LECTURE 14: MICROSCOPY // CHAPTER 4 // Flashcards
Physical evidence
- volatiles – fire accelerants, explosives, pepper sprays, cosmetic-lubricants
- very small particles – gunshot residues
- non-volatile trace evidence – fibres, paint, rubber, plastic, metal, glass, soil
Light microscopy
- light source
- sample holder
- magnifier/ lenses
- viewer
- series of lenses
- movement or switching changes magnification
- image viewed using eye pieces or cameria set-up
- different set-ups for different samples
- transmission
- reflectance
- brightfield
- darkfield
- phase contrast
Some microscope terms
- depth of field
- field of view
- focal length
- working distance
- magnification
- resolution
Stereomicroscope (dissecting scope)
Used in trace evidence for
- routine searching of items for trace evidence
- viewing layer sequence in paint chips
- manipulating and preparing small sample for subsequent analysis
Compound microscope
- higher mags than stereo • up to 1000x magnification - very narrow depth of field • limited to thin or narrow samples - good for fine detail - majority of use in PE is fibres • e.g. scales on wool fibre • delusterant on synthetic fibres
Comparison microscope
- 2 compound microscopes
- viewing bridge adjustable
- allows for comparison ‘side by side’
- up to 1000x
- major of use in PE is fibres
• comparison of recovered and control fibres side by. side
Polarised light microscope (PLM or petrological microscope)
- use two polarising filters
- regularly-spaced, patterned or crystalline samples rotate the light
- some of this rotated light passes through the second polarising filter, so these regularly spaced areas show up coloured against a black background
- up to 500x mag
- difference in the way materials interact with the polarised light give changes in appearance
- in PE used for:
• mineral ID
• ID of fibre types
• confirmation of glass fragments
Scanning electron microscope (SEM) with energy dispersive X-ray spectrometry (EDS)
- Higher resolution •Higher magnification
• up to 500,000x - Secondary e’s -> topography
- Backscattered e’s -> relative elemental distribution
- X-rays characteristic of
- Grey scale images- NO colour
- More complex and expensive instrument than light microscopes
- Used for many substrates
• Must be dry (no solvent or water)
• Stable (most of our work done under high vacuum)
• EDS- elements higher than elements
compound magnification
magnification occurs in 2 stages & the total magnification is the product of the magnification of the 1st lens & the 2nd lens
lens
is a translucent material that bends light in a known & predictable manner.
The size & position of an image produced by a lens can be determined through geometry based on the
focal length
focal length
the distance between the 2 points of focus on either side of the lens & determines much of the image quality.
Refraction
occurs as light passes from 1 medium to another when there is a difference in the index of refraction between the 2 materials.
Refractive index (RI) =
the relative speed at which light moves through a material with respect to its speed in a vacuum.
