sweat. The primary component of sweat is water. Therefore, if a fingerprint were left alone, it would eventually dry.

There are also solid components of a fingerprint, including organic compounds like amino acids, glucose, proteins, and fatty acids as well as inorganic components like potassium, sodium, ammonia, and chloride salts. The basic premise of fingerprinting is to make a chemical react with a property in the fingerprint to make it visible. "Developing" a fingerprint is the process of making it visible.

An inked print refers to a print that is taken from a person and recorded directly onto a fingerprint card, as previously described. The correct procedure for obtaining an inked fingerprint is simple. Ink is placed on the area to be printed—the finger, for example—which is then rolled onto a card. The most common fingerprint card allows rolled prints to be taken from all 10 fingers. Using this method, prints can also be obtained from a palm or foot. The fingerprint cards are then filed away, usually by the agency that obtained the print, for comparison purposes when a print for evidence turns up. The person taking an inked print must be careful not to use too much ink and not to press too hard on the print card. Doing either will cause the print to smudge, making it useless. "Inkless" systems, which utilize ink that easily wipes off, eliminate these concerns altogether. Chemically treated paper systems use no ink whatsoever. Livescan systems use biometric hardware to create direct digital images from the suspect's finger surfaces.

Although new technology is making inked prints, of any type, a thing of the past, there is still some use for them. Agencies that do not have access to fingerprint databases may rely solely on fingerprint cards for comparison purposes. Smaller agencies that do not have the funds to run computer databases will continue to rely on manual methods for fingerprint comparison.

Evidence often involves latent prints. There are three broad categories of latent prints: plastic, or impression, prints; contaminated, or visible, prints; and latent, or invisible, prints. The term latent prints can refer to all three of these categories, as a whole, or specifically to invisible prints. As shown in chapter 1, plastic, or impression, prints are created when a finger touches pliable material, such as a newly painted surface, a layer of dust, putty, or adhesive tape. Contaminated, or visible, prints occur after a finger, contaminated with foreign matter, like blood, touches a clean surface, as shown in the photograph. Lastly, latent, or invisible, prints refer to the prints that are left as a result of the small amounts of perspiration and oil that are found on friction ridges.

Not all latent prints are able to be developed. The quality of latent prints relies on numerous conditions. The first condition to be considered is the surface that the print is deposited on. Plastic prints will last for years if undisturbed. Invisible prints on smooth surfaces, like glass or porcelain, can be developed after a similar period of time. Prints left on porous material like paper, however, are more variable in how long they can survive. It is possible that prints left on documents will fade or deteriorate. This can happen if they are subject to high humidity or become wet. Otherwise, invisible prints left on paper are fairly stable and can be developed years after they were left.

The second condition affecting the quality of latent prints is the nature of the material contaminating the fingerprint. Visible prints

Latent Material Defects
Bloody print (Courtesy of the author)

left by powder, dust, or soot are easily destroyed, whereas prints left in blood, ink, or oil will last longer periods of time under favorable conditions.

The third condition to consider is the person leaving the fingerprint. One must consider any physical or occupational defects of the subject leaving the print. For example, the fingerprints of an individual who has worked with their hands, like a bricklayer, may have very unusual qualities. Their fingerprints are not destroyed, but typically they do contain several points that can be used as identification factors, such as calluses.

A fourth condition to consider is how the object bearing the prints was handled. If a suspect picks up a glass and the glass slips, it is possible that the print will be a bit off. This is because the distance between friction ridges is very small, and the ridge detail was lost.

The final condition is with regard to latent print quality, especially when dealing with contaminated or visible prints, is the amount of material contaminating the fingerprint. The friction ridges that make up fingerprints are basically a series of ridges and valleys. If too much of a contaminant—blood, for example—lies on the friction ridges, it will fill these valleys, resulting in a smudged print. These prints are of little or no value as fingerprint evidence.

At times it may seem very difficult to find a fingerprint; however, they are of such value that the investigating officer should take the time to recover any prints left behind. The officer should search all areas surrounding the crime scene thoroughly and initially collect as many prints as possible. It may be of assistance to shine a flashlight at an oblique angle to the surface being examined.

The fact that a glove may have been worn should not be of concern, as it is very possible that the glove itself may leave a print that is unique to it. Additionally, it may be possible to develop a fingerprint left on the inside of a glove recovered at a crime scene, as was the case in a 1996 burglary of an office building in Los Angeles, where the clumsy burglar left his rubber gloves. In this case the observant police officer noticed a pair of rubber gloves left inside the office building. The officer stored them as evidence in the hope that they might reveal the perpetrator. Using a chemical method, the investigator was able to develop a print detailed enough to obtain a conviction.

An officer investigating a crime scene should also pay close attention to areas that are commonly touched. If the crime occurred in a vehicle, the investigator should look on areas like the back of the rearview mirror, on buttons, the trunk lid, and steering wheel. If the crime scene is in a residence, less obvious places such as the undersides of toilet seats, handles of dresser drawers, and the backs of movable cabinets should not be ignored. It may help to have someone familiar with the area, like the resident, walk through so they can determine if anything is out of place.

Lastly, if there is any doubt, the investigator should always err on the side of caution and lift the print, because the fingerprints can always be ruled out as the victim's. Once destroyed, however, the evidence will be impossible to recover. After the fingerprints are collected as evidence, the investigator will need to compare them to other prints, possibly inked prints, to determine if they are a match.

To make a fingerprint match, a fingerprint examiner uses a combination of ridge characteristics, or "points," collectively known as "minutiae." The characteristics are ridge endings, bifurcations, lakes or deltas, independent ridges, dots or islands, spurs, and crossovers. A bifurcation is when a single ridge splits into two ridges. Ridge endings occur at the point at which a ridge ends. Delta regions are roughly oval-shaped areas where ridges flowing in different directions meet. Dots resemble pinpoints, and islands, which are slightly larger than dots, are isolated ridges. Dots and islands are local ridge characteristics that occur at either a ridge bifurcation or a ridge ending. Spurs resemble their namesake, the spurs worn by horse riders. Crossovers are ridges that cross other ridges.

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