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 ImpressionsWhat 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 ImpressionsFeatures 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 ImpressionsFeatures 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 StandardSFS1. 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.