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Fingerprint Analysis - The Basics
Every person has minute raised ridges of skin on the inside surfaces of their hands and fingers and on the bottom surfaces of their feet and toes, known as 'friction ridge skin'. The friction ridges provide a gripping surface - in much the same way that the tread pattern of a car tire does. Friction ridge skin is also the only skin on the body without hairs.
Friction ridges do not run evenly and unbroken across our fingers, hands, toes and feet. Rather, they display a number of characteristics known as minutiae. The principle categories of minutiae are as follows:
- ridge ending - a ridge that ends abruptly;
- bifurcation - a single ridge that divides into two ridges;
- lake or enclosure - a single ridge that bifurcates and reunites shortly afterward to continue as a single ridge;
- short ridge, island or independent ridge - a ridge that commences, travels a short distance and then ends;
- dot - an independent ridge with approximately equal length and width;
- spur - a bifurcation with a short ridge branching off a longer ridge; and
- crossover or bridge - a short ridge that runs between two parallel ridges.
(Some fingerprint experts consider that there are only two categories of minutiae - ridge endings and bifurcations - with all other categories being combinations of these.)
The Principles of Fingerprint Identification
There are two fundamental principles underlying the use of fingerprints as a means of identifying individuals - immutability and uniqueness.
Immutability. More than a century of accumulated fingerprint study and experience has demonstrated that friction ridge patterns do not change naturally during the life of a person. The pattern of minutiae starts developing in the third month of pregnancy and is fully formed by the fourth month. During a person's lifetime, the pattern remains the same, apart from changing in size or by accident, mutilation or skin disease, until death. In fact, the friction ridge patterns will remain after death until the body decomposes.
Uniqueness. Friction ridge detail forms in a purely random manner during fetal development in the womb. There is sufficient variability in the arrangement of minutiae to ensure that no two friction ridge patterns are identical, whether they are on different fingers of the same person or on the fingers of different people. (The same principle covers all friction ridge skin.) While this principle is difficult to prove empirically, no two fingerprints have ever been found to be identical in over a century of the use of fingerprinting. And the number of people to have been fingerprinted worldwide is now in the hundreds of millions. Additionally, studies have demonstrated that while identical twins share the same DNA profile markers, they can nevertheless be differentiated by their fingerprints.
A further premise underlying the use of fingerprinting to identify individuals is that while ridge patterns display immense variability, they can be grouped into pattern categories to facilitate the classification, filing and accessing of very large volumes of fingerprint records. Most English speaking countries have used the 'Henry System' developed by Sir Edward Henry at the end of the nineteenth century. The three basic categories of ridge patterns in this system are: loops, whorls and arches.
Friction ridges have very small pores along their length that continuously exude perspiration. The perspiration forms a layer along the top of the ridges. When a person touches an object, a moist impression of the friction ridge pattern is left on that object. The phrase 'latent fingerprint' is generally used to refer to a fingerprint, or partial fingerprint, which has been left at the scene of a crime.
The degree to which a latent fingerprint is visible depends on the nature of the object touched and the conditions at the time the object was touched. Impressions made on smooth non-porous surfaces such as metal, glass or plastics are sometimes visible to the naked eye. Such prints can be developed with powder that adheres to the moisture in the fingerprint. Powders that contrast in colour with the background are used to make the fingerprint visible. The developed fingerprints are recorded by photography, and sometimes also by 'lifting' the impression with adhesive tape.
Impressions made on porous objects such as paper, cardboard and unfinished timber are generally invisible. These prints can be detected and developed with special lighting, lasers, x-rays and a range of chemical processes. Once developed, the fingerprint is generally recorded by photography.
The amount and type of matter on fingers at the time an object is touched also affects the degree to which a latent print is visible. For example, if blood or paint are present on the skin when an object is touched, then the resultant fingerprints will be visible regardless of the nature of the surface.
Points of Identification
Human skin is elastic, and no two impressions of the same fingerprint will be exactly identical. Therefore, there is little value in measuring the angle formed by a bifurcation or the precise distance between two particular minutiae on fingerprints.
However, fingerprints can be compared to each other by examining the minutiae to determine whether:
- the same minutiae are present (eg a bifurcation);
- the minutiae flow in the same direction (eg the bifurcation is on a ridge running horizontally and the two divided ridges are to the right of the bifurcation); and
- the minutiae occupy the same relative positions to each other (eg the bifurcation is separated from an enclosure below it by six intervening ridges).
Where minutiae on two different fingerprint impressions meet these criteria, they are referred to as points of similarity. Where minutiae do not meet these criteria, they are referred to as points of dissimilarity. When sufficient minutiae are located in the same true relative sequence or unit relationship, then an identification is assumed, and the points of similarity are referred to as points of identification.
As soon as a fingerprint examiner identifies a single unexplainable point of dissimilarity between two fingerprint impressions, then he or she assumes there is not a match between the two fingerprints.
However, there is no international standard for the number of points of identification required for a match between two fingerprints. (Some countries have set minimum numbers of points of identification for a match. However, the International Association for Identification has found that there is no valid basis for such an approach.) Rather, the fingerprint examiner offers an expert opinion, based on his or her training and experience and a thorough examination of all the details of the fingerprints, as to whether there is a match or identification. This opinion will be one of the following:
- there is a match between two fingerprints;
- there is not a match between two fingerprints; or
- the comparison is inconclusive.
The fingerprint examiner does not make judgments about the likelihood or probability of a match - there is no such thing as a 50% or 80% match. Factors that may lead to an inconclusive comparison include latent prints being smudged, not sufficiently complete, or overlaid one over another. Alternatively, dirt or other materials on the finger at the time the latent impression was made may result in it being dissimilar.
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Last Update 01/05/09