Fingerprinting
Fingerprints – An Old and Traditionally Valuable Type of Evidence
Fingerprints are among the oldest and most important kinds of evidence used for human identification. The individuality of fingerprints is so impressed into the public consciousness that fingerprint analogies are regularly used in dvertising interchangeably with “unique” or specifically individual.” Although the terms “DNA fingerprints” and“DNA fingerprinting” are often invoked by the popular press, and even some DNA scientists, to imply individuality, most forensic scientists feel it is not proper terminology. The use of friction ridge skin patterns on fingertips as a means of personal identification dates back many centuries. A convincing fingerprint match is universally accepted as certain evidence that identifies a particular person. Fingerprints are often essentially two-dimensional but they can be in soft receiving surfaces and be three-dimensional (plastic or impression prints). |
We often think of fingerprints as being used primarily to help locate, identify, and eliminate suspects in criminal cases. The initial driving force behind the development of fingerprints was not solving crimes but unambiguous identification of individuals. Fingerprints, along with dentition patterns (aka teeth marks) are also important in making unequivocal identifications of human remains when more conventional methods of postmortem identification cannot be
used. Fingerprints may also be thought of as one member of a class of biometric identifiers that includes retina or iris patterns of the eye, face thermography, hand geometry, and others. As the technology for rapid scanning and storage of these biometric patterns has developed, they have become more important as the basis for security systems, accessing information and secure areas, and to prevent incorrect identifications. |
About Fingerprints –Their Nature and the History and Development of their Use
It is important to understand something about the nature of fingerprints, and the properties that contribute to making them individual. We also look at how the use of fingerprints became so important in criminal records and investigations. The development of fingerprints as a means of personal identification is a major thread in the development of forensic sciences in the later 19th and earlier 20th
centuries.
Three features of fingerprints underlie their use as a means of personal identification: (1) every fingerprint is unique to an individual; (2) fingerprints do not change throughout life, unless there is some significant damage to the dermal layer of skin; and (3) they can be classified for convenient file searching.
It is important to understand something about the nature of fingerprints, and the properties that contribute to making them individual. We also look at how the use of fingerprints became so important in criminal records and investigations. The development of fingerprints as a means of personal identification is a major thread in the development of forensic sciences in the later 19th and earlier 20th
centuries.
Three features of fingerprints underlie their use as a means of personal identification: (1) every fingerprint is unique to an individual; (2) fingerprints do not change throughout life, unless there is some significant damage to the dermal layer of skin; and (3) they can be classified for convenient file searching.
It has been known for a long time that the fingertips, palms of the
hands, and soles of the feet of humans and other primates have friction ridge skin. The skin on those areas forms a complicated pattern of “hills and valleys.” The“hills” are called ridges, and the “valleys” furrows. This friction ridge skin on the fingertips form any one of a number of basic fingerprint patterns – arches, loops and whorls. Within each basic pattern there are a number of possible variations – such as plan and tented arches, and ulnar and radial loops. Loop and whorl patterns contain definable features called the core and delta of the fingerprint. They are important in 10-print fingerprint classification and in fingerprint comparison. The patterns form early in embryonic development and remain constant throughout the embryonic life, birth, and life of the individual. A person’s genetic composition almost certainly plays some part in determining the sizes and basic shapes of the patterns and ridges, but it cannot be the only factor. We know this because identical twins, who come from the same fertilized egg and thus have identical genetic makeup, have distinguishable fingerprints. |
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Within a fingerprint pattern there are a number of features called minutiae. In fingerprint comparisons, these are the crucial features examiners use to actually compare the fingerprints to decide if they are or are not from the same source. Fingerprint ridges form the minutiae by
(1) ending abruptly (forming an ending ridge),
(2) splitting into two ridges (forming a bifurcation), or
(3) being short, like the punctuation mark at the end of a sentence (forming a dot).
(1) ending abruptly (forming an ending ridge),
(2) splitting into two ridges (forming a bifurcation), or
(3) being short, like the punctuation mark at the end of a sentence (forming a dot).
