The goal of fingerprint examination is to achieve individual identification, that is, to establish that a fingerprint came from a particular individual. The scientific basis for latent print identification consists of the following two premises:
1. The friction ridges formed during fetal development on the palmar surfaces of the hands and the plantar surfaces of the feet persist throughout the life of the individual except when damaged by scarring or disease.
2. No two areas of friction ridges on the hands or feet of any person or persons are duplicated.
The second premise is supported by the fingerprints of identical twins. Although DNA provides the instructions for producing every physical characteristic of the human body, the fingerprints of otherwise identical twins are different. Therefore, it must be concluded that fingerprint patterns are entirely random.
The examination of latent fingerprints involves several steps, progressing from simple to increasingly sophisticated. It begins with an initial visual inspection of the surface under strong light. Alternative light sources are then applied. Some prints will exhibit inherent luminescence when exposed to a laser or a forensic light source. Next the surface properties are evaluated for absorbency. One simply needs to imagine what a drop of water would do if placed on a given surface—remain intact or be absorbed. Fingerprint examiners generally process absorbent surfaces with chemical reagents and treat nonabsorbent with materials such as powder, superglue, or dyes.
Once the print has been enhanced through one mean or another, it must be photographed, lifted, or examined directly. As with firearms identification, fingerprint comparisons begin by determining whether class characteristics are consistent between samples. As previously discussed, there are three basic types of friction ridge detail: loops, whorls, and arches. These patterns constitute the class characteristics that the fingerprint examiner looks for. If, for example, the left thumb of the fingerprint from the crime scene is a loop pattern and the suspect is determined to have a whorl pattern for his or her left thumb, there is no need to look further at the suspect's prints. The suspect could not have left the latent print found at the crime scene.
A fingerprint card with the suspect's prints is the standard against which the questioned (crime scene) prints are compared. The comparison of class characteristics can be done without the aid of magnification when the prints being compared are clear and complete. When they are not clear or complete, the fingerprint examiner must then resort to a stereomicroscope or a hand lens, both of which have low-power magnification. The less detail that is visible, the more difficult this process will be.
Assuming that the class characteristics are consistent, the examiner then begins to look for the individual characteristics, or minutiae, discussed in chapter 2. There are six methods that can be used for the actual comparison: the overlay method, photographic strip method, Seymour trace method, polygon method, Osborn grid method, and conventional method.
The overlay method uses a transparent sheet printed with a fingerprint, usually with a different color ridge detail than the prints being compared with it. To determine identity or nonidentity, the fingerprint examiner lays the transparent sheet over the fingerprint sample. Obviously the base fingerprint and the overlay both must be to the same scale. The overlay transparency is then shifted back and forth and up and down as the examiner compares ridge detail.
The photographic strip method uses photographic enlargements of the two prints being compared. First the enlargement of the latent print is cut into lateral strips, then it is placed on top of the other enlargement and fixed in a secure frame. The strips are then removed and replaced one at a time to reveal the underlying detail on the inked impression. The visual comparison proceeds one strip at a time.
The Seymour trace method involves copying both the inked and the latent print onto tracing paper. The two tracings are then compared by overlaying them and using back lighting.
The polygon method utilizes photographic enlargements that are pinpoint punched at corresponding characteristics. The enlargements are then flipped over, and the analyst draws lines to connect the pinpoints. If the polygon created on both enlargements matches, an identification is declared.
The Osborn grid method involves making photographic enlargements of the two fingerprints to be compared with a superimposed grid on each. The grid cells are then compared cell by cell. If all available characteristics in each cell match, an identification is declared.
The conventional method has shown to be the best and most reliable method of comparison over the years. It involves a side-by-side visual comparison of the latent print and the fingerprint card using a fingerprint glass. The fingerprint glass consists of a magnifying lens mounted in a metal stand. This frees the examiner from having to hold the lens. The examiner begins by using a probe to mark a minutia observed in the print; the probe has a needle tip that is physically stuck into the print at the area of interest. A second fingerprint glass over the impression on the inked print card allows the examiner to look for the minutia observed in the questioned print. If the corresponding minutia is found, it is marked with a second probe. The examiner can then move back and forth between the questioned print and the inked impression from the suspect to confirm that the same detail is present. This process is repeated point by point until the examiner has obtained sufficient correspondence to make an identification.
With the advent of digital imaging the examiner can now do side-by-side comparisons on a monitor, as well as the overlays. Additionally, various software is available that allows questioned prints to be digitally enhanced. The software allows various points to be electronically marked and measurements of spatial orientation to be made on screen. The comparison results can then be printed and the print retained in the case file.
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