Forensic Microscopy Lecture

Datum

Fixed reference point for all 3D measurements

Artifact

human-made or modified portable object

Feature

non-portable artifact

Organic and environmental remains

remnants that indicate human activity

matrix

material surrounding an object

provenience

origin and derivation of an item in 3D space in relation to datum and other items

Evidence

information, whether personal testimony, documents or material objects that is given in a legal investigation to make a fact or proposition more or less likely

what are the four types of crime scene searches

- spiral

- strip

-lane

- grid

what is a magnification system

• a single lens used to form an enlarged image of an object is a simple magnification system

• compound magnification system consists of 2 stages of magnification, and the total magnification is the product of the magnification of the first and 2nd lens

Virtual Image

the enlarged produced image from the lens, a real image is one that could be projected onto a screen

what is a lens

translucent material that bends light in a known and predictable manner

size and position of an image produced by a lens can be determined through geometry based on the focal length of the lens

focal length

focal length is the distance between two points of focus on either side of the lens

resolution

minimum distance 2 objects can be seperated and still be seen as 2 objects mag

Lens

• - magnification with one lens no indefinite

• - as magnification increases lens diameter decreases to bend the light more to make larger image

• - practical limit for simple lens is 10x to 15x

Compound magnifying systems

use 2 lenses to further enlarge lens and exceed limits imposed by simple lens

total magnification of the microscope is product of the 2 lens

lenses of up to 40x can be used in a compound microscope

empty magnification

• lenses in a compound microscope have resolution limits, can continue magnifying but resolution is not improved

Eyepiece or Ocular

lens that one looks when viewing an object microscopically

area seen when looking through the eyepieces is called field of view

objective

objective lens is closest to the object or specimen being studied - most important part

numerical aperture is an angular measure of the lens light gathering lability and its resolving quality

tube length is the distance from the lowest part of the objective to the upper edge of the eyepiece (standardized 160 mm)

coverslip

thin plates that are placed on top of mounted specimens, protects the specimen and objective from damage

The stage

platform where the specimen sits during viewing

can move 360 degrees but not move back in forth

condenser

used to obtain bright, even field view and improve image resolution

lenses located below the stage that focus or condense the light onto the specimen field of view

a condenser diaphragm is used to eliminate excessive light and adjust contrast

field diaphragm allows more or less light into the lens system of the microscope

illumination

2 types: Critical and Kohler

1. 1. Critical: concentrates light on the specimen with the condenser lens

2. 2. Kohler: illumination set the light rays parallel throughout the lens system, allowing them to evenly illuminate the specimen

Refraction

refraction or bending of light is one characteristic that allows lenses to focus beam of light into single point

occurs when light passes from one medium to another when there is a difference in index of refraction between 2 materials

Refractive Index

relative speed at which light moves through material with respect to its speed in a vacuum

RI of vacuum =1

N = C/v

N: Index of refraction

C: speed of light

v: velocity of light in that material

Snell’s Law

Angle of refracted light is dependent on both the angle of incidence and the composition of the material into which it is entering

• - normal is a line perpendicular to the boundary between 2 substances

• - N1*sin(q1) = N2*sin(q2)

polarized light microscopy

exploits optical properties of materials to discover details about the structure and composition materials

Isotropic materials

Demonstrate the same optical properties in all directions

have only 1 RI

Anisotropic materials

optical properties that vary with the orientation of the incoming light and the optical structure of the material

PLM uses the fact that anisotropic materials divide light rays into parts to yield information about the material

polarizer is a special filter that lets light pass through only a preferred direction

light that vibrates only one direction is called polarized light

PLM uses 2 filters

Polarizer

• - has its preferred vibration direction set left to right

• - beneath the stage

• - analyzer is polarizing filter that is aligned opposite of the polarizer