1. distinguish between patent, latent, and plastic impressions
Chapter 15 Cast and Impressions By the end of this chapter you will be able to:
distinguish between patent, latent, and plastic impressions
describe how to make foot, shoe, and tire impressions
use track width and wheel base information to identify vehicles
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Forensic Science: Fundamentals & Investigations, Chapter 15

2. Introduction
People, vehicles, and objects leave evidence of their presence at an accident or crime scene.
This evidence is often in the form of an impression.
Forensic Science: Fundamentals & Investigations, Chapter 15

3. 3 Types of Impressions Patent – visible, two-dimensional
Latent – invisible, but can be visualized through special dusting and electrostatic techniques
Plastic – three-dimensional prints left in soft materials (snow, mud, soil)
Forensic Science: Fundamentals & Investigations, Chapter 15

4. Patent impressions A dust shoe print on the fabric of a chair.
A fingerprint on wallpaper.
Forensic Science: Fundamentals & Investigations, Chapter 15

5. Latent impression
Forensic Science: Fundamentals & Investigations, Chapter 15

6. Plastic impression Shoe print in mud.
Forensic Science: Fundamentals & Investigations, Chapter 15

7. Individual or Class Evidence?
Impressions can be either class or individual evidence
Class – tread pattern, brand, size
Individual – split on shoe sole or unusual wear
Forensic Science: Fundamentals & Investigations, Chapter 15

8. Shoe Impressions
The size of a shoeprint can tell the size of foot of the person.
The depth of a foot or shoe impression can tell something of the person’s weight.
The type of shoe can tell something of the person’s job or personality.
CSIs use databases that contain the names of specific manufactures and tread patterns to identify different types of shoes.
Forensic Science: Fundamentals & Investigations, Chapter 15

9. Shoe Wear Patterns
Each person wears down his/her shoes differently, so shoe wear patterns can provide clues about a person:
Body weight.
The way a person walks (on toes or heels, toes pointed in or out, etc).
The surface on which the person usually walks.
Debris that became embedded in the tread or unique holes and cuts in the tread.
Forensic Science: Fundamentals & Investigations, Chapter 15

10. Shoe wear patterns (cont.)
Gait – length of strides can indicate if person was walking or running
Limp/Injury – can be indicated if one foot is angled differently or if one foot makes a deeper impression than the other
Trail of prints – can assist in recreating events
Different gaits are also caused if a person is carrying a heavy weight
Forensic Science: Fundamentals & Investigations, Chapter 15

11. Collection of Shoe Impression Evidence
Photographing Impressions:
Take photos before impressions are touched or altered.
Fill view finder with impression.
Lens should be perpendicular to impression in order to reduce distortion.
Take multiple photos from at least two different angles.
Forensic Science: Fundamentals & Investigations, Chapter 15

12. Photographing Impressions (cont).
5. Place an identifying label and a ruler in position with the impression.
Use oblique lighting when possible (sunlight causes shadows).
Use flash if needed.
If impression is faint, spray with color-contrasting paint.
Forensic Science: Fundamentals & Investigations, Chapter 15

13. Lifting Latent Impressions
Latent impressions are left when a shoe or bare foot walks on a smooth surface.
Bare foot – leaves thin layer of body oil
Shoe – leaves thin film of substances from the plastic in the sole or dirt
CSIs have to know where to look to find latent impressions
Forensic Science: Fundamentals & Investigations, Chapter 15

14. Lifting Latent Impressions (cont).
Different methods to make latent prints visible include:
Luminol makes bloody footprints visible.
Dusting (similar to dusting a fingerprint) reveals an impression.
Electrostatic lifting and gel lifting (impression shown above) techniques
Forensic Science: Fundamentals & Investigations, Chapter 15

15. Luminol Shoe print visualized by Luminol
Forensic Science: Fundamentals & Investigations, Chapter 15

16. Electrostatic Lifting
A dry shoe deposits a small amount of dust with each step.
Apply an electrostatic charge to lifting film and place it over the latent print; the film picks up and holds the dust. (Like static cling)
View the film with a special light source, and the impression becomes visible.
Electrostatic charges can lift impressions from paper, carpeting, wood surfaces, linoleum, asphalt, and concrete.
Forensic Science: Fundamentals & Investigations, Chapter 15

17. Gel Lifting
Gel lifter = a layer of thick gel between paper backing and plastic cover sheet.
The gel conforms to uneven surfaces.
Peel off the plastic cover sheet and press firmly over print; then lift off and replace cover sheet.
The gel lifter is then photographed to reveal the print.
Forensic Science: Fundamentals & Investigations, Chapter 15

