Fingerprints have been used as a means of identification for over 100 years.
Fingerprints are made up of an intricate system of ridges which creates an uneven surface on the palms of the hand and soles of the feet. It's the arrangement of the ridges and their certain unique properties which a fingerprint expert will use to make the identification.
This section will help you to understand how fingerprints develop and the process in which a fingerprint expert identifies a print.
Development of Fingerprints
Fingerprints develop early in foetal life before birth. Volar pads (bumps) form on the babies' fingers and palms between 6 and 13 weeks of its life. Where these bumps occur, how the baby moves around inside the womb and how fast and big the baby grows all effect how the fingerprint patterns and ridges form.
The details of a person's prints are unique to them and only them. Even Identical twins do not have identical fingerprints.
A person's fingerprints will remain the same throughout their life. If superficial damage occurs the skin will grow back in exactly the same arrangement as at birth. This is why fingerprints are a reliable means of identification at all stages of a person's life. They are even one of the last features to decompose after death.
If you look at your own hands you will see that the surface is covered with lots of tiny lines - these are called ‘ridges'. Ridges cover all of your palms and fingers and also the soles of your feet and your toes.
The skin on these parts of the body is quite different from that covering the rest of your body and is called ‘Friction Ridge Skin'. It helps us to grip things and helps our body cool down when we are too hot by sweating. This is done through tiny holes on top of the ridges called ‘sweat pores'.
The ridges are also subject to certain breaks or interruptions which are called ridge features or characteristics.
There are two main types of characteristics:
- Ridge End - the ridge stops suddenly
- Bifurcation - a single ridge flows along, forks in two and then continues into separate ridges [or two ridges join and continue as one ridge]
The ridges of a fingerprint flow into discernable shapes or patterns that can be sorted into various types. Patterns are the first level of detail used in the identification process and recognition of the various fingerprint patterns are essential to the fingerprint officer.
There are 3 main pattern types named after the general shape they resemble:
Identifying a Print
When we touch something we leave behind some of our sweat and this in turn can leave behind an impression of the ridge detail on your fingerprint, thus leaving a fingerprint impression.
A fingerprint can be left on many types of surfaces. It can be made visible by brushing it with a powder or treating it with chemicals in a lab. Similarly, if the fingers are coated with ink or another substance such as paint, oil or blood, than a permanent impression may be left on a particular item.
When a fingerprint is found at a scene of crime, the impression left by the owner is referred to as a ‘mark'.
A ‘print' is taken by the police from a person they suspect of committing the crime. This is normally referred to as a ‘Tenprint'. Tenprints are the rolled impressions made on a fingerprint form taken under controlled conditions, normally at a Police Office.
A fingerprint examiner will compare and evaluate the crime scene ‘mark' against the ‘print' .
The identification of a fingerprint is based upon the agreement of details between the unknown crime scene mark and the known print on a fingerprint form by careful Analysis, Comparison, Evaluation and Verification (ACE-V) of the information held within both.
The first step, analysis, is an intelligence gathering process to ascertain and establish the amount of friction ridge detail present within the unknown mark and also consider the following factors:
The surface on which the unknown mark was discovered (glass, paper, polythene bag etc).
The substance that the unknown mark was made by. This may be sweat from the person who left the mark, blood, oil or any other foreign substance capable of leaving an impression.
The method by which the scene of crime officer or laboratory technician recovered the unknown mark to make it visible for identification. These include fingerprint powders, ninhydrin (a chemical which reacts with sweat and is used on paper) or superglue which is used to recover marks on plastics. The development medium used may have an effect on the appearance of the ridges and the quality of the print.
If there was any pressure placed on the unknown mark when it was deposited or movement as this can affect the appearance of the ridges.
The final aspect of analysis to be considered is the anatomical features within the unknown mark.
Ridge Flow and Pattern
The fingerprint expert will then analyse the print in more detail looking at the actual flow of the ridges to determine if there is any discernable pattern type.
They will consider any clues in the print that may indicate which finger, thumb or area of palm that could have left the impression. Pattern and digit determination allow the expert to prioritise those fingerprints to be compared.
The expert will also look at the quality and clarity of the unique features and characteristics that are revealed in the print.
Quality and Clarity
At the conclusion of the analysis stage the fingerprint expert will have made a decision as to the suitability of the print for further examination.
The second stage of the process is comparison. Once the fingerprint officer has analysed the unknown mark and accumulated all the information possible it is then compared to a print on a fingerprint form.
Comparison of Patterns
Firstly the patterns are compared. The overall fingerprint pattern does not have sufficient uniqueness to determine identification, and only functions to narrow the number of possibilities of donors. However, at this stage of the comparison, differences in pattern type may be sufficient to exclude the unknown mark from the print.
Comparison of Ridge Characteristics
If the patterns are found to be in agreement the comparison moves to the next level - the ‘comparison of ridge characteristics'. The ridge features are examined to ascertain whether they are in the same position, in the same order and have the same relationship to each other with none in disagreement in both the known prints and unknown marks. This is known as the coincident sequence.
If any unaccountable disagreement is found the identity cannot be established and the known print will be discounted from the comparison process.
However, if the initial sequences agree in both impressions, the expert will proceed to compare the relative position and location of further characteristics in both prints, all the time looking for any disagreement in the sequence of characteristics.
The fingerprint expert will also be taking into account the unique features of the ridges themselves looking for agreement between any visible distinctive ridge edge shapes and minute detail.
This process will continue until the expert is satisfied that the comparison process is complete.
Evaluation and Verification
After comparing the unknown mark and known print the fingerprint expert will make their evaluation. They will weigh up all of the information available as a result of the comparison process and determine whether there is agreement between the two. This then enables them to form an opinion regarding the identity of the print.
The most crucial aspect of the identification process is the verification element.
This is an independent and complete analysis, comparison and evaluation of both prints which is again carried out by fully qualified examiners. The verification process is the key to the reliability of fingerprint evidence.
It demonstrates that the original conclusions are valid through consistent results from the different experts who have analysed, compared and evaluated the information available in both impressions.