The traces, actually called fingermarks, consist of tiny amounts of moisture which form patterns corresponding to the ridges and lines on the fingers and other parts of the hand. Non-absorbent materials such as plastics and painted surfaces produce better marks than absorbent ones like fabrics. Marks are normally invisible unless they have been left by paint or blood. So a police fingerprint expert coats likely surfaces with very fine dust, often powered aluminium. The particles stick to the moisture traces, making them visible. Sticky tape is then places on the mark to lift away an impression of the pattern, which can be taken away and photographed. Some fingermarks are now photographed on site.
Modern technology is now helping the police to obtain marks from some previously bags and smooth leather.
One method called vacuum metallization involves putting the surface into a container from which the air is expelled, creating a vacuum. A layer of gold, then a layer of zinc, is evaporated onto the surface. The gold is deposited uniformly over the area, but it is absorbed by the ridges of moisture which make up the fingermark pattern. Zinc will only condense onto another metal, so it adheres to the gold-coated areas, enhancing them to provide a contrast with the uncoated fingermarks. The pattern of marks is then photographed.
Once the photograph is obtained, it is compared with fingerprints of known criminals held on police files. There are four main types of fingerprint pattern. The patterns are divided up into such features as ‘forks’, ‘lakes’, ‘spurs’ and ‘islands’.
For an identification to be presented in court, a number of recognizable features of the mark of a single finger or thumb must correspond with the same number of features on the print. The number varies between countries, but can be as high as 17. If the mark shows more than one finger the court will usually accept fewer features per finger. Most fingerprint officers and detectives regard more than eight features as enough to confirm identity. Although this would not be presented in court, it would be enough to concentrate investigation on a suspect.
Picture Credit : Google