Chance impressions, or what is more commonly known as latent fingerprints, are the oftentimes invisible patterns made by fingerprints that are usually left at crime investigations or on objects recovered from crime scenes, and forensically analyzed by latent fingerprint experts with the application of chemical or physical methods.
The use of fingerprinting as a means to identify criminals spread throughout Europe and North America during the early twentieth century after British police officer Sir Edward Richard Henry introduced the use of fingerprints to solve crimes in the 1890s. As scientists studied fingerprint identification in more detail, they discovered that the ridge arrangement of fingerprints is unique and permanent, unless accidentally or intentionally altered. As crime-detection methods improved, law enforcement officers discovered that any hard, smooth surface touched by hands could produce fingerprints made by the oily secretions found on skin. When these so-called latent fingerprints were dusted with powders or chemically treated, the resultant pattern (or impression) could be observed, photographed, and stored for later use.
Latent fingerprints, which today are important pieces of forensic evidence , are created either artificially, naturally, or as a combination of the two. They are artificially created when fingers become covered with a foreign residue such as grease or oil. Latent fingerprints are naturally created when very small sweat pores on friction skin (that is, the top of skin ridges located on the inner surface area of fingers and hands) excrete perspiration. This perspiration, along with oils from touching other parts of the body and hair or from contact with external substances, remain on these ridges, so when an object is touched by a finger a duplicate recording of these characteristics is usually left on the surface. These hidden (or latent) impressions can be made visible when latent print examiners apply chemicals, lasers and other light sources, powders, or other physical means.
Latent fingerprint evidence is generally divided into two categories: porous evidence, such as cardboard, paper, and unfinished wood, that readily allows for the preservation of latent fingerprints because residue soaks into the surface; and non-porous evidence, such as glass , finished wood, and plastic, which does not easily permit the preservation of latent fingerprints because substances only lie on the surface and can be intentionally or accidentally wiped away.
A positive identification of a latent fingerprint is normally achieved when, according to the expertise of a latent print examiner, the amount of similarity between the latent print (found at a crime scene, for example) and the inked fingerprint (taken from a suspect) is sufficient to make a corresponding match. Such matches are not based only on the degree of similarity (or number of matching points) between the two prints, but on various examinations made of the skin ridges. The resulting conclusion by the examiner is based on that person's experience, training , and understanding of the science behind latent fingerprint identification. A trusted latent print examiner must be knowledgeable in all areas of the science of fingerprint identification including classification methods, history, and procedures for locating, processing , and preserving latent prints at the crime scene or in the laboratory. As part of their duties, these examiners also give expert witness testimony with regards to latent fingerprints in all phases of the legal process.
New electronic procedures have been developed to detect and analyze latent fingerprints for crime detection. One such procedure is called digital imaging , the method of placing latent fingerprints into a digital format with the use of such equipment as digital cameras , computers, and scanners. Latent print examiners then use digital enhancement imaging software to adjust various features of the digital information such as brightness, contrast, and density in order to improve the quality of the evidence. Such electronic images are then input into an Automated Latent Print System (ALPS) computer, a database system that searches latent prints for possible matches. The ALPS computer assists the examiner in locating and retrieving records of known prints so that a list of possible matches can be made. Once a list is generated, another examiner (independent from the original investigation) conducts a scientific examination to verify the original identification.
On the federal level, the Latent Print Unit of the Federal Bureau of Investigation (FBI) conducts investigative work concerning the examination of latent fingerprints. When submitted as evidence to the FBI Laboratory, latent prints are input into the Integrated Automated Fingerprint Identification System (IAFIS) computer. The IAFIS then compares them against data from the FBI Criminal Justice Information Services (CJIS) Division, the largest international repository of fingerprint records.
Such efforts help to identify crime evidence involving latent fingerprints and solve serious crimes throughout the nation.
see also Automated Fingerprint Identification System (AFIS); Chemical and biological detection technologies; Computer forensics; Digital imaging; DNA fingerprint; FBI crime laboratory; Fingerprint; Fingerprint analysis (famous cases); Ink analysis; Integrated Ballistics Identification System (IBIS); Ridge characteristics.