The Scanning Electron Microscope (SEM) is a type of electron microscope capable of producing high-resolution images of a sample surface. SEM images have a characteristic three-dimensional appearance and are useful for judging the surface structure of the sample. SEM uses an electron beam generated in a vacuum to magnify very small objects. It is an effective instrument for the analysis of physical evidence because it enables a researcher to analyze a substance by both its topographical–the appearance of its surface, and compositional–the elements that make up the material, qualities. Its main advantage is its capability to resolve small detail. The SEM’s increased resolution and large depth of field enable it to magnify objects by more than 50,000 times their normal size. The SEM generates images through perceiving secondary electrons that are released from the surface due to excitation by the primary electron beam. In the SEM, the electron beam is blasted across the sample, with detectors building up an image by mapping the detected signals with beam position.

The SEM operates by scanning a tightly focused electron beam over a sample. Electrons in the beam scatter off of the sample and onto a cathode ray tube, or screen. Every point on the sample corresponds to a pixel or picture element on the screen. The more electrons that hit a particular element of the screen, the brighter the pixel appears. As the electron beam examines over the entire sample, a complete image of the sample is displayed on the monitor. SEMs have a range of magnification between 20X to 200,000X. Normally, the TEM resolution is approximately an order of magnitude better than the SEM resolution, nevertheless, since the SEM image relies on surface processes rather than transmission it is able to image bulk samples and has a much greater depth of view, and so can produce images that are a good representation of the 3D structure of the sample. SEM reveals new levels of detail and complexity in the amazing world of microorganisms and miniature structures. The scanning electron microscope, which is designed for precisely analyzing the surfaces of solid objects, makes use of a beam of focused electrons utilizing approximately 5–25 kilovolts energy, as an electron probe that is scanned in a regular manner over the specimen. The electron source and electromagnetic lenses that produce and focus the beam are comparable to those described for the transmission. Scanning electron microscope can amplify objects 100,000 times and is used to detect the minute gunpowder particles present on the hand of a person who has recently fired a gun. These particles can also be chemically analyzed to identify their origin from a particular type of bullet. Also, X-rays released from the electron beam/sample interaction can be detected by an Energy Dispersive Spectroscope (DES), which allows samples to be analyzed for their elemental composition. In the forensic field, SEM/DES is used to detect and analyze gunshot residue and other trace materials, such as paint, glass and light filaments. An electron microscope requires a good vacuum for the production and proliferation of the electron beam, which in the past meant that the specimen under inspection had to be placed also in vacuum. Nonetheless, it is now possible to view specimens inside gaseous surroundings so that wet samples in a water vapor environment can also be examined. The conductive coating of specimens according to traditional practice is no more essential because the gaseous layer around the specimen becomes ionized and suppresses charge buildup. The gas itself can also be used as detection medium giving rise to novel detection and imaging techniques. These innovations have resulted in the scanning electron microscope. The entire work has been based on the improvement and use of a prototype SEM, which has generated results of high quality. In many respects, these results still remain unique and unrivaled to date. The SEM is rapidly gaining approval by the scientific, technical and industrial community as evidenced from the large number of publications arising from its use. The many attempts and history that have led to the present development of SEM have been reviewed and surveyed elsewhere. These works include all main modes of imaging, electron dynamics and gas dynamics. The optimum design and integration of detectors with electron optics and differential pumping comprises the basic philosophy of SEM. Vital to these is also the establishment of new detection methods such as the gaseous detection device (GDD). With this, both the secondary and the backscattered electron signal can be detected, in a variety of ways. The progress of a proper theoretical and practical background has also constituted the basis for much of the progress in the past, present and future.