chapter 4 l.
Download
Skip this Video
Download Presentation
Chapter 4

Loading in 2 Seconds...

play fullscreen
1 / 31

Chapter 4 - PowerPoint PPT Presentation


  • 1413 Views
  • Uploaded on

Chapter 4. The Microscope and Forensic Identification of Hair and Fibers. Objectives (1 of 2). Students should gain an understanding of: The parts of a compound microscope and how it works The use of a comparison microscope to compare two objects

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about 'Chapter 4' - chandler


Download Now An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
chapter 4

Chapter 4

The Microscope and Forensic Identification of Hair and Fibers

objectives 1 of 2
Objectives (1 of 2)
  • Students should gain an understanding of:
    • The parts of a compound microscope and how it works
    • The use of a comparison microscope to compare two objects
    • The large working distance and the larger depth of field afforded by the stereomicroscope
    • Differentiation of amorphous and crystalline materials by use of a polarized light microscope
objectives 2 of 2
Objectives (2 of 2)
  • The structure of hair and the microscopy techniques used to identify human hair
  • The characteristics of natural fibers, human-made fibers, and the fabrics made with both types of fibers
  • The use of microspectrophotometers and scanning electron microscopes in the forensic lab
introduction
Introduction
  • Trace evidence: small, often microscopic, objects that are readily transferred between people and places
  • Microscopic comparison of fibers and hairs: started at the FBI laboratory in the early 1930s
  • Capabilities of forensic laboratories: greatly expanded with the development of modern analytical instruments
magnifying small details
Magnifying Small Details
  • Forensic scientists need to analyze many different types of materials
  • Early labs relied on the light microscope
  • This microscope offered less than 10 times magnification
refraction
Refraction
  • Refraction: magnifying glass bends (refracts) light rays as they pass through air and back through the lens
  • Focal length: depends on the change in refractive index
  • Refractive index: ratio of the velocity of light in a vacuum to the velocity of light in any other medium
types of microscopes 1 of 9
Types of Microscopes (1 of 9)
  • A microscope has at least two lenses:
    • Objective (lower) lens: produces a magnified and inverted version of the object
    • Ocular (smaller) lens: produces a virtual image in the viewer’s brain
  • Magnifying power = power of the objective lens × power of the ocular lens
  • The ability to distinguish extremely small objects depends on the wavelength of light used to illuminate the object
types of microscopes 2 of 9
Types of Microscopes (2 of 9)
  • Compound microscopes have six parts:
    • Base: stand on which it sits
    • Arm: support for the tube body
    • Body tube: hollow tube that holds the objective and eyepiece lenses
    • Stage: platform that supports the specimen
    • Coarse adjustment: knob that focuses the microscope by raising and lowering the body tube
    • Fine adjustment: knob that adjusts the height of the body tube in smaller increments
types of microscopes 3 of 9
Types of Microscopes (3 of 9)
  • The optical system of a compound microscope has four parts:
    • Illuminator: electric lighting (e.g., tungsten, fluorescent, halogen)
    • Condenser: part that focuses light rays through a lens at the center of the stage
    • Eyepiece: part you look through
    • Objective: second lens of the microscope
  • A higher numerical aperture (NA) allows for more detail
  • Anything beyond 1000× is considered “empty magnification”
types of microscopes 4 of 9
Types of Microscopes (4 of 9)
  • Comparison microscopes
    • Are used to compare two specimens
    • Consist of two compound microscopes connected by an optical bridge
    • Provide a single eyepiece through which the examiner sees both images side by side
    • Can be lighted from below the stage or via a vertical or reflected illumination system
types of microscopes 5 of 9
Types of Microscopes (5 of 9)
  • Stereoscopic microscopes
    • Are the most commonly used microscope in crime labs
    • Include two eyepieces
    • Produce a three-dimensional image with a right-side-up, frontward orientation
    • Offer a large working distance
    • Can be lighted from below or vertically from above
types of microscopes 6 of 9
Types of Microscopes (6 of 9)
  • Polarizing microscopes
    • Can provide information on the shape, color, and size of minerals
    • Can distinguish between isotropic and anisotropic materials
    • Include two polarizing filters, a polarizer lens (fixed below the specimen), and an analyzer lens (fixed above the specimen)
    • Through analysis of plane-polarized light, can determine whether the sample exhibits pleochroism
    • Are used to identify human-made fibers and paint
types of microscopes 7 of 9
Types of Microscopes (7 of 9)
  • Microspectrophotometers
    • Optical microscopes have been attached to spectrophotometers.
    • The lamp emits radiation that passed through the sample.
    • Light is separated according to its wavelength and the spectrum formed is observed with a detector.
    • These devices can determine the composition of unknown materials.
types of microscopes 8 of 9
Types of Microscopes (8 of 9)
  • Microspectrophotometers
    • Can measure the intensity of light reflected from a sample, the intensity of light emitted when a sample fluoresces, or the intensity of polarized light after it has interacted with a sample
    • Allow for more precise measurements of a sample while eliminating interference from surrounding material
    • Are useful for analysis of synthetic fibers
