Scanning electron microscopes (SEM) provide forensic investigators with critical imaging and analytical capabilities that are not available from other techniques. In particular, they can resolve features as small as a nanometer, allowing useful magnifications of 100,000 times or more.

Even at very high magnifications, scanning electron microscopes have large depth of field, allowing them to capture well-focused images of complex three-dimensional objects. In conjunction with X-ray microanalysis, they can determine the elemental composition of the samples as small as a few micrometers or map the distribution of elements in larger samples with micrometer spatial resolution. They are widely used for the analysis of gunshot residue; tool mark investigations; ink and paper analysis; paint, hair, and fiber matching; and the identification of other trace evidence, such as pollen and diatoms, that can link a suspect to a crime scene.

SEM IMAGING
Unlike optical microscopes, which form a real image from transmitted or reflected light, a scanning electron microscope forms a virtual image on an electronic display. A finely-focused beam of electrons scans the sample surface in a raster pattern and the brightness of each point in the displayed image represents the strength of the imaged signal generated at the corresponding point in the sample when it is illuminated by the beam.

The imaged signals are created by interactions between beam electrons and sample atoms as the beam penetrates into the sample. The depth of penetration is determined by the accelerating voltage of the beam and the density of the sample, and ranges from a few nanometers to a few micrometers. SEM is generally regarded as a surface imaging technique. Different interactions yield signals, each with different characteristics. The most common signals include: secondary electrons (SE), used primarily for high resolution and high topographic contrast; backscattered electrons (BSE), used for material contrast (atomic number); and characteristic X-rays, used to determine elemental distribution and composition.