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 ‘tenprints'. 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' to determine if they came from the same person.
The identification of a fingerprint is based upon the unique 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:
Substrate - The surface on which the unknown mark was discovered (glass, paper, polythene bag etc.).
Matrix - 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.
Development medium - 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.
Distortion - 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.