Microscopes (Forensic Science)
Modern microscopes vary considerably from the earliest instruments developed in the seventeenth century, but the function of microscopes remains much the same as it was several hundred years ago. 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 400 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.
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Light Microscopes (Forensic Science)
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 40 and an ocular lens with a 10 magnification would yield a total magnification of 400 .
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...
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Comparison Microscopes (Forensic Science)
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 ballistic 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 evidence samples, such as fibers, to samples from known sources.
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Electron Microscopes (Forensic Science)
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...
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Further Reading (Forensic Science)
Blackledge, Robert D., ed. Forensic Analysis on the Cutting Edge: New Methods for Trace Evidence Analysis. Hoboken, N.J.: John Wiley & Sons, 2007. Collection of essays on forensic methods focuses on advances in technology and includes discussion of the use of microscopes.
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. Provides a complete overview of forensic investigations, including the use of various forms of microscopes.
Petraco, Nicholas, and Thomas Kubic. Color Atlas and Manual of Microscopy for Criminalists, Chemists, and Conservators. Boca Raton, Fla.: CRC Press, 2004. Highly illustrated manual describes analytical tests and testing procedures involving the use of microscopy.
Saferstein, Richard, ed. Forensic Science Handbook. 2d ed. Vol. 2. Upper Saddle River, N.J.: Prentice Hall, 2005. The fifth chapter of this book provides an excellent review of the principles of microscopy and the use of microscopes in forensic investigations. Includes detailed descriptions of how light microscopes are constructed.
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Microscopes (World of Forensic Science)
A microscope is the instrument that produces the high magnification image of an object that is otherwise difficult or impossible to see with the unaided eye. A microscope's resolving power allows the user to differentiate two objects from one another that could not be distinguished with the naked eye.
Microscopes assume a central role in forensic science. Forensic evidence, particularly trace evidence, is often so tiny as to escape detection with the naked eye. But the magnified examination of samples can reveal a great deal of detail. For example, examination of gunshot residue using a scanning electron microscope can allow an investigator to determine the shape of the spent residue and even its elemental composition, both of which are critical to the identification of the gunpowder used. The microscope can aid in matching the residue on a victim to residue present on a suspect. As another example, examination and identification of fibers would be impossible without the use of light microscopy.
Microscopic examination of documents can reveal information that cannot otherwise be seen. The high magnification and analysis possible using specialized techniques of scanning and transmission electron microscopy can reveal the presence of material that is otherwise undetectable in the elements that make up a sample.
Today's sophisticated use of microscopes in forensic analysis had its beginnings hundreds of years ago. In ancient and classical civilizations, people recognized the magnifying power of curved pieces of glass. By the year 1300, these early crude lenses were being used as corrective eyeglasses.
In the seventeenth century Robert Hooke published his observations of the microscopic examination of plant and animal tissues. Using a simple two-lens compound microscope, he was able to discern the cells in a thin section of cork. The most famous microbiologist of this century was Antony van Leeuwenhoek (1632723). Using a single lens microscope that he designed, Leeuwenhoek described microorganisms in environments such as pond water. His were the first descriptions of bacteria and red blood cells.
By the mid-nineteenth century, refinements in lens grinding techniques had improved the design of light microscopes. Still, advancement was mostly by trial and error, rather than by a deliberate crafting of a specific design of lens. It was Ernst Abbe who first applied physical principles to lens design. Abbe combined glasses that bent light beams to different extents into a single lens, reducing the distortion of the image.
The resolution of the light microscope is limited by the wavelength of visible light. To resolve objects that are closer together, the illuminating wavelength needs to be smaller. The adaptation of electrons for use in microscopes provided the increased resolution.
In the mid-1920s, Louis de Broglie suggested that electrons, as well as other particles, should exhibit wavelike properties similar to light. Experiments on electron beams a few years later confirmed this hypothesis. This was utilized in the 1930s in the development of the electron microscope.
There are two types of electron microscope: the transmission electron microscope (TEM) and the scanning electron microscope (SEM). The TEM transmits electrons through a sample that has been cut so that it is only a few molecules thin. Indeed, the sample is so thin that the electrons have enough energy to pass right through some regions of the sample. In other regions, where metals that were added to the sample have bound to sample molecules, the electrons either do not pass through as easily, or are restricted from passing through altogether. The different behaviors of the electrons are detected on
The combination of the resolving power of the electrons, and the image magnification that can be subsequently obtained in the darkroom during the development of the film, produces a total magnification that can be in the millions.
Because TEM uses slices of a sample, it reveals internal details of a sample. In SEM, the electrons do not penetrate the sample. Rather, the sample is coated with gold, which causes the electrons to bounce off of the surface of the sample. The electron beam is scanned in a back and forth motion parallel to the sample surface. A detector captures the electrons that have bounced off the surface, and the pattern of deflection is used to assemble a three dimensional image of the sample surface.
In the early 1980s, the technique called scanning tunneling microscopy (STM) was invented. STM does not use visible light or electrons to produce a magnified image. Instead, a small metal tip is scanned very close to the surface of a sample and a tiny electric current is measured as the tip passes over the atoms on the surface. When a metal tip is brought close to the sample surface, the electrons that surround the atoms on the surface can actually "tunnel through" the air gap and produce a current through the tip. The current of electrons that tunnel through the air gap is dependent on the width of the gap. Thus, the current will rise and fall as the tip encounters different atoms on the surface. This current is then amplified and fed into a computer to produce a three dimensional image of the atoms on the surface.
Without the need for complicated magnetic lenses and electron beams, the STM is far less complex than the electron microscope. The tiny tunneling current can be simply amplified through electronic circuitry similar to circuitry that is used in other electronic equipment, such as a stereo. In addition, the sample preparation is usually less tedious. Many samples can be imaged in air with essentially no preparation. For more sensitive samples that react with air, imaging is done in vacuum. A requirement for the STM is that the samples be electrically conducting, such as a metal.
Scanning tunneling microscopes can be used as tools to physically manipulate atoms on a surface. This holds out the possibility that specific areas of a sample surface can be changed.
Other forces have been adapted for use as magnifying sources. These include acoustic microscopy, which involves the reflection of sound waves off a specimen; x-ray microscopy, which involves the transmission of x rays through the specimen; near field optical microscopy, which involves shining light through a small opening smaller than the wavelength of light; and atomic force microscopy, which is similar to scanning tunneling microscopy but can be applied to materials that are not electrically conducting, such as quartz.
SEE ALSO Fibers; Fluorescence; Microscope, comparison; Polarized light microscopy; Scanning electron microscopy; Scanning electron microscopy; Trace evidence.