Types of Impressions - Mrs. Types of Impressions 1. Latent Impressions: hidden impressions ......

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  • http://science.howstuffworks.com/impression-evidence.htm

  • 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 is pressed against another material/object with enough force to leave an imprint of its shape.


    •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 evidence Things 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 knowledge Conduct 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