Prints and impressions (PowerPoint)

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					Physical Evidence



        Prints and impressions
        (tires, shoes, fingers)
Footprint at scene said
to Match Avila’s OC Register, 4/5,05

   Samantha was kidnapped in July 2002 while
    playing in front of her home with a friend. Her
    body was found the next day. She had been
    sexually assaulted, then suffocated.
   Sgt. Robert Kennedy, an RCMP expert on
    footprint comparison, testified that a
    footprint found near the body had
    distinct characteristics that matched Avila’s right foot.
   Mark Johnson, an Orange County Sheriff’s crime lab forensic
    specialist, testified that shoeprints found near the body had a
    pattern matching a size 12 Fila Disrupter tennis shoe.
   Mark Johnson also testified that tire prints found at the scene
    were of the same size and design as the Michelin tires on
    Avila’s 1994 Ford T-Bird.
Prints
   Prints are made when an object
    deposits a two-dimensional
    pattern

     –   Patterns are deposited by
         raised sections, or “ridges”
     –   Deposits can be visible or
         latent
     –   Latent prints become visible
         through development

   Visible or developed prints are
    photographed and (if possible)
    lifted

   Given sufficient “points of ID”
    prints can be individually
    matched to a person or object
Impressions

   Impressions are made when
    pressure from an object creates
    a three-dimensional image on a
    soft surface
   Impressions should FIRST be
    photographed; then, if possible,
    cast
   Given sufficient “points of
    identification”, impressions can
    be individually matched to a
    person or object
   Unusual impression: pattern of
    pedestrian’s clothing on vehicle
    paint
Shoes

   Class characteristics
    –   Identify brand and model
        from outsole design
        features
    –   Computer-assisted
        matching


   Individual
    characteristics
    –   “Points” of comparison,
        from questioned to
        known
Tires

   Class characteristics
    –   Can identify tire brand and model
    –   Can identify vehicle brand and
        model using that tire
    –   Can identify vehicle brand and
        model
            Having the measured track
             (width) and wheelbase (length)

   Individual characteristics
    –   May identify specific tire from
        wear and damage (“points of
        identification”)
Fingerprints

   What are they?
    –   Outlines of ridges where sweat
        glands are present
   How are they deposited?
    –   Inked
    –   Stained or dirty hands
    –   Impressions on soft materials
        (“plastic prints”)
    –   Body secretions (sweat and
        other chemicals)
Developing fingerprints

   Making visible (“developing”)
     – Special light sources (u.v., laser)
     – Dusting with materials that adhere
       to body secretions
          Light & fluorescent powder
     – Chemicals to darken body
       secretions
          Ninhydrin
          Superglue fuming 
   Photograph before lifting
   Special conditions
     – Fingerprints on paper
     – Fingerprints on bodies
     – Fingerprints on greasy surfaces
Identification

   Common patterns
    –    Loop
    –    Whorl
    –    Arch


   Common minutiae
     –   Islands
     –   Dots
     –   Bifurcation
     –   Ridge endings
Comparison

   Compare pattern (loop, arch,
    whorl)
   If similar, then compare
    minutiae (where ridges start,
    stop and separate)
     –   Islands, dots, bifurcations,
         ridge endings
   Set number of points (6-15)
    must be identical in type and
    location with known print to
    make an identification
   “Locations” found as on a
    map and by counting ridges
AFIS
   Old system required having a
    suspect in mind, with a 10-print
    card on file
     – Questioned print compared to
        card

   New system – AFIS

     –   Digitizes 10-print cards from
         arrested persons
     –   Digitizes prints recovered at
         crime scenes
     –   Compares both, looking for
         possible matches
     –   Makes suggestions

   Final match must be done in the
    conventional way, by comparing
    minutiae

				
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posted:9/4/2011
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