Other Marks Flashcards
(31 cards)
What is PPE?
PPE - Physical Pattern Evidence
PPE - > Objects/materials which have retained physical characteristics of other objects/materials through direct physical contact
PPE Examples
Surfaces of objects contain structures/imperfections that form patterns
Impressions / striations
- Ridges on finger surfaces
- Ridges on lip surfaces
- Structures/logos on shoes
- Imperfections on screwdriver head
- Imperfections inside firearm barrel
- Palms
- Feet
- Tyres
- Bites
- Tools
How does PPE Work?
Patterns readily transferred from the donor surface to the receiving surface by contact/interaction between them - these traces are obtained by PPE
Every physical trace that…
- Contains any recognisable/exploitable patterns
- Left by physical direct contact
Importance of PPE in Forensic Science
PPE is critical/fundamental for forensics:
1) Easily created/released during physical interaction of objects/people
2) unintentional
What can PPE be used for?
- Used to provide evidence to identify criminal/objects used and establish associations to the crime scene
- Criminals may interact with crime scenes in many ways -> interactions cause transfer of material/modification of surfaces and cause PPE creation
PPE taxonomy
- Nature of source
- Formation procedure
Source type
People/objects produce and release PPE.
Divided into:
- personal traces - finger, palm, bite, barefoot, ear, lips impressions
- traces of objects - tools, firearms, shoes, tyre impressions, and striations
Different evidential values in crime reconstruction:
1. Fingerprints - personal traces
Direct link to a person
2. Footprints - traces of objects
Direct link to an object (shoes). More evidence is needed to link to a person - DNA.
Formation mechanism
- Impression evidence
The result from a patterned object making pressure contact with a receiving surface (finger, shoe, lip) - Striation evidence
The result of a tool/object moving against a receiving surface (tool/barrel)
Formation of impression -> vertical pressure
Formation of striation -> contact movement against surface
What is impression evidence split into?
- indentations
- imprints
What are indentations?
Impressions with distinct 3D characters. Usually found on soft receiving surfaces and are deformations resulting from contact.
What are imprints?
Impressions effectively 2D (no depth). Hard/flat surfaces and due to deposition/removal of material.
What is imprint evdience split into?
- positive
- negative
What is a positive imprint?
Due to deposition of material from patterned object to contact surface (residue).
Fingerprint in blood
What is a negative imprint?
Due to the removal of material from the contact surface by part of the patterned object that touches it.
Fingerprint after powder enhancement
Explain exploitation of pattern marks
- Recovery - deposition surface/mark (crime scene)
- Enhancement + Photography (crime scene and lab)
- Analysis of pattern characteristics (lab and ACE-V)
- Comparison with reference print (lab and ACE-V)
- Evaluation - assessment of evidential value (lab and ACE-V)
What are the general principles for pattern marks?
On a crime scene, pattern marks can be deposited on every surface, including:
- indoor surfaces/floors -> 2D impressions
- human bodies -> 3D, Photographed as 2D
How is PPE comparative evidence?
Involves a comparison between a questioned element and a reference element.
1 - The mark has to be compared to a print to produce evidence
2 - Marks have to be imaged to be exploited - hence different photographic steps
How do you carry out preservation at a crime scene?
1st Steps are the most important …
Rule - Take photos at every procedure type
- Visible traces -> before applying further enhancement
- All traces -> before sampling (after enhancement)
What is needed for on-scene Photography?
- Camera tripod for stability/longer exposures
- Adequate illumination -> side light, gazing illumination
- Camera parallel to the ground
- Ruler to allow measurements and comparison
If the image is distorted is it useless for comparison.
How are marks recovered?
If the surface isn’t movable, marks are usually recovered by removing or duplicating their features. Different collection techniques are used and depend on the mark/support:
- indentations are casted
- imprints are lifted
What casting materials are used?
- Dental Stone
The same material is used in casts/construction. Used to cast shoe/tyre marks. - Silicon-Based
Liquid silicone is used in dentistry. Used to cast finger/tool marks.
What are lifting methods?
1 - Electrostatic
Uses special electrostatic units. Involves electrostatically lifting dust onto a lifting film.
2 - Adhesive Sheets
Made of gelatine, used to mechanically lift dust from planar surfaces.
What are the enhancement techniques which are used?
1 - Optical
Photographic techniques and special illumination
2 - Physical
Interaction between mark residues and a coloured revealing agent -> powdering
3 - Chemical
Interaction between mark residues and reagents which lead to colour complex formations or between residues and reagents that leads to a colour complex or between residues and staining agents
What similarities/differences do pattern evidence characteristics search for?
- mark vs print
- class and individual
The agreement only established in:
- the presence of corresponding characteristics
- absence of inexplicable differences
If differences observed -> need a logical explanation
