SEM
Definition
Scanning electron microscope. Focused electron beam raster-scans the sample surface; resolution down to a few nanometres with great depth of field. Paired with EDX for elemental data.
- Full name
- Scanning electron microscope
- Probe
- Focused electron beam raster-scanned over the surface
- Resolution
- Down to a few nanometres
- Key strength
- Very large depth of field versus optical microscopy
- Common pairing
- EDX for elemental composition
Common questions
Why is SEM preferred over optical microscopy for rough surfaces?+
Its depth of field is far greater than any optical objective, so a fracture surface, tool mark, or fired cartridge case stays in sharp focus across its full topography instead of only a thin plane, which lets examiners image three-dimensional surface detail directly.
Does SEM require a coated or conductive sample?+
Traditional high-vacuum SEM needs a conductive coating, usually gold or carbon, on non-conductive samples to prevent charging artifacts. Variable-pressure or environmental SEM modes can image uncoated, non-conductive, or slightly hydrated evidence directly.
What does adding EDX to an SEM examination provide?+
EDX detects characteristic X-rays generated by the electron beam to identify the elements present at a chosen spot or area, turning a purely morphological image into one with quantitative elemental data, which is essential for confirming gunshot residue particles.
Related terms
- Comparison Microscope
- Two compound microscope bodies connected by an optical bridge that presents one half of each body's image in a single split eyepiece...
- Depth of Field
- Axial thickness of the specimen that appears acceptably in focus at one time. High in stereoscopic microscopes, very low in oil-immersion compound...
- EDX / EDXRF
- Energy-dispersive X-ray detector attached to an SEM. Identifies elements from characteristic X-rays emitted when the beam strikes the sample. Backbone of GSR...
- Fluorescence Microscope
- Uses UV or short-wavelength excitation to make fluorophores emit longer-wavelength light. Used for semen, saliva, fibres and ink discrimination.
- Numerical Aperture (NA)
- The product of the refractive index of the medium between objective and specimen and the sine of the half-angle of the maximum...
- Polarising Microscope
- Compound microscope fitted with a polariser below and an analyser above the stage. Used to study birefringent materials like fibres, minerals and...
- Resolution
- The ability of a mass analyser to distinguish two ions of nearly equal m/z, defined as m/Δm at a given peak. A...
- Stereoscopic Microscope
- Two separate light paths producing a true 3D image at low magnification (typically 7x to 40x). Used for trace recovery, gross examination...
- TEM
- Transmission electron microscope. Electrons pass through an ultra-thin section; resolution below 1 nm. Rare in routine forensic casework, common in research.