chapter 7 forensic microscopy. terms virtual image real image
TRANSCRIPT
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Chapter 7
Forensic microscopy
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Terms
• Virtual image
• Real image
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Physical properties of light
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Compound microscopes
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Compound microscope terminology
• Transmitted illumination (below)• Reflected illumination (above)• Condenser• Iris diaphragm• Parfocal• Monocular• Binocular• Depth of focus• Field of view
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Compound microscope
• 40-450x magnification (up to 1000x)
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Comparison microscope
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Stereoscopic microscope
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Stereoscopic microscope
• Most frequently used in forensics
• 10-125x range
• Large working distance– Good for bulky artifacts
• Wide field of view & great depth of focus (3-D images)– Trace evidence on artifacts
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Polarizing microscopy
• Polarized light
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Polarizing microscope• Polarizer
• Analyzer
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Polarizing microscopy
• Analyzer out of path = plane polarized light; gives specimen’s “true color”
• Analyzer in path = crossed polars; gives specimen’s interference color
Polyester, plane-polarized light
Polyester, under crossed polars
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Polarizing microscopy
• Improves specimen contrast
Stained plant fibers, left – stained; right – polarized light
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Polarizing microscopy
• Birefringence
Calcite produces two images when it is placed over the blue pencil. One of the images appears normally as when viewing an object through clear glass. The other pencil image appears displaced, due to the nature of doubly-refracted light. When anisotropic crystals refract light, the resulting rays are polarized and travel at different velocities. One of the rays travels with the same velocity in every direction through the crystal and is termed the ordinary ray. The other ray travels with a velocity that is dependent upon the structural properties of the crystal
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Polarizing microscopy
• Many crystalline substances are birefringent– Forensic analysis of
soil components
• Many synthetic fibers are birefringent
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Microspectrophotometer
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Principles of spectroscopy
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Principles of spectroscopy
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Microspectrophotometer
• Infrared spectromicroscopy
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Scanning electron microscopy
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Scanning electron microscopy
• High magnification (up to 100,000x)
• Excellent focal depth (3-D)
• Used with X-ray analyzer (Fig 7-12), can identify elemental composition of a specimen– Has a suspect fired a gun?
• Pollen identification
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Online microscopy resources
• Molecular expressions
• Olympus Microscopy Resource Center
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For next class
• Read chapter 7– Read chap. 7 Case Reading
• Reading #1 (reading pack)