Impression Evidence “What does it tell us?”
Mar 29, 2015
Impression Evidence
“What does it tell us?”
Essential Question:
How can an impression be used to solve a crime?
ImpressionsAn 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. May prove that a crime has been committed
2. Establish key elements of a crime3. Link a suspect with a crime scene or a
victim4. Establish the identity of a victim or
suspect5. Corroborate verbal witness testimony 6. Exonerate the innocent 7. 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 surfacesLuminol:
◦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 impressionsBite marks
Tire impressionsTool marks
Bite MarksCan be as complex as a fingerprint
◦ Individual evidenceDental 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 MarksFeatures 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 ImpressionsThe most commonly missed evidence at a crime
sceneClass characteristics:
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:Number of suspectsCorroborate or dispute alibis and witness
testimonyCan link two crimesLink a suspect to a victim or crime scenePoint of entry/exitPosition of the suspect, victim, and/or
witnessPathway through the crime sceneDirection of travelTime 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 leaveOn 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 scene2. 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 dirt4. T or F. Approximate age is determined
through bite marks.5. T or F. Bite marks are individual evidence.
Tire ImpressionsWhat they tell you:Type of tireMake or model of the tireDirection of travel after and/or
before the crimeNumber of vehicles presentMay eliminate a suspected tireMatch 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 MarksTypes 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 MarksWhat they tell you:Type of tool usedSize and shape of the tool used (class
characteristics)Rule out suspected toolsShape and pattern of imperfections
(individual characteristics)◦ Can be matched to tools in suspect’s
possessionEntry/exit to crime scene
Tool MarksHow are tool mark impressions collected?◦Pictures of tool mark◦Casting using plaster or silicone
Admissibility of EvidenceProbative value3 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.