Microscopes in forensic investigations

DEFINITION: Research instruments designed to allow the visualization of objects too small to be seen by the naked eye.

SIGNIFICANCE: Microscopes enable forensic scientists to observe details that are normally invisible to the naked human eye, allowing them to conduct thorough analyses of evidence samples that are frequently crucial to investigations.

Modern microscopes differ considerably from the earliest instruments developed in the seventeenth century, but their function remains much the same as it was then. Microscopes are designed to magnify items that are too small for the human eye to see.

Two important concepts to understand regarding the general operation of microscopes are magnification and resolution (also called resolving power). Magnification is the increase in size of the image of the object that a microscope allows. This is expressed as a numeral with a multiplication sign; for example, a 400x image is four hundred times normal size. Resolution is the ability of the microscope to distinguish two items as unique; it is associated with the amount of detail that may be discerned within the image. Ideally, microscopes provide not only high levels of magnification but also high resolution. Among the many different types of microscopes in existence, forensic investigators most frequently rely on light microscopes, electron microscopes, and comparison microscopes in their work.

Light Microscopes

Light microscopes function by using a combination of lenses to alter the path of light to provide magnification and resolution. A light source passes light either through (transmitted) or around (reflected) the sample. A glass lens, called an objective lens, then detects a portion of the transmitted or reflected light and focuses it on a second lens, called the ocular lens. Because only a portion of the initial light passing through the sample is viewed, the size of the object is magnified. The total amount of magnification is determined by the product of the magnification of the individual lenses; for instance, an objective lens with a magnification power of 40x and an ocular lens with a 10x magnification would yield a total magnification of 400x.

Several types of light microscopes are available. Compound light microscopes, also sometimes called biological microscopes, use lenses placed in series to increase the magnification of objects. These microscopes may have either one or two ocular lenses. They are useful for examining samples that easily allow the passage of light and may be fixed in place on glass or plastic slides, such as bacteria, hair, or fibers. Another common form of the light microscope is the stereoscopic microscope, which uses lenses placed at different angles to produce a three-dimensional image, although usually at a lower magnification level than that of the biological microscope. The position of the stereoscopic microscope’s light sources illuminates the surface of the specimen, making it useful for examining detail in larger objects. Forensic scientists often use stereoscopic microscopes for the initial sorting of evidence, such as fibers and hair, from other materials.

Comparison Microscopes

Comparison microscopes represent a special adaptation of light microscopes for use in forensic investigations. A comparison microscope is basically two light microscopes that have been combined into a single instrument with an optical bridge. This configuration allows the investigator to examine two samples at the same time. This is especially useful in ballistics tests, when forensic researchers compare the microscopic details on the surfaces of bullets, which are specific to the weapons from which they were fired. Forensic scientists may also use comparison microscopes when comparing other kinds of samples, such as fibers, to samples from known sources.

Electron Microscopes

Although they are called microscopes because they serve to produce magnified images of objects, electron microscopes are very different in construction from light microscopes. In place of photons of light, electron microscopes use negatively charged electrons to obtain images. Because electrons are negatively charged, their paths may be controlled by magnets and electrical fields. In an electron microscope, after leaving the source, the electrons interact with the sample and are then detected by an imaging system. The imaging system relays the pattern of electrons to a computer, which then presents an image of the specimen. Electron microscopes have significantly higher magnification power and resolving power than do the best light microscopes; some can magnify images more than one million times and possess the ability to resolve individual atoms.

Several different types of electron microscopes have been developed, but the major ones used in forensic research are the transmission electron microscope (TEM) and the scanning electron microscope (SEM). In a TEM, the electrons pass through the specimen to a detector. As these electrons move through the sample, they interact with the atoms and molecules within the sample, which distorts the path of the electrons. The electrons are then collected by a collector, which in turn presents an image to the operator. TEMs are useful for examining the internal structures of cells and other samples. They have the highest magnification and resolving power of any kind of microscope.

Often the electron microscope of choice in a forensics lab is the SEM, which uses a beam of electrons to reveal the surface of a microscopic sample. As the electrons leave the source, they interact with the atoms and molecules on the surface of the specimen. Some of the electrons are scattered by the surface molecules, and other electrons cause X-rays to be released. The scattered electrons and X-rays are then collected by an imaging system, which provides a picture of the specimen’s surface. SEMS are often used in conjunction with atomic force microscopes (AFMS). While SEMS have a better dept of field, AFMS show contrast better and are three-dimensional. Because different surfaces reflect electrons differently, forensic scientists using SEMs first coat some specimens in a metal, often gold, before scanning.

Bibliography

Blackledge, Robert D., ed. Forensic Analysis on the Cutting Edge: New Methods for Trace Evidence Analysis. Hoboken, N.J.: John Wiley & Sons, 2007.

James, Stuart H., and Jon J. Nordby, eds. Forensic Science: An Introduction to Scientific and Investigative Techniques. 2d ed. Boca Raton, Fla.: CRC Press, 2005.

Petraco, Nicholas, and Thomas Kubic. Color Atlas and Manual of Microscopy for Criminalists, Chemists, and Conservators. Boca Raton, Fla.: CRC Press, 2004.

Saferstein, Richard, ed. Forensic Science Handbook. 2d ed. Vol. 2. Upper Saddle River, N.J.: Prentice Hall, 2005.

Therassa, Megha, et al. "A Comprehensive Review on Application of Atomic Force Microscopy in Forensic Science." Journal of Forensic and Legal Medicine, vol. 105, July 2024, doi.org/10.1016/j.jflm.2024.102717. Accessed 15 Aug. 2024.