Forensic Trace Evidence: Hair, Fibers, Toolmarks, Paint, and Glass Analysis

Trace Evidence

Small, often microscopic material found at a crime scene or on a victim/suspect.

Locard's Exchange Principle

The concept that when two objects come into contact, there is always an exchange of materials.

Class Characteristic

Property of evidence that can only be associated with a group and never a single source.

Individual Characteristic

Property of evidence that can be attributed to a common source with an extremely high degree of certainty.

Comparison Microscope

Two compound microscopes connected by an optical bridge, allowing two samples to be viewed side-by-side.

Keratin

The main protein composition of hair.

Follicle

The structure from which the hair grows.

Cuticle

The outer protective layer of hair, composed of overlapping scales.

Cortex

The intermediate layer of hair, composed of elongated cells, containing pigment (melanin).

Medulla

The central core of the hair shaft (may be continuous, interrupted, fragmented, or absent).

Medullary Index (MI)

The ratio of the medulla diameter to the hair shaft diameter (MI < 1/3 for human hair, MI > 1/2 for animal hair).

Anagen Phase

The initial growth phase where the hair follicle is actively producing hair.

Telogen Phase

The final resting phase before the hair is shed.

Root/Follicular Tag

Tissue adhering to the root of the hair, often containing mitochondrial DNA (or nuclear DNA if the entire root is present).

Natural Fiber

Fiber derived entirely from animal or plant sources (e.g., cotton, wool, silk, linen).

Manufactured/Synthetic Fiber

Fiber derived from natural or synthetic polymers (e.g., rayon, nylon, polyester, acrylic).

Polymer

A substance composed of long chains of repeating molecular units (monomers).

Birefringence

A double refraction characteristic of some fibers, examined under polarized light to distinguish between different types.

Striation

A scratch mark or series of lines left by the passing of a tool across a surface.

Impression/Indentation

A negative print or mark left when a tool is pressed into a softer material.

Lands and Grooves

The internal components of a gun barrel that impart twist/spin to a projectile; rifling (analogous terms for toolmarks).

Comparison (Forensic)

The process of matching unique marks on a piece of evidence (like a bullet, toolmark, or fingerprint) to a known source.

Binder

The non-volatile portion of paint that forms the film; provides adhesion and durability.

Pigment

Fine powder material that gives paint its color and opacity.

Solvent

The liquid medium that dissolves or disperses the binder and pigment.

Layer Structure

The unique sequence and composition of paint layers, especially in automotive paint.

Amorphous Solid

A solid in which the atoms and molecules are arranged in a random manner, not a crystal structure (like glass).

Refractive Index (RI)

The ratio of the velocity of light in a vacuum to its velocity in a specified medium (a method of characterizing glass).

Becke Line

A bright halo observed near the border of a particle immersed in a liquid of different refractive index; used to determine RI.

Radial Fracture

Cracks that radiate outward from the point of impact (on the opposite side of the impact).

Concentric Fracture

Cracks that form in a rough circle around the point of impact (on the same side as the impact).

Tempered Glass

Glass that is rapidly cooled during manufacturing, causing it to fragment into small, blunt pieces when broken.

Wallner Lines

Stress markings on the edge of broken glass fragments; they can help determine the direction of force.