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Chapter 7

Chapter 7. Forensic microscopy. Terms. Virtual image Real image. Physical properties of light. Compound microscopes. Compound microscope terminology. Transmitted illumination (below) Reflected illumination (above) Condenser Iris diaphragm Parfocal Monocular Binocular Depth of focus

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Chapter 7

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  1. Chapter 7 Forensic microscopy

  2. Terms • Virtual image • Real image

  3. Physical properties of light

  4. Compound microscopes

  5. Compound microscope terminology • Transmitted illumination (below) • Reflected illumination (above) • Condenser • Iris diaphragm • Parfocal • Monocular • Binocular • Depth of focus • Field of view

  6. Compound microscope • 40-450x magnification (up to 1000x)

  7. Comparison microscope

  8. Stereoscopic microscope

  9. 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

  10. Polarizing microscopy • Polarized light

  11. Polarizing microscope • Polarizer • Analyzer

  12. Polyester, plane-polarized light Polyester, under crossed polars Polarizing microscopy • Analyzer out of path = plane polarized light; gives specimen’s “true color” • Analyzer in path = crossed polars; gives specimen’s interference color

  13. Polarizing microscopy • Improves specimen contrast Stained plant fibers, left – stained; right – polarized light

  14. 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

  15. Polarizing microscopy • Many crystalline substances are birefringent • Forensic analysis of soil components • Many synthetic fibers are birefringent

  16. Microspectrophotometer

  17. Principles of spectroscopy

  18. Principles of spectroscopy

  19. Microspectrophotometer • Infrared spectromicroscopy

  20. Scanning electron microscopy

  21. 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

  22. Online microscopy resources • Molecular expressions • Olympus Microscopy Resource Center

  23. For next class • Read chapter 7 • Read chap. 7 Case Reading • Reading #1 (reading pack)

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