Impressions evidence - Effingham County Schools

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Impression Evidence
“What does it tell us?”
Essential Question:
How can an impression be
used to solve a crime?
Impressions
An object or surface takes the form of
another through direct contact.
 Two types:

◦ 2 dimensional: transfer of an imprint to a firm
substrate (glass, paper, floors, etc.)
 EX: lip print on a glass window
◦ 3 dimensional: transfer of an imprint to a soft
substrate (mud, grass, snow, etc.; usually
outdoors)
 EX: footprint in mud
Reasons for examining
impressions: it is evidence!
1.
2.
3.
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May prove that a crime has been committed
Establish key elements of a crime
Link a suspect with a crime scene or a
victim
Establish the identity of a victim or suspect
Corroborate verbal witness testimony
Exonerate the innocent
Give detectives leads to work within the
case
Ways to Recover Impressions:

Mechanical processing:
◦ Brush and Powder method
 Tape lift
 Used for 2D impressions
 Ex: footprints on a waxed floor

Casting:
◦ Making a replica of the impression
◦ Filling a 3D form with a material mold that will not destroy the
impression
◦ Used for 3D impressions
 Ex: Tool marks on a window sill or tire impression in the mud

Electrostatic Lifting:
◦ A charge is used to lift dry materials from the surface to a black film
 Ex: Shoe impression on a dusty floor

Oblique angle photography:
◦ method of documenting impressions
Ways to Enhance Impressions:

Dyes:
◦ Used on glass or marble surfaces

Luminol:
◦ Chemical with luminescent properties activated
upon contact with the iron in blood
 Ex: may be applied to enhance a bloody footprint

Alternative light:
◦ Used on porous surfaces like wood or paper
◦ Ex: Black light

Dusting:
◦ Powder and brush
Think:

Individually answer this question:
Which technique for collecting impressions
would be best for lifting bite marks from a
victim? Explain.
You have 2 mins!
Pair & Share:

Discuss your answer with a partner
Do you both agree on the same technique?
You have 2 mins!
Types of Impressions:
Foot impressions
Bite marks
Tire impressions
Tool marks
Bite Marks

Can be as complex as a fingerprint
◦ Individual evidence

Dental records & X-rays are often used to
identify individuals
◦ Show position of teeth, fillings, etc.

What they may tell you:
◦ Can identify a suspect or victim
◦ Most often indicate assault or sexual attack
◦ Are common with domestic violence
Bite Marks
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Features to analyze:
Type of bite mark (human or animal)
Characteristics of the teeth (position, evidence of
dental work, wear patterns, etc.)
Color of area to estimate how long ago the bite
occurred (old or recent bite)
Swab for body fluids for DNA tests
Did you know?
The most famous incident where bite mark evidence led to a conviction, was in the
case of the notorious serial killer, Ted Bundy. He was responsible for an
undetermined number of murders between 1973 and 1978 and was finally tied to
the murder of Lisa Levy through bites that he had inflicted on her body.
Bite Marks
Shoe Impressions


The most commonly missed evidence at a crime scene
Class characteristics:
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Size
Manufacturer
Model
Type
Individual characteristics:
 Gait-manner of moving the foot
 Wear of shoes, esp. outsoles
 Outsoles: outer soles of shoes at bottom
 Damage
 cuts, gouges, chunks…
 debris
 nails/screws
Shoe Impressions
What they tell you:
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Number of suspects
Corroborate or dispute alibis and witness
testimony
Can link two crimes
Link a suspect to a victim or crime scene
Point of entry/exit
Position of the suspect, victim, and/or
witness
Pathway through the crime scene
Direction of travel
Time period (from temporary impressions such
as in snow or dew)
Shoe Impressions
What they tell you continued:
 Size: approximate height of an individual
◦ Put heel to wall and measure left foot from
wall to tip of the big toe.
◦ The length of an adult’s foot is about 15% of
his/her height
◦ This percentage varies in growing individuals
Shoe Impressions
Features to analyze:
• Tread patterns, size, and depth
• Wear patterns caused by the way a
person walks
• Material defects or damage (nicks, cuts,
etc.)
• Other trace materials, such as soil, tar,
rocks, and paint that would indicate
where a person has been
Ticket to leave

On a sheet of paper, WRITE THE QUESTION
& ANSWER to these true or false questions:
1. T or F. Impressions are the most apparent pieces
of evidence at a crime scene
2. T or F. There are two types of impressions:2D and
3D impressions.
3. T or F. Casting can be used to lift a shoe
impression in the dirt
4. T or F. Approximate age is determined through
bite marks.
5. T or F. Bite marks are individual evidence.
Tire Impressions
What they tell you:
 Type of tire
 Make or model of the tire
 Direction of travel after and/or before the
crime
 Number of vehicles present
 May eliminate a suspected tire
 Match a tire to a suspect
Tire Impressions

Features to analyze:
◦ Wheelbase
 Distance between the center of the front tire and the
center of the back tire.
◦ Stance = Track width
 Distance between the two front tires or the two back
tires
◦ Class characteristics
 Ex: size, tread design
◦ Individual characteristics
 Ex: Cuts, gouges, wear,
debris (rocks, nails)
Tool Marks
Types of Tool Marks:
 Compression: substrate caught between two
opposing forces (pinching or shearing marks)
◦ Wrench, pliers

Sliding: a tool scrapes across a substrate causing
parallel striations
◦ Screwdriver
Tool Marks
What they tell you:
 Type of tool used
 Size and shape of the tool used (class
characteristics)
 Rule out suspected tools
 Shape and pattern of imperfections (individual
characteristics)
◦ Can be matched to tools in suspect’s possession

Entry/exit to crime scene
Tool Marks
 How
are tool mark impressions
collected?
◦ Pictures of tool mark
◦ Casting using plaster or silicone
Admissibility of Evidence
Probative value
 3 standards:

◦ Relevance
 Proves or disproves a fact of the crime
◦ Competence
 Reliability of the evidence
◦ Materiality of evidence
 Proves an essential fact of the case
Georgia Performance Standard

SFS1. Students will recognize and classify various
types of evidence in relation to the definition
and scope of Forensic Science.
◦ d. Evaluate the relevance of possible evidence at
the site of an investigation.

SFS4 Students will evaluate the role of ballistics,
tool marks and evidence of arson in forensic
investigation.
◦ c. Recognize the forensic significance of tool
marks, footwear and tire impressions in an
investigation.

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