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
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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
- 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)
- 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
- 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
- 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: 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)
- 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)
- 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)
- 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)
- 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)
- 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)
- 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)
- 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)
- 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)
- 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)
- 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)
- 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)
- 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)
- 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)
- 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)
- 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)
- Collect hair evidence by hand
- Wide, transparent sticky tape
- Lint roller
- Evidence vacuum cleaner
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)
- 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)
- 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)
- 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)
- 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)
- 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)
- 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)
- 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)
- 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