The application of microscopy techniques for purposes of civil or criminal law. Edmond Locard (1877–1966) stated that every contact leaves a trace. This is known as the Locard exchange principle, the basis for much of forensic microscopy. Since the early 1800s, the microscope has been used to help solve crimes. Today, it remains one of the most used tools in the crime laboratory. Criminal evidence ranging in scale from micrometer-sized particles to hair and paint chips can be found and removed to a laboratory for microscopic analysis, where its history may be deduced to help solve the crime. Microscopy can provide insight into the identity and origin of a material, what has happened to it and when, and the routes it may have taken between a crime victim, suspect, and crime scene (Fig. 1). See also: Criminalistics
The low-powered stereomicroscope is probably the most used microscope in the crime lab. The magnification typically ranges from 7× to 40×, and the image is upright (not reversed) and three-dimensional. With additional lenses, the magnification can range from below 3× to above 100×. This type of microscope can be used with a variety of lighting techniques such as transmitted, fluorescence, diffuse, coaxial, and oblique illumination. See also: Fluorescence microscope; Microscope; Optical microscope
The stereomicroscope is used in drug analysis to help identify marijuana and to evaluate powders and chunks of material that may need to be separated before analysis. In examining firearms and tool marks, it is used to evaluate striations or imprint marks, look for trace evidence, evaluate bullet holes for muzzle blast, and search for unburned gunpowder particles. In addition, when a bullet passes through an object, some of that object may be transferred to the bullet and detected microscopically. In forensic biology, the stereomicroscope is used to find and measure small bloodstains, to evaluate a bloodstain to determine if it came from the inside or outside of a garment, and to remove stains from hidden places. When examining documents, it can be used to evaluate typefaces, examine strikeovers and ink lines that cross over, and find erasures or alterations. For trace evidence, the stereomicroscope is the tool of choice for finding and characterizing hairs, fibers, paint, glass, soil, and building material. See also: Forensic biology
The biological microscope is used for examining biological fluids, human tissue, hairs, fibers, drug crystals, food, and other stains. A typical biological microscope has 4×, 10×, 20×, and 40× objectives on a revolving nosepiece. It is normally equipped with 10× eyepieces giving a final magnification of from 40× to 400×. Many additional lenses can be added to this style of microscope to increase the range of magnification from less than 20× to greater than 1500×. In addition, these microscopes can be modified to include darkfield, phase contrast, fluorescence, dispersion staining, darkfield epi-illumination, and reflected coaxial illumination. When darkfield epi-illumination or reflected coaxial illumination is used, the biological microscope is transformed into a metallurgical microscope, which can be used to examine surface detail and opaque objects. See also: Phase-contrast microscope; Reflecting microscope
A forensic pathologist typically uses the biological microscope to examine tissue samples from injuries to determine if the injuries occurred before, during, or after death. In the forensic biology section of the laboratory, the microscope is typically used to identify sperm in rape cases. This process involves differential staining and examining the slide at greater than 200×. When sperm are identified from samples collected from a victim, this is proof that sexual activity has taken place. The legal issue of consent is not addressed by the presence of sperm. Other body fluids such as vaginal secretions, saliva, and feces might have been mixed with semen, and microscopy can give vital clues as to the origins of these other body fluids.
Some crime labs use crystal tests to identify drugs such as phencyclidine, amphetamines, cocaine, barbiturates, heroin, morphine, and codeine. In a typical crystal test, a small amount of the drug and a liquid reagent are placed on a microscope slide. After a short time, unique crystals for that drug can be recognized at 100× with a biological microscope.
Polarized light microscope
The polarized light microscope is a biological microscope with special modifications, including a polarizer, analyzer, rotating stage, accessory slot, Bertrand lens, and a flip-up condenser. This microscope is typically used to identify synthetic fibers, minerals, glass, starch, and many other types of particles. It has the advantage of measuring optical properties without altering the sample. Six major synthetic fiber types (acrylic, acetate, rayon, olefin, nylon, and polyester) can be quickly differentiated with the polarized light microscope (Fig. 2). A significant amount of training and experience, as well as reference standards, is necessary to master the use of this instrument, since most universities do not teach this subject. The McCrone Research Institute in Chicago has trained many forensic scientists in the use of the polarized light microscope. See also: Birefringence; Polarized light microscope
A comparison microscope consists of two identically equipped compound microscopes joined together with an optical bridge, giving a split-screen image. The comparison microscope is used to compare bullets, cartridge cases, and toolmarks. This microscope normally uses low-powered objectives and reflected, diffused, or oblique illumination. The comparison microscope is also used to compare trace evidence such as hairs and fibers. It uses the same range of magnifications as the biological microscope.
One such case illustrating the comparison of hair, as well as DNA analysis, involved an elderly male victim who was stabbed in a robbery at a hamburger stand. Clumps of long dark hair that appeared to have been pulled out during a struggle were found in several locations at the crime scene. Microscopic examination of that hair showed more than 50 anagen roots. Anagen roots are those still attached to the scalp with a blood vessel and a nerve, indicating that the clumps of hair were forcibly removed during the struggle. The hair from the crime scene had a maximum length of 17 in. The maximum length of the victim's gray hair was less than 1 in. The police collected and the crime lab compared hair from several long-haired suspects with negative results. Finally, a female drug user was identified by a relative as the murderer. A hair sample was collected and submitted to the hair examiner, who testified that the hairs from the crime scene probably came from the suspect. A blood sample from the suspect along with 15 roots from the crime scene hairs were submitted to a DNA laboratory (Fig. 3). The DNA matched, and the suspect was convicted. This case became the first California DNA case to successfully get through the Appeals Court.
Scanning electron microscope
The scanning electron microscope with energy dispersive x-ray spectrometer (SEM/EDX) is routinely used to identify the elemental content of very small samples. The most common sample analyzed with the SEM/EDX is gunshot residue. This is done by dabbing the shooter's hands with sticky tape attached to an SEM stub (a substrate for mounting specimens). The stub is then examined for gunshot residue (GSR) particles (usually 1–6 micrometers) that contain lead, barium, and antimony. Many of the SEM/EDX microscopes are connected to computers that have programs that can search automatically for GSR particles. See also: Scanning electron microscope
Digital forensic microscopy
Most microscope systems can be modified to capture digital images, which can be sent by e-mail to forensic scientists for examination and comment. An automated system for glass refractive index measurement uses a phase-contrast microscope, hot stage, video system, and computer to compare glass samples. The National Integrated Ballistics Identification Network (NIBIN), sponsored by the Federal Bureau of Investigation (FBI) and Bureau of Alcohol, Tobacco and Firearms (ATF), uses digitally captured images (through the microscope) of cartridge cases or bullets to link shooting incidents with guns taken from suspects in distant jurisdictions.
The infrared spectrophotometer with a microscope attached is a common tool in crime laboratories. It is used to perform spectrophotometric analysis on very small samples such as paints, plastics, fibers, and many other substances. The microspectrophotometer is used to measure the spectrum of the visible colors, ultraviolet radiation, and fluorescence that are present in microscopic objects. An automated version of the microspectrophotometer using a remote control stage and computer can search tape lifts taken from suspects, victims, and crime scenes. First a fiber is chosen and entered into the apparatus. The tape lifts are then searched for that target fiber.
Another digital system is a portable microscope with fiber-optic cables that is used to acquire high-quality microscopic images such as a paint transfer on a bumper or hood of a vehicle without dismantling.