Exam 2 - Firearms & Trace Evidence Flashcards
Locard’s Exchange Principle
Every contact leaves a trace - putting two items together will likely leave a mark on one another.
Kinds of evidence at court (two distinct groups)
- Damage based evidence
2. Non-damage based evidence
Damage based evidence
Evidence from footwear and instruments. An item has to acquire damage in order to leave behind a unique impression.
Non-damage based evidence
Fingerprints. Any damage to it’s features (like a scar) will not be used in its classification as it may fade away with time.
No two fingerprints have been found to be the same (even in identical twins). So no 2 items (no matter how similar) are likely to acquire the same random damage features.
Footwear Impression
An impression is the retention of the characteristics of an item by another object.
Shoes impress over soft grounds and leave behind an impression.
Mechanisms of an impression
- On a two-dimensional flat surface (tiled floor or piece of paper). Material can be deposited and remain for a considerable length of time - due to a static electrical charge produced on the under-sole transferring particles to a surface or wet deposits on the under-sole left behind on the surface.
- A three-dimensional impression is formed if the surface was soft and the under-sole sank into it (sand).
2-D impressions
Visible only when there is contrast between background and impression.
- Impression on white paper; use black powder to make it visible because of the color contrast.
- Non-porous surfaces like glass, linoleum and tile; use dyes or chemicals.
- Impressions in dust can be treated with methods involving electrostatic treatment.
- Impressions in blood.
Search for impressions without causing damage
Shine a light obliquely (a light source positioned close to the surface giving a low angle of incident light) across the surface of interest.
