Impression Evidence

When an item like a shoe or a tire comes into contact with a soft surface, it leaves behind a pattern showing some or all of its surface characteristics known as an impression. The collection and analysis of impression evidence found at the scene of a crime can often be very important to an investigation.

The major types of impression evidence are shoeprints, tire tracks , tool marks and the marks that are found on a fired bullet. Impressions can be found in a variety of surfaces including dust, carpet, mud, and, very significantly, blood . Collection of an impression is a specialized forensic task because, unlike a hair or bullet, an impression cannot just be packaged and taken back to the lab. Impression evidence is often fragile; a tire track may deteriorate or even be destroyed by rainfall, for example. There is a need for the forensic scientist to retain as much information as possible when collecting impression evidence.

Impressions may be found in either two or three dimensions. An object like the sole of a shoe will leave a two dimensional impression on a hard surface such as a tiled floor. The impression comes from static charge on the sole transferring particles from the sole to the surface. Sometimes wet deposits on a sole will adhere to such a surface. A three-dimensional impression is made when the surface over which the shoe passed was soft and the sole actually sank into it. The method used to collect the impression evidence depends largely on how the impression was made and on what kind of surface. The impression is photographed on-site and then a plaster cast may be made, or the impression may be dusted with fingerprint powder. Dyes can be used to bring up impressions in non-porous surfaces, such as linoleum, although these are absorbed by a porous surface like carpet. Impressions made in dust can be very fragile. Contact with a brush or spray would destroy them. However, electrostatic treatment allows the impression to be lifted onto a more stable surface for transport back to the lab. Many new methods are being developed for the collection and enhancement of impression evidence so that the maximum information can be extracted.

see also Casting; Crime scene investigation.