Chapter 4The 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

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• 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: 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)

• 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)

• 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)

• 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