Forensic Science: The Basics Microscopy Chapter 6 Jay A. Siegel,Ph.D. Power point presentation by Greg Galardi, Peru State College, Peru Nebraska Presentation by Greg Galardi, Peru State College CRC Press, Jay A. Siegel Ph.D. 1
Microscopes Most often encountered instrument in science labs Give examiners up close look at evidence Can be used for trace evidence examination Do not destroy evidence Shortens time of analysis of evidence in Shortens time of analysis of evidence in hands of knowledgeable examiner Variety of microscopes are available Presentation by Greg Galardi, Peru State College CRC Press, Jay A. Siegel Ph.D. 2
Types of Microscopes Simple magnifier- magnifying glass Compound microscopes- basic, stereo, polarized light, comparison and microspectrophotometer Electron microscope Presentation by Greg Galardi, Peru State College CRC Press, Jay A. Siegel Ph.D. 3
How Objects are Magnified Most basic types of microscopes use convex lenses Convex lens is wider in middle and tapers towards top and bottom Shape of lens depends on degree of magnification (practical limit is 50 power) Convex lens refract light rays as they pass through lens Forms a virtual image Presentation by Greg Galardi, Peru State College CRC Press, Jay A. Siegel Ph.D. 4
How Objects are Magnified Improvement on magnification can be achieved by using two convex lenses First lens magnifies object and virtual image is formed Second lens magnifies virtual image even more greatly Magnification of object is the product of magnification of each lens (10x20 =200 times) Presentation by Greg Galardi, Peru State College CRC Press, Jay A. Siegel Ph.D. 5
Compound Microscope Microscope made from two convex lenses is a compound microscope Parts include Eyepiece (ocular lens) Objective Stage Light source Many microscopes are parfocal- once objective lens is focused, then another lens is rotated into focus Presentation by Greg Galardi, Peru State College CRC Press, Jay A. Siegel Ph.D. 6
Compound Microscope Quality microscopes have two focus controls Coarse Fine All microscopes have a type of light system Transmitted light Reflected light Presentation by Greg Galardi, Peru State College CRC Press, Jay A. Siegel Ph.D. 7
Compound Microscopes Most important characteristics are Magnification-product of magnification of ocular Magnification-product of magnification of ocular and objective lens Resolution- described by numerical aperture Higher N.A.>better resolution Empty magnification - Any attempt above N.A. does Empty magnification - Any attempt above N.A. does not improve resolution Field of view- how much to view at one time Depth of focus- how far inside an object the image will be in focus Presentation by Greg Galardi, Peru State College CRC Press, Jay A. Siegel Ph.D. 8
Microscopes Derived from compound Microscope Comparison Microscope Stereo Microscope Polarized light microscope Microspectrophotometer Presentation by Greg Galardi, Peru State College CRC Press, Jay A. Siegel Ph.D. 9
Comparison Microscope Enables examiner to view to objects, side by side, at a time Consists of two compound microscopes connected by a bridge Evidence examined in this way includes fired bullets and cartridges, hair, and fibers This type of observation also allows for two images to be overlaid on one another, especially useful in examining stria on bullets Presentation by Greg Galardi, Peru State College CRC Press, Jay A. Siegel Ph.D. 10
Stereo Microscope Allows for low magnification (25-50) power of evidence, ability to manipulate object and ability to view it in three dimensions Has long working distance which allows for use of needles, forceps, etc If equipped with a trinocular head, this allows for real time digital viewing and photography of object on computer screen Presentation by Greg Galardi, Peru State College CRC Press, Jay A. Siegel Ph.D. 11
Polarized light microscope One of most powerful tools in forensic science laboratory Capable of identifying some particles and objects without further analysis Light is propagated from light bulb or other source Contains two polarizing filters- one beneath stage (polarizer) and one beneath ocular (analyzer) Presentation by Greg Galardi, Peru State College CRC Press, Jay A. Siegel Ph.D. 12
Polarized light microscope Materials examined fall into two categories: Isotropic- not effected by polarized light (gasses, liquids Isotropic- not effected by polarized light (gasses, liquids and many solids are examples) Anisotropic behave differently when plane polarized light reaches it from different planes. For example, certain fibers tend to have different colors when exposed to parallel planes of long axis versus perpendicular planes of long axis (solids with ordered crystals, crystal structures, acrylic fibers and certain plastics) If polarizer and analyzer are aligned at 90 degrees (crossed polars) no light will exit analyzer Presentation by Greg Galardi, Peru State College CRC Press, Jay A. Siegel Ph.D. 13
Microspectrophotometer Used to measure characteristics of materials as they interact with light Two types of microspectrophotometers: Microscope with light source that may be Microscope with light source that may be ultraviolet, visible or infrared Conventional infrared spectrophotometer that has a microscope mounted as an accessory Presentation by Greg Galardi, Peru State College CRC Press, Jay A. Siegel Ph.D. 14
Scanning Electron Microscopy Conventional magnification is limited by distortions created by lens due to curvatures and limits imposed by convex lenses Magnification limit of light is 1000 power SEM uses electrons instead of light to magnify and image Can magnify an image from 10 to 200,000 times Can even measure x-rays- energy dispersive x-ray analysis (EDX) Presentation by Greg Galardi, Peru State College CRC Press, Jay A. Siegel Ph.D. 15
Scanning Electron Microscopy Object mounted on metal stub and placed in evacuated sample compartment Object must be able to conduct electricity, so it may be coated with gold or carbon Beam of electrons aimed at object, which absorbs most of them Presentation by Greg Galardi, Peru State College CRC Press, Jay A. Siegel Ph.D. 16
Scanning Electron Microscopy Secondary electrons and backscattered electrons are captured, amplified and aimed at a cathode ray tube (CRT) which has phosphorescent materials that glow when struck with electrons Electron beam is scanned across object and Electron beam is scanned across object and a magnified image of object shows up on CRT Presentation by Greg Galardi, Peru State College CRC Press, Jay A. Siegel Ph.D. 17
Scanning Electron Microscopy EDX- energy dispersive x-ray analysis- when primary electron beam strikes object, energy from beam causes x-rays from nuclei present in object to be emitted X-ray analyzer captures x-rays and displays them by frequency and quantity Assigns element identities Gunshot residue (GSR) samples are an Gunshot residue (GSR) samples are an example of items analyzed in this manner Presentation by Greg Galardi, Peru State College CRC Press, Jay A. Siegel Ph.D. 18