18. Gel lifter Gel lifter Shoe print taken from gel lifter
Forensic Science: Fundamentals & Investigations, Chapter 15

19. Casting Plastic Impressions
If imprint is made in sand or dirt, a Plaster of Paris cast is made.
Apply a light film of hairspray to prevent the impression from collapsing under the weight of the plaster, then pour in the plaster.
Wait for the Plaster of Paris to harden.
Forensic Science: Fundamentals & Investigations, Chapter 15

20. Plaster of Paris cast
Forensic Science: Fundamentals & Investigations, Chapter 15

21. Foot Length and Shoe Size
The shoe model must be identified first, then an estimate of shoe size can be obtained.
A person’s height is generally related to his/her foot size, but it is impossible to predict exact height from shoe size.
Considered class evidence.
Forensic Science: Fundamentals & Investigations, Chapter 15

22. Tire Treads and Impressions
Tire evidence:
Can indicate type of vehicle that left the mark
Link a suspect or victim to a crime scene
Can reveal the events that took place.
Examples:
Speed a car traveled when it left the road
Direction car traveled when fleeing a crime scene
Determine who is at fault at the scene of a fatal accident.
Forensic Science: Fundamentals & Investigations, Chapter 15

23. Tire Treads and Impressions
Tires can leave patent, latent, or plastic tread patterns.
Patent – if vehicle has tracked through a fluid such as oil, tar, or blood
Latent – left on asphalt by manufacturer’s oils used to keep tires soft and pliable
Plastic – left on off-road surfaces such as mud, lawns, sand, or snow
Forensic Science: Fundamentals & Investigations, Chapter 15

24. Anatomy of a Tire
To channel water away and provide traction, a tire’s tread is divided into ridges (elevated) and grooves (indentations).
Every model of tire is unique.
A single tread can usually indicate the general type of vehicle.
Forensic Science: Fundamentals & Investigations, Chapter 15

25. Recording Tread Impressions
Ridges and grooves of discovered tire impressions are counted across the entire width.
Unique characteristics such as wear or pebbles embedded in the grooves are noted.
The impression from a crime scene may be matched to a suspect’s vehicle.
A print of a suspect’s tire impressions is taken
Ink is painted onto the suspect vehicle’s tire, and the vehicle is driven over paper or cardboard.
The print is then compared to the impression from the crime scene.
Forensic Science: Fundamentals & Investigations, Chapter 15

26. Identifying a Vehicle
Identifying tread patterns may not be enough to link a suspect with a crime scene. Other things can help with this:
Front and rear track widths.
Wheelbase measurement.
Turning diameter.
Databases can be checked to find the vehicle with these specifications.
Forensic Science: Fundamentals & Investigations, Chapter 15

27. Accident Reconstruction
Drivers may not recall the exact series of events, or the story may not be consistent with the evidence.
Debris patterns and tire marks can help reconstruct the events of an accident.
The ultimate goal of accident reconstruction is to determine who was at fault.
Forensic Science: Fundamentals & Investigations, Chapter 15

28. Accident Reconstruction
Tire marks are helpful in:
Hit and run accidents
Head-on collisions
Quick getaways from the scene of a crime
There are three basic types of tire marks:
Skid marks
Yaw marks
Tire scrub
Forensic Science: Fundamentals & Investigations, Chapter 15

29. Skid Marks Formed when someone brakes suddenly and locks wheels.
Provides evidence of the distance at which brakes were applied.
Calculation of velocity (speed) can be made from skid marks.
Forensic Science: Fundamentals & Investigations, Chapter 15

30. Yaw Marks
Produced when a vehicle travels in a curved path faster than the vehicle can handle and skids sideways.
Tires and road surface melt from extreme temps.
Audible squeal and often smoke occurs.
Forensic Science: Fundamentals & Investigations, Chapter 15

31. Tire Scrubs
Produced by damaged or overloaded tires during or immediately after impact.
Usually curved, irregular in width.
May have striations that look like stripes.
Determine area of impact.
Forensic Science: Fundamentals & Investigations, Chapter 15

32. Summary
Impression evidence—whether patent, latent, or plastic—will be considered class evidence unless it has individualizing features.
Debris patterns and tire marks can be clues to speed, direction, and vehicle identification.
Documentation (including photos early on) is extremely important in an investigation.
Forensic Science: Fundamentals & Investigations, Chapter 15