Chapter 7 The Microscope

Virtual image – a magnified image as seen through a lens Real image – an image seen directly Two lens for viewing a. Objective b. Eyepiece (Ocular)

5 major types of microscopes Used in viewing forensic specimens 1. Compound 2. Comparison 3. Stereoscopic 4. Polarizing 5. Microspectrophotometer

Compound Microscope Illuminator = Transmitted illumination or vertical or reflected illumination Condenser = Abbe is the most simple (collects light rays from the base) Objective lens = Closest to the specimen and is parfocal Ocular lens = Monocular or binocular

Numerical Aperture The ability of the objective lens to resolve details into separate images instead of one blurred image (which happens up close) Magnification power offers a. Field of view b. Depth of focus

Comparison Microscope Two compound microscopes side by side Produces a circular field with the two halves reserved for the specimens To compare bullets, cartridges, and opaque objects Vertical illumination is used To compare hairs or fibers Transmitted illumination is used

Stereoscopic Microscope Provides magnification from 10x to 125 x Presents a 3 dimensional image Image is right side up Two monocular compound microscopes spaced and aligned to present a 3-D image

Stereoscopic cont. Most frequently used in the crime lab Wide field of view Great depth focus Large working distance for specimens Used for paint, soil, gunpowder residues, marijuana

Polarizing Microscope Plane-polarized Light which emerges from a crystalline substance and vibrates in only 1 plane Polarizer A device that polarizes light in this way Contains an analyzer A second type of polarizing crystal through which a polarized beam passes

Polarizing cont. If analyzer and polarizer are at 90 degree angles then no light is seen Makes a specimen readily distinguishable Useful for birefringement minerals in the soil

Microspectrophotometer Combination of computerized spectrophotometer and microscope Allows for viewing a small particle under the microscope Allows for the a beam of light directed at the particle to obtain an absorption spectrum

Microspectro. cont. Visual inspection of color is essential in examining paint, fiber, ink evidence Used to distinguish counterfeit and authentic currency by comparing spectral patterns of inked lines on currency Useful in determining fiber and paint chip chemicals

Scanning Electron Microscope (SEM) A beam of electrons on the specimen from a tungsten filament Electrons are focused by electromagnets The upper layers of the specimen rebound electrons Rebounded electrons collected by a cathode ray tube or the TV

SEM cont. Emitted electrons are converted into an image High magnification, high resolution, great depth of focus 10x to 100,000 x Can be used to determine elemental composition of a specimen by coupling with an X-ray Can be used to tell if a suspect has recently fired a gun