Chapter 4

1. Chapter 4 The Microscope and Forensic Identification of Hair and Fibers

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

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

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

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

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

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

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

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”

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

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

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

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.

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

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

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.

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.

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

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

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?

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

22. Forensic Applications of Microscopy: Hair (7 of 8) • Collect hair evidence by hand • Wide, transparent sticky tape • Lint roller • Evidence vacuum cleaner

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.

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.

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.

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.

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.

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

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

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

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