Impression Evidence is when an object comes into contact with another surface or object giving it the potential to leave something behind or collect a trace in the process.
This transfer of evidence may come in the form of an impression or physical trace.
The underlying principle that explains this transfer of evidence is Locard’s Exchange Principle.
In this section of the course, we will focus on the impression evidence that is potentially created when items come into contact with one another.
The first is class characteristics, which means an item can be placed within a group of like items, but not linked to a single source. Examples could include fibers from a shirt, spent shotgun pellets, and plant material.
The second is individualized characteristics, indicating there is something unique about the item that links it to a single, specific source. Examples might be finger/footprints, DNA, or fired bullets.
Impression evidence can fall under either one of these types.
For example, a fingerprint is an impression left when an individual touches an object. This impression is considered an individualized characteristic because it is unique to a particular individual.
On the other hand an impression caused by a fabric will only provide class characteristic information.
Just a few types of common impression evidence are: shoe, tire, tool marks including firearms and typed documents, dental impressions, fabric impressions, friction ridge skin from suspect or victims, and handwriting or questioned documents.
Each of these has special aspects to collection and analysis, and an expert in one field may not be an expert in another.
Object traces are of nonhuman origin, meaning they are not produced by the transfer of human features such as finger, palm, and ear prints.
Manufactured products tend to have markings either on them or in them. Such markings may be the result of the manufacture process and represent class characteristics.
For example:
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It’s important to distinguish which features come from the production process or class characteristics and which come from the specific-use history of the source or individualized characteristics.
Breaks, scratches, and wear are all types of activity history.
Such traces include fingerprints, palm and bare footprints, as well as ear prints.
When an instrument is pushed or forced into a material this is called compression.
However, sometimes minute imperfections, markings, or damage to a tool can be traced back to a specific source, making it individualized evidence.
For example, imagine a chip out of the end of a screwdriver matching up to a pry mark on a door jamb, or the broken head of a hammer matching bruises on a victim.
Additionally, human “imperfections” and general human variation make things like fingerprint or tooth impressions individualized and potentially useful in the progression of a case.
IMG SOURCE:
http://img.medscape.com/pi/emed/ckb/pathology/1603817-1607640-1680207-1769223tn.jpg
http://www.utforensic.org/docs/bitemark2.jpg
As you look at the following images can you identify class and/or individualized characteristics?
Each of these has special aspects to collection and analysis, and an expert in one field may not be an expert in another.
IMG SOURCE: http://img.medscape.com/pi/emed/ckb/pathology/1603817-1607640-1765532-1787375tn.jpg
Impressions epitomize the one and two way sharing of materials between human beings and their environment.
Impression evidence can be two or three dimensional and this appearance will dictate which documentation and collection methods are most appropriate. Therefore, it is essential to be able to locate and identify them to avoid destroying evidence.
For example, a handwritten document has the potential to yield both fingerprint and handwriting impression evidence. However, applying chemicals to develop fingerprints could destroy necessary minutiae for questioned documents.