The term ‘impression evidence’ in crime scene investigation refers to marks, prints or any form left on a surface such as (soil, cement, wood, or metal) of the crime scene that can be used as evidence. Impression evidence is formed when one object is pressed against another material. The most common types of impression evidence found in the crime scene are footprints, tyre tracks, bite marks and tool marks.

Investigating officials can use impression evidence to link the suspects to the crime. A crime scene investigator must know how to detect and analyze the impressions to link them to a culprit. Careless collection of evidence will result to inaccurate findings. A competent forensics investigator can detect the slightest hint of impressions and process them to solve a case.

Impression evidence is usually found at the following places

1. Footwear impressions in dirt, mud, or snow;
2. Fingerprints;
3. Glove prints;
4. Tool mark impressions from prying, breaking, cutting, and scraping a surface;
5. Tyre track impressions in dirt, mud, or snow;
6. Bite marks found on a victim or food items;
7. Lip and ear impressions;
8. Fabric impression/transfers;
9. Various typed documents.

There are three types of fingerprints that can be found:

1. latent,
2. patent, and
3. plastic.

They are made of the sweat and oil on the skin’s surface. This type of fingerprint is invisible to the naked eye and requires additional processing in order to be seen. This processing can include basic powder techniques or the use of chemicals. Latent prints are generated due to sweat, or the presence of salts, oils and tiny particles of dust.

They can be found on a wide variety of surfaces: smooth or rough, porous (such as paper, cloth or wood) or nonporous (such as metal, glass or plastic). This type of fingerprint is easily visible to the human eye. Visible prints are formed when blood, dirt, ink, paint, etc., is transferred from a finger or thumb to a surface.

They are three-dimensional impressions and can be made by pressing your fingers in fresh paint, wax, soap, or tar. Like patent fingerprints, plastic fingerprints are easily seen by the human eye and do not require additional processing for visibility purposes.

Footwear impression marks – the mark made by the outside surface of the sole of a shoe (the out-sole) – are distinctive patterns often found at crime scenes. They are among the most commonly found evidence at crime scenes and present more frequently than fingerprints. Footwear impressions are created when footwear is pressed or stamped against a surface such as a floor or furniture, in which process, the characteristics of the shoe is transferred to the surface. The gait of each individual is different. The unique features of each person’s gait sculpt their footwear in individual and repeatable patterns. As a result, items of footwear, identical at the time of purchase, will rapidly take on the characteristics of the anatomy and movement patterns of the feet that wear them. Footwear also reflects the effects of the physical environments into which they step. Thus, within perhaps as short a time as a few hours, each shoe, boot, slipper, or other kind of footwear can become a unique reflection of the wearer.¹⁾

Taking of shoe mark When an individual item of footwear shows characteristics that are unique, it can be associated with the crime scene to the exclusion of all others of the same class, and this constitutes making a positive identification.

An image of a shoe mark can be obtained using photography, gel, or electrostatic lifting or by making a cast when the impression is in soil. Subsequently, in the forensic laboratory, the image of the shoe mark is compared with the shoe-prints and shoe impressions of known shoe samples. Interactive image enhancement operations are available in Photoshop and other image processing software that are available to the footwear examiner.²⁾

The FBI Laboratory maintains a footwear database that is a computerized reference collection of more than 14,000 shoe out-soles from hundreds of different footwear manufacturers. Many police departments and crime laboratories throughout the United States use similar computerized footwear databases. Examiners search questioned footwear impressions through these databases to determine the brand and/or model name of the footwear that left an impression at the crime scene. This information may lead to the suspect of the particular crime.

Investigators should be aware that footprints may not be readily apparent at a crime scene, and those that do exist may not be on the floor. Footprints can be found on doorsteps, floors, carpets, furniture, or even on victims themselves, both on their clothing or in the form of bruises. To make footprints visible, investigators examine surfaces with oblique lighting with the crime scene darkened or may use chemicals or an electrostatic detector device.