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Types of Impressions
1. Latent Impressions: hidden impressions requiring special techniques to be visualized.
2. Patent Impressions: 2D impressions that are already visible
3. Plastic impressions: 3D impressions cast in soft materials, such as soil and snow or blood
What is impression evidence?Let me explain…
• Impression evidence can be defined as objects or materials that have retained the characteristics of other objects through direct contact.
• Impressions are created when one object ispressed against another material/objectwith enough force to leave an imprint ofits shape.
IMPRESSIONS: INDIVIDUAL OR CLASS EVIDENCE?
•Depending on how it is made, impression evidence may be either class evidence or individual evidence
•Class evidence example: particular tread pattern in shoes or in tires may identify the brand and size, but it does not identify a specific individual or tire. .
•Individual evidence example: distinguishing characteristics, such as split on a shoe sole or unique feature or unusual wear on a car tire
Collecting the evidenceThings to remember…..
• Take photos as soon as possible.
• Take multiple photos of the impression from at least two different orientations.
• Place an identifying label and a ruler in position with the impression for the photo.
• Use oblique lighting when possible.
How do we collect latent impressions?Glad you asked…..
You must make the print visible by using one of the following techniques:
• Saturating the print with luminol which will make bloody footprints glow in the dark.
• Dusting the latent print reveals an impression.
• Electrostatic lifting – applying an electric charge on a piece of lifting film, which is then placed over the latent print. The film picks up and holds the dust of the latent print
• gel lifting – gel lifter is a thick gel sandwiched between paper backing and a plastic cover sheet. It conforms to uneven surfaces. Best used on oil or moist impressions. Used for latent impressions.
• Casting
Electrostatic dusting and lifting?Sure I’ll explain….
• Electrostatic dusting can reveal dust left with each step and create an impression.
• Electrostatic charges can lift impressions from paper, carpeting, wood surfaces, linoleum, asphalt, and concrete.
• Other methods to recover latent impressions include gel lifters.
• These methods can provide information about the person who left the shoe print.
Most popular and successful method is the use of snow print wax for impressions left in snow
Available in a spray can
Results in fragile, thin cast that shows great detail
After drying is filled with dental casting mixture called dental stone
Paint thinner, spray paints, paraffin and sulfur can be used as well
3D impression to preserve physical
evidence
Plaster of Paris Soft, even after drying
Made up of large particles that may cause the loss of crucial detail
Require the use of forms and reinforcing materials
Dental plasters and stones Dense, uniform, smaller particle size
Quick drying and show more detail
Expensive
What’s the 411?Information we can gather from footprints…
• 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.
• The number of people at crime scene, movement, entrance/exit.
• Databases contain the names of specific manufactures and tread patterns used to identify different types of shoes.
• Debris that became embedded in the tread or unique holes and cuts in the tread
• Multiple prints can show person gait or walking habits
Factors that personalize a person’s footwear:
Body weight
The way a person walks Weight distribution
Direction of toes (straight, pointing out or in)
The surface on which the person walks
Unique holes, cuts, and debris embedded in the tread
12
What’s the 411?All about tire tread evidence…
• Tread pattern is unique design of tire surface. Usually found in road accident scenes or in the access and escape routes of other crime scenes.
• Tread help investigators identify the type of vehicle that left the mark.
• Tires can leave patent, latent, or plastic tread patterns.
• Link a suspect or victim to a crime scene, and can reveal the events that took place.
*Tire databases are available help investigators determine the brand and model
of the tire that left the impression , which can be used to determine the type of
vehicle that made the tracks.
Forensic Science: Fundamentals & Investigations, Chapter 15 14
Tire treads—ridges and grooves channel water away and provide traction
Patent tread patterns—impressions made after tire runs through a fluid material
Latent tread patterns—impressions from tire oils used to keep tires soft and pliable
Plastic tread patterns—three dimensional impressions left in soft surfaces
How do you record tire impressions?It’s simple….
• Ridges (elevated areas) and grooves (indented areas) of discovered tire impressions are counted across the entire width.
• Analyze
1. Tread Pattern
2. Width & depth of the tread pattern
• Unique characteristics such as wear or pebbles embedded in the grooves are noted.
• A print of a suspect’s tire impressions (through one revolution) is taken.
After the evidence is recorded….Identify the vehicle
• Identifying tread patterns may not be enough to link a suspect with a crime scene. Other things can help with this:
1. Front and rear track widths.
(measured from center of each tire to
opposite tire.)
1. Wheelbase measurement.
2. Turning diameter (how tight a vehicle
Can make a U-turn.
• Databases can be checked to find the vehicle with these specifications.
Forensic Science: Fundamentals & Investigations, Chapter 15 18
Track widths—From center of left tire to center of right tire
Wheelbase length—From center of front axle to center of rear axle
Forensic Science: Fundamentals & Investigations, Chapter 15 19
Count ridges and grooves across the tire width
Note unique characteristics—wear or pebbles embedded in the grooves
Create a print of the suspect’s tire impressions through one revolution
Compare impressions from the crime scene and suspect’s tire
Identifying tread patterns may not be enough to link a suspect with a crime scene
20
Turning diameter
Databases can be checked to find the vehicle with these specifications
Forensic Science: Fundamentals & Investigations, Chapter 15 21
Drivers may not recall the exact series of events before, during, and after an accident
People, vehicles, and objects, however, can leave evidence of their actions at the scene of an accident
Debris patterns and tire marks can be clues to speed, direction, and vehicle identification
There are three basic types of tire marks:
Skid marks—can be clues to the distance when brakes were applied and the vehicle’s speed. When a car turns a track is created by outer front tire due to added stresses
Yaw marks—can show a sideways skid.
Tire scrub—can determine the area of impact.
How are each of these formed?
Applying your knowledgeConduct a case study
• The O.J. Simpson Case
• The Meredith Kercher Case
• Lacey Rutheford-Coos County
• Russell Williams 2010
• Cecil Sutherland
• United States V Allen