types of microscopes 9 of 9
Types of Microscopes (9 of 9)
  • Scanning electron microscopes
    • Can magnify 100,000×
    • Have a depth of focus more than 300× that of an optical microscope
    • Use electrons rather than light
    • Offer much greater resolution than with a light microscope
forensic applications of microscopy hair 1 of 8
Forensic Applications of Microscopy: Hair (1 of 8)
  • An individual hair cannot result in definitive identification of a person unless it has a DNA tag attached.
  • Hair samples can exclude suspects.
  • Hair is often contributing evidence that connects a suspect to a crime scene or connects multiple crime scene areas to each other.
forensic applications of microscopy hair 2 of 8
Forensic Applications of Microscopy: Hair (2 of 8)
  • Hair is composed primarily of keratin, which makes hair resistant to physical change.
  • Each strand grows out of a follicle.
forensic applications of microscopy hair 3 of 8
Forensic Applications of Microscopy: Hair (3 of 8)
  • Three parts of a hair:
    • Cuticle: scales of hardened, flattened, keratinized tissue that are unique to animal species
    • Cortex: orderly array of cortical cells that allows for comparison of hair samples
    • Medulla: rows of dark-colored cells organized in a pattern specific to the animal species
forensic applications of microscopy hair 4 of 8
Forensic Applications of Microscopy: Hair (4 of 8)
  • Hair growth stages:
    • Anagenic: hair follicle is actively producing the hair; follicle is attached to the root
    • Catagenic: transition stage in which the root is pushed out of the follicle
    • Telogenic: hair naturally becomes loose and falls out
forensic applications of microscopy hair 5 of 8
Forensic Applications of Microscopy: Hair (5 of 8)
  • Ask two questions when hair evidence is found at a crime scene:
    • Is the hair human?
    • Does it match the hair of the suspect?
forensic applications of microscopy hair 6 of 8
Forensic Applications of Microscopy: Hair (6 of 8)
  • When analyzing hair, the investigator must:
    • Distinguish between animal and human hair
    • Assess the hair color, length, and diameter
    • Compare features of the hair samples, including their distribution, color, and shape of pigment granules
forensic applications of microscopy hair 7 of 8
Forensic Applications of Microscopy: Hair (7 of 8)
  • Collect hair evidence by hand
    • Wide, transparent sticky tape
    • Lint roller
    • Evidence vacuum cleaner
forensic applications of microscopy hair 8 of 8
Forensic Applications of Microscopy: Hair (8 of 8)
  • Microscope examination might reveal two pieces of information:
    • Area of body from which the hair originated
    • Race of the hair’s owner
  • Microscopy cannot determine the age or sex of the hair’s owner.
forensic applications of microscopy fibers 1 of 8
Forensic Applications of Microscopy: Fibers (1 of 8)
  • Most fibers do not degrade at a crime scene.
  • Fibers are easily transferred from one object or person to another.
  • Fibers provide evidence of association between a suspect and a crime scene.
  • Fiber evidence must be carefully secured to avoid its loss or cross-contamination.
  • Most fiber evidence can only be placed within a class.
forensic applications of microscopy fibers 2 of 8
Forensic Applications of Microscopy: Fibers (2 of 8)
  • Natural fibers are derived from plant or animal sources.
  • Cotton is the most widely used natural fiber.
forensic applications of microscopy fibers 3 of 8
Forensic Applications of Microscopy: Fibers (3 of 8)
  • Yarn is classified into two types:
    • Filament: continuous length of human-made fiber
    • Spun: short lengths of fibers that are twisted or spun together
  • Physical properties of yarn include its texture, number of twists per inch, number of fibers per strand, blend of fibers, color, and pilling characteristics.
forensic applications of microscopy fibers 4 of 8
Forensic Applications of Microscopy: Fibers (4 of 8)
  • Woven fabrics consist of intertwining of two sets of yarns.
  • They are woven on a loom.
  • Basic weaves are plain, twill, and satin.
forensic applications of microscopy fibers 5 of 8
Forensic Applications of Microscopy: Fibers (5 of 8)
  • A wide variety of synthetic fibers have replaced natural fibers in fabrics, garments, and rugs.
  • There are two types of synthetic fibers:
    • Cellulosic: produced from cellulose-containing raw materials such as trees and plants
    • Synthetic: produced from chemicals made from refined petroleum or natural gas
forensic applications of microscopy fibers 6 of 8
Forensic Applications of Microscopy: Fibers (6 of 8)
  • Plastics: malleable materials easily formed into different products
  • Polymers: huge molecules formed by chemically linking together smaller molecules
  • Production of synthetic fibers:
    • Produced by melt spinning process
    • Shapes of holes in spinneret determine cross-sectional shape of the polymer
forensic applications of microscopy fibers 7 of 8
Forensic Applications of Microscopy: Fibers (7 of 8)
  • Step 1 in comparison of synthetic fibers: examination with a comparison microscope
    • Pay special attention to the fibers’ color, diameter, cross-section shape, pitting or striations, and presence of dulling agents
    • Advantages of comparison microscopy:
      • Fiber is not destroyed
      • Technique is not limited by the sample size
      • Microscopes are readily available
forensic applications of microscopy fibers 8 of 8
Forensic Applications of Microscopy: Fibers (8 of 8)
  • Step 2 in comparison of synthetic fibers: analysis of chemical composition
    • Try to place fiber in a specific polymer subclass
    • Use refractive index to identify synthetic fibers