Friday, August 04, 2006

Fingerprint



A fingerprint is an impression normally made by ink or contaminants transferred from the peaks of friction skin ridges to a relatively smooth surface such as a fingerprint card. These ridges are sometimes known as "dermal ridges" or "dermal papillae". The term fingerprint normally refers to impressions transferred from the pad on the last joint of fingers and thumbs, though fingerprint cards also typically record portions of lower joint areas of the fingers (which are also used to effect identifications). Friction skin ridges are not unique to humans, however, and some species of primate also have friction skin ridges on "fingers" and paws in configurations sometimes similar to human friction ridge skin. Some New World monkeys also have friction ridge skin on their tails, possibly associated with use of their tails for gripping during climbing, and the knuckle-walking great apes have friction ridge skin on the dorsal surfaces of their fingers. Friction skin ridges on humans are commonly believed to provide traction for grasping objects. In the over 100 years that fingerprints have been examined and compared, no two areas of friction ridge skin on any two fingers or palms (including between identical twins) have been found to have the same friction ridge characteristics.


Fingerprint identification

Fingerprint identification (sometimes referred to as dactyloscopy) is the process of comparing questioned and known friction skin ridge impressions (see Minutiae) from fingers, palms, and toes to determine if the impressions are from the same finger (or palm, toe, etc.). The flexibility of friction ridge skin means that no two finger or palm prints are ever exactly alike (never identical in every detail), even two impressions recorded immediately after each other. Fingerprint identification (also referred to as individualization) occurs when an expert (or an expert computer system operating under threshold scoring rules) determines that two friction ridge impressions originated from the same finger or palm (or toe, sole) to the exclusion of all others.

Latent prints

Although the word latent means hidden or invisible, in modern usage for forensic science the term latent prints means any chance or accidental impression left by friction ridge skin on a surface, regardless of whether it is visible or invisible at the time of deposition. Electronic, chemical and physical processing techniques permit visualization of invisible latent print residue whether it is from natural secretions of the eccrine glands present on friction ridge skin (which produce palmar sweat, but no oils), or whether the impression is in a contaminate such as oil, blood, paint, ink, etc.

Patent prints

These are prints which are obvious to the human eye and are caused by a transfer of foreign material on the finger, onto a surface. Because they are already visible they need no enhancement, and are photographed instead of being lifted. Where possible, the item containing the print is taken away and looked at by forensic scientists.

Plastic prints

A plastic print is a friction ridge impression from a finger or palm (or toe/foot) deposited in a material that retains the shape of the ridge detail. Commonly encountered examples are melted candle wax, putty removed from the perimeter of window panes and thick grease deposits on car parts. Such prints are already visible and need no enhancement, but investigators must not overlook the potential that invisible latent prints deposited by accomplices may also be on such surfaces. After photographically recording such prints, attempts should be made to visualize other non-plastic impressions deposited in natural finger/palm secretions (eccrine gland secretions) or contaminates.

Classifying fingerprints

There are three basic fingerprint patterns: Arch, Loop and Whorl. There are also more complex classification systems that further break down patterns to plain arches or tented arches. Loops may be radial or ulnar, depending on the side of the hand the tail points towards. Whorls also have sub-group classifications including plain whorls, accidental whorls, double loop whorls, and central pocket loop whorls.

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