MicroscopyChapter 6

Objectives• To be able to describe the light path

through a simple lens

• To be able to define a compound microscope and describe the light path through it

• To be able to name the parts of a compound microscope

• To be able to describe how a comparison microscope is constructed

Objectives• To be able to describe how a stereo

microscope is constructed

• To be able to define plane polarized light

• To be able to describe how a polarized light microscope works

• To be able to describe how a scanning electron microscope works

• To be able to define and describe energy dispersive x-ray analysis

Introduction

• The instruments you will encounter most often in a forensics lab is the microscope

• Most evidence is of the trace variety– There is a small amount of it so it must be

conserved

• Examination with a microscope does not destroy evidence

• Sometimes the only instrument needed is a microscope

Types of Microscopes• The two major types of microscopes are

compound and electron microscopes

• Microscopes discussed in this chapter include:– Simple magnifier (magnifying glass)– Compound (basic, stereo, polarized light,

comparison, microspectrophotometer)– Electron

• Table 6.1 lists common evidence types and the microscopes that are used

Lenses: How Objects are Magnified• The most simple type of all microscopes is

the simple convex lens

• Convex lenses bend (refract) light rays as they pass through the lens

• Light rays form a virtual image

• The shape of the lens determines the magnification

• The thicker the lens is in the middle, the higher the magnification– However, thickness causes distortion

Lenses: How Objects are Magnified

• The practical limit of a single lens is 50x

• Improvements can be made by using two convex lenses– The first lens magnifies the object, the second

magnifies the virtual image

• The total magnification is the product of the magnification of each lens– 10x and 20x = 200x

The Compound Microscope 1• A microscope made from two convex lenses

is called a compound microscope– Lens 1 = eyepiece, lens 2 = objective

• The evidence sits on the stage and a light source shines through the object

• The body tube is above the stage with the objective lenses mounted beneath it

• Most microscopes are parfocal - once an object is in focus, lenses can be changed and the object will remain in focus

The Compound Microscope 2• At the top of the body tube is the eyepiece

(ocular lens)– Single lens = monocular– Two lenses (both eyes) = binocular– Three lenses (for photomicrographs) = trinocular

• A course and fine focus are used to focus the object

• The diaphragm is beneath the stage and controls the amount of light that reaches the object

• Filters can limit the wavelengths of light that reach the object

The Compound Microscope 3

• Reflected light microscopes are used for opaque objects such as bullets– The light source is mounted above the stage

• The most important characteristics of compound microscopes are:– Magnification, resolution, field of view, and depth

of focus

The Compound Microscope 4• Magnification: the product of the

magnification of the ocular and objective lens (up to 1000x)

• Resolution: The ability of a lens to separate details of an object into distinct images rather than one blurred image

• Field of view: How much of an object is visible at one time

• Depth of focus: how far inside the object the image will be in focus

Microscopes Derived from CM’s• The compound microscope can be modified

in a number of useful ways to accommodate special circumstances

• Examples of modified compound microscopes include:– Comparison microscope– Stereo microscope– Polarized light microscope– Microspectrophotometer

The Comparison Microscope

• Evidence often needs to be microscopically compared– Fired bullets, hairs, fibers, etc.

• The comparison microscope enables the examiner to view two objects, side-by-side, at the same time

• The comparison microscope consists of two compound microscopes connected with a comparison bridge

The Stereo Microscope• Stereo microscopes typically have low

magnification (25-50x), the ability to manipulate the material, and the ability to see it in three dimensions

• It is the most versatile and commonly used in forensics labs

• It has a long working distance– Allows the examiner to get hands and tools under

the lenses

• Stereo microscopes consist of two monocular compound microscopes aligned with slightly different viewing angles to create a 3D image

Scanning Electron Microscopy• Electrons microscopes use electrons

instead of light to magnify an image

• A scanning electron microscope (SEM) can magnify from 10 to 200,000 times

• A beam of electrons is aimed at the object and are backscattered

• Backscattered electrons are captured, amplified, and aimed at a cathode ray tube (TV) to create an image