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Impression Evidence What does it tell us?. Essential Question: How can an impression be used to solve a crime?

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Presentation on theme: "Impression Evidence What does it tell us?. Essential Question: How can an impression be used to solve a crime?"— Presentation transcript:

1 Impression Evidence What does it tell us?

2 Essential Question: How can an impression be used to solve a crime?

3 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

4 Reasons for examining impressions: it is evidence! 1. May prove that a crime has been committed 2. Establish key elements of a crime 3. Link a suspect with a crime scene or a victim 4. Establish the identity of a victim or suspect 5. Corroborate verbal witness testimony 6. Exonerate the innocent 7. Give detectives leads to work within the case

5 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

6 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

7 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!

8 Pair & Share: Discuss your answer with a partner Do you both agree on the same technique? You have 2 mins!

9 Types of Impressions: Foot impressions Bite marks Tire impressions Tool marks

10 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

11 Bite Marks 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.

12 Bite Marks

13 Shoe Impressions The most commonly missed evidence at a crime scene Class 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

14 Shoe Impressions What they tell you: 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)

15 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 adults foot is about 15% of his/her height This percentage varies in growing individuals

16 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

17 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.

18 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

19 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)

20 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

21 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 suspects possession Entry/exit to crime scene

22 Tool Marks How are tool mark impressions collected? Pictures of tool mark Casting using plaster or silicone

23 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

24 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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