Labcode|1|: Fingerprints

Forensic scientists have used fingerprints in criminal investigations as a means of identification for many years. Fingerprint identification is one of the most important criminal investigation tools due to two features: their persistence and their uniqueness. Persistence refers to the fact that fingerprints do not change over time. The friction ridges which create fingerprints are formed while inside the womb and grow proportionally as the baby grows. Permanent scarring is the only way a fingerprint can change. Uniqueness acknowledges that fingerprints are unique to an individual. Even identical twins have different fingerprints.

In this LabCode article we will learn:

1. What is a fingerprint?
2. How to search for a fingerprint?
3. How to get a fingerprint?
4. How to use a finger print for identification?

1. What is a fingerprint ?

first we need to know what are we talking about exactly: what is the fingerprint? The Oxford dictionary define fingerprint as "An impression or mark made on a surface by a person’s fingertip, able to be used for identifying individuals from the unique pattern of whorls and lines on the fingertips".There is two causes wich make figerprint identification the most important criminal investigation tools:their persistence and their uniqueness. Persistence refers to the fact that fingerprints do not change over time. The friction ridges which create fingerprints are formed while inside the womb and grow proportionally as the baby grows. Permanent scarring is the only way a fingerprint can change. Uniqueness acknowledges that fingerprints are unique to an individual. Even identical twins have different fingerprints.
There are three types of fingerprints that can be found: latent, patent, and plastic. Latent fingerprints are made of the sweat and oil on the skin’s surface. This type of fingerprint is invisible to the naked eye and requires additional processing in order to be seen. This processing can include basic powder techniques or the use of chemicals. Patent fingerprints can be made by blood, grease, ink, or dirt. This type of fingerprint is easily visible to the human eye. Plastic fingerprints are three-dimensional impressions and can be made by pressing your fingers in fresh paint, wax, soap, or tar. Like patent fingerprints, plastic fingerprints are easily seen by the human eye and do not require additional processing for visibility purposes.

2. How to search for a fingerprint?

Locating a fingerprint often requires a vigilant and calculated search. However, in circumstances where the print is visible to the naked eye, finding a fingerprint is relatively easy. The more intricate searches take place when the print is present on a surface but not visible. The type of fingerprint left behind usually determines the amount of time and effort investigators must put into locating the print.
Investigators often follow a two-phase process when searching for fingerprints. The first phase involves looking for patent and plastic prints since they are visible.
Often times, a flashlight is used during this phase. The second phase involves a blind search for latent prints, according to Scientific Evidence[Paul C. Giannelli, Edward J. Imwinkelried, Andrea Roth, and Jane Campbell Moriarty, Scientific Evidence, p. 949 § 16.03 (5th ed. 2012)]. To narrow the search, investigators usually focus on the entry and exits points that the suspect used and any items that appear to have been disturbed, such as overturned lamps or possible weapons.
The type of surface being searched for fingerprints often determines the technique employed by investigator:

• Nonporous Surfaces: A powder technique is usually used to identify latent prints on nonporous surfaces such as glass, marble, metal, plastic, and finished wood.When powder is distributed on the surface, it adheres to the residue deposited from the finger’s touch, allowing investigators to find the print. Often times, to avoid smudging the print, a magnetic powder technique is used in which the powder is poured on the surface and then spread evenly over the surface using a magnetic force instead of spreading the powder with a brush.The color of the powder should contrast with the surface that is being searched to allow better visibility.

• Porous Surfaces: The powder technique is not as effective on porous surfaces such as fabric, unfinished wood, and paper. Instead, investigators often use chemical methods to locate the print such as iodine fuming, silver nitrate, or ninhydrin. When one of these chemicals comes into contact with the chemicals present in the fingerprint residue (natural oils, fats), the print become visual.

• Human Skin:Locating and identifying fingerprints left on human skin is incredibly difficult. According to Scientific Evidence, the first major obstacle is finding the print since the oily residue left by fingers that creates the fingerprint itself is often present on human skin, making it difficult to create a contrast between the surface (skin) and the print. Further, after a print is left on human skin, the oily residue often disperses and is absorbed into the skin, blurring the print. Two hours is the maximum amount of time that a print on skin may be viable.

• Textured Surfaces:Surfaces that are not flat or have a rough surface, such as a painting with brush strokes or a golf ball will make the process of identifying and collecting fingerprints more difficult, but not impossible.

3. How to get a fingerprint?

Here we will talk about tho most used method: using the powder.

• The application: Powders may be applied with a fingerprint brush, a brush with extremely fine fibers designed to hold powder, and deposit it gently on the fingerprint to be revealed, without rubbing away the often delicate residue of the fingerprint itself. They may also be applied by blowing the powder across the fingerprint, or by pouring the powder onto the print, and then blowing away the excess. Magnetic powders are also used, where a fine magnetic powder is held by a magnetic applicator,across the fingerprint. As no bristles touch the surface, this often damages the print less than other methods of developing the print.

• Photographing the fingerprint: After the print is located, it is vital that it is photographed before it is lifted. A photograph captures where the print was located in comparison to other objects and captures the orientation of the print. Further, a photograph can serve as a key piece of identification of a patent or plastic print and can be used to compare and possibly match the print to its source. Photographing the print’s location at the crime scene also guards against tampering of evidence.

• Lifting a print: can be accomplished on either flat surfaces or round surfaces. Lifting a print usually involves a rubber tape with an adhesive surface which is applied to the fingerprint, leaving an imprint on the tape. Often times, a flat object, such as a ruler, will be slowly swiped across the top of the tape to ensure that there are no bubbles or ripples in the tape that will affect the imprint. Next, the tape is carefully peeled off the surface and a plastic cover is placed on the adhesive side of the tape to prevent disruption of the print. Identification information and a description of the location of the print should be written on the back of the tape or card.

            4. How to use a finger print for identification?

Fingerprints come from ridges on the skin. These ridges form patterns. Loops. Arches. Whorls. If one print shows a loop and another a whorl, they came from different people. But if two prints both show loops, they may or may not have come from the same person. To find out, we need to take a closer look.

The ridges on our fingers do all sorts of strange things. They split. They come to an end. They form islands. These are the minutiae — a technical term in the world of fingerprints. We claim two fingerprints are the same if we can count enough minutiae that match.

In a perfect world we’d want a perfect match. But a fingerprint taken from a door handle usually isn’t perfect. So we’re willing to accept “enough” matches. And here’s where the math comes in.
We want enough matches to make the chance of a mistake very, very unlikely — say, one in a million billion. And most mathematical models predict a chance this small or smaller with around a dozen matches. So many fingerprint experts use this “twelve-match guideline” — plus or minus a few matches. When a fingerprint expert takes the witness stand and says he’s absolutely certain two fingerprints match, he means he’s found about twelve or more matching minutiae.