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• Field of view

Compound microscope

• 40-450x magnification (up to 1000x)

Comparison microscope

Stereoscopic microscope

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

Polarizing microscopy

• Polarized light

Polarizing microscope• Polarizer

• Analyzer

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

Polarizing microscopy

• Improves specimen contrast

Stained plant fibers, left – stained; right – polarized light

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

Polarizing microscopy

• Many crystalline substances are birefringent– Forensic analysis of

soil components

• Many synthetic fibers are birefringent

Microspectrophotometer

Principles of spectroscopy

Principles of spectroscopy

Microspectrophotometer

• Infrared spectromicroscopy

Scanning electron microscopy

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

Online microscopy resources

• Molecular expressions

• Olympus Microscopy Resource Center

For next class

• Read chapter 7– Read chap. 7 Case Reading

• Reading #1 (reading pack)