Computer Storage Systems and Fingerprints
Computer storage and retrieval systems for fingerprints were originally developed for law enforcement applications. Efforts to develop the systems began in the early 1960’s as a collaboration between the FBI and scientists at the National Bureau of Standards. The law-enforcement-based automated system is called the Automated Fingerprint Identification System or AFIS. AFIS is a complicated, expensive set of computer hardware and software for image processing and storage. A central mainframe or large server holds the database. There are multiple workstations for scanning, input, and searching. Large systems have workstation distributed over dozens of locations in a state. AFIS can be view in perspective as one of the three types of major electronic databases for law enforcement purposes that are now available. The others are CODIS (Combined DNA IndexingSystem) which holds DNA profiles and NIBIN (National Integrated Ballistics InformationNetwork) which holds searchable image information from fired bullets and cartridge cases. |
Each database holds two types of files or profiles. One is the knowns. In AFIS, this file contains the prints of known individuals. Any questioned specimen, image, or profile can be searched for in the known file and if it is found, its source is identified as a match. The other type is often called the “forensic” file. It consists of images or profiles from unsolved cases. It contains evidence fingerprints that have not yet been associated with an individual. The file is valuable to investigators, however, in that it allows cases that are not obviously related to be connected because they have the same fingerprints. This kind of connection allows investigators to share information and leads, thus increasing the probability of apprehending a suspect.
As noted earlier, AFIS should be seen today as part of a larger picture that includes an array of automated systems for human biometrics– the use of some type of body metric (part) for identification. Today, systems based on retinal scans, iris scans, and rapid fingerprint scanning followed by quick comparison with database records are available for security purposes. Criminal identification and related law enforcement applications of automated fingerprint systems remain very important but AFIS and other biometric systems are rapidly finding their way into use as public and private security methods. These systems can help control entry and/or access into computers or structures, can identify persons for security purposes to prevent identity theft, and can help control welfare or social services fraud. |
Collection and Preservation of Fingerprint Evidence
Most of the time collection of fingerprint evidence involves collecting and submitted the intact item that bears or might bear the fingerprint. At scenes, fingerprints are frequently not visible, but latent (invisible, see next section). Latent prints must be developed using a physical or chemical technique to make the print clearly visible for comparison.
At a scene, logic may suggest collecting items for fingerprint processing at the laboratory. These items would be those that might have been touched or handled by people involved in the case. The purpose might be to try and identify fingerprints so a cold search can be done in AFIS to try and identify or eliminate possible suspects. Another purpose might be to find out who among several people involved in a case touched or handled an item.
At times, fingerprints may be on items that would be difficult or impractical to collect for processing at the laboratory. Classically powder-dusted fingerprints were collected by tape lifting. This is still a good technique but powder dusting may not be the best choice for developing certain latent prints. Regardless of the technique used, latent prints developed at the crime scene should be carefully photographed and the developed print then documented and collected if possible. The fingerprint should be photographed from a distance to show where the print lies in relationship to the surrounding objects and up close to capture enough of the fine detail for comparison. With most techniques
other than powder dusting, collection will involve submitting the intact item or object.
Most of the time collection of fingerprint evidence involves collecting and submitted the intact item that bears or might bear the fingerprint. At scenes, fingerprints are frequently not visible, but latent (invisible, see next section). Latent prints must be developed using a physical or chemical technique to make the print clearly visible for comparison.
At a scene, logic may suggest collecting items for fingerprint processing at the laboratory. These items would be those that might have been touched or handled by people involved in the case. The purpose might be to try and identify fingerprints so a cold search can be done in AFIS to try and identify or eliminate possible suspects. Another purpose might be to find out who among several people involved in a case touched or handled an item.
At times, fingerprints may be on items that would be difficult or impractical to collect for processing at the laboratory. Classically powder-dusted fingerprints were collected by tape lifting. This is still a good technique but powder dusting may not be the best choice for developing certain latent prints. Regardless of the technique used, latent prints developed at the crime scene should be carefully photographed and the developed print then documented and collected if possible. The fingerprint should be photographed from a distance to show where the print lies in relationship to the surrounding objects and up close to capture enough of the fine detail for comparison. With most techniques
other than powder dusting, collection will involve submitting the intact item or object.
Essentially three types of prints may be encountered at crime scenes and/or on items of evidence: visible (patent), plastic (impression), and latent. A visible (patent) print is one that needs no enhancement or development to be clearly recognizable as a fingerprint. Such a print is often made from grease, dark oil, dirt, ink, blood, or other visible material. It may be suitable for comparison with no additional processing.
A plastic (impression) print is a recognizable fingerprint indentation in a soft receiving surface such as butter, Silly Putty, tar, drying paint, and so on. These prints have distinct three-dimensional character but are immediately recognizable and often require no further processing. A latent print is one that requires additional processing to be rendered clearly visible and thus potentially suitable for comparison. The processing of latent prints to render them visible is called development (or enhancement or visualization). Great progress has been made in this area by the clever applications of chemical and physical principles, coupled with a better understanding of the composition of latent fingerprint residues. |