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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
Chapter 4

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Slide 1

Chapter 4

The Microscope and Forensic Identification of Hair and Fibers

Slide 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

Slide 3

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

Slide 4

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

Slide 5

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

Slide 6

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

Slide 7

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

Slide 8

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

Slide 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”

Slide 10

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

Slide 11

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

Slide 12

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

Slide 13

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.

Slide 14

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

Slide 15

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

Slide 16

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.

Slide 17

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.

Slide 18

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

Slide 19

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

Slide 20

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?

Slide 21

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

Slide 22

Forensic Applications of Microscopy: Hair (7 of 8)

  • Collect hair evidence by hand

    • Wide, transparent sticky tape

    • Lint roller

    • Evidence vacuum cleaner

Slide 23

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.

Slide 24

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.

Slide 25

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.

Slide 26

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.

Slide 27

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.

Slide 28

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

Slide 29

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

Slide 30

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

Slide 31

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


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