Advertisement

The popularity of television shows like CSI and theprime time reporting of court cases have catapultedforensic microscopy into the public spotlight. “Muchof what is seen on CSI and other related TV shows looksreal and the writers and producers try to simulatereal science,” explains Chuck Zona, of the Collegeof Microscopy, Westmont, Ill. But what is the realscience behind forensic microscopy? Are the technologiesas quick and powerful as they are portrayed on TV?

The birth of forensic science

Forensic science examines physical evidence as it relatesto the law. This discipline can be tracked back toEdmund Locard, director of the technical police laboratoryin Lyon, France, in the early 1900s. He formulatedLocard’s Exchange Principle, which states thatwhenever any two objects come into contact, there isalways transference of material from each object tothe other. The job of the forensic scientist is tothen study the material that is left behind at a crimescene to determine the perpetrator of the crime.

Tools of the trade
Today, forensic scientists can choose from a varietyof techniques to study this evidence, but perhaps themost important technique has been forensic microscopy.Forensic microscopy encompasses the identificationand classification of a wide range of materials andsubstances: impressions such as fingerprints and footprints,fractured fragments such as broken tools and torn paper,trace evidence such as hairs and fibers, genetic markers,bullets, and handwriting.

Skin cells illuminated by plane-polarized light at400X magnification. Image: McCrone Associates, Inc.and the McCrone Atlas of Microscopic Particles.

The tools employed in forensic microscopy vary fromthe basic low power hand magnifiers to high power electronmicroscopes. Binocular (stereo) microscopes are commonlyused in document examination, as in trying to determinewhether one pen stroke passes over another. Scanningelectron microscopes are used for morphological andelemental analysis, and to identify gunshot residue(GSR). Transmission electron microscopes are used forpathogen analysis and for the examination of paintpigments. Infrared or IR microscopes are used in drugidentification, while phase contrast microscopes areused to characterize materials such as glass and biologicalfluids.

Polarized light microscopy
Even with all of this variety, polarized light microscopy(PLM) is by far the dominant technique used in forensicmicroscopy and key in the forensic analysis of smallunknown particles, commonly known as trace evidenceanalysis.

PLM is a contrast-enhancing technique that uses birefringentmaterials to improve the quality of the images obtainedby the microscope. Polarized light microscopes havea high degree of sensitivity and can be employed bothquantitatively and qualitatively.

In terms of sophistication, polarized light microscopescan be categorized into three levels: small, highlycapable microscopes designed for education or basicanalytical work; laboratory grade, which allows fullpolarized light forensic analysis; and high grade laboratoryinstruments, which are also used for research.

Comparison microscopy
The comparison microscope is the primary tool usedin trace evidence comparisons in forensic science laboratories.It is used in document analysis and in determiningwhether a particular bullet was fired from a particulargun.

The instrument consists of two microscopes which canbe viewed simultaneously on a split screen. This enablesthe direct comparison of trace evidence under the samelighting conditions and magnification. The images canalso be positioned so that they appear to overlay eachother. For document analysis, a different filter canbe used on each document, and when the images are overlaid,the third, darker color corresponds to those areaswhere the two segments overlap.

Cross-section of blond and brunette human hairs illuminatedby plane-polarized light at 400X magnification. Image:McCrone Associates, Inc. and the McCrone Atlas of MicroscopicParticles

In determining whether a bullet came from a particulargun, the bullets and shell casings are compared side-by-side. “Bycomparing the unique marks and striations from thesamples, an examiner may be able to determine if twobullets were fired from the same gun,” explainsKevin Boulay, Leeds Forensic Product Manager, of LeedsPrecision Instruments, Inc., Minneapolis, Minn. Leedsis the manufacturer of a Firearms and Toolmarks comparisonmicroscope.

“The comparison microscope is also used to compare aunique signature of deformation made by an object,known as toolmarks,” explains Boulay. “Acommon example of when a toolmark comparison microscopewould be used is when a padlock is cut during a crime.The padlock and bolt cutter that were suspected ofbeing used in the crime would be compared with thecomparison microscope to try to determine if the uniquemarks left by the bolt cutters match deformation onthe padlock.”

Forensic microscopy’s unique view
While the actual science used in forensic microscopy is the same as in other microscopy arenas, the approach and attitude are different.

“ A key difference between forensic science and traditional natural sciences is that in traditional science, the subject specimen is often known and microscopy is used to quantify its properties,” explains Bill Fester, Director of Marketing for the Scientific Equipment Group of Olympus America, Inc., Melville, N.Y. “In forensic science, it can be quite the opposite—the microscope is used to study a specimen’s properties in order to determine what it is and where it may have come from.”

“ Forensic microscopy takes on a little different perspective,” agrees Zona. “Because of the potential effect on someone’s liberty (criminal law) or to someone’s livelihood, wealth or business (civil law), forensic microscopy should be approached from the understanding that what is done and concluded from the microscopy needs to be right, needs to be provable, needs to be without reproach, and needs to be fair to the parties.”

There are other distinctions as well. “Forensic microscopy has two important distinctions: first is the need to record documentation to provide information and evidence. Second, is the requirement to provide measurements of a sample for relative size references,” says Stan Schwartz, VP of Nikon Instruments, Melville,N.Y.

Looking to the future
The optical technology of forensic microscopy is quite mature. Forensic scientists have expressed their needs to manufacturers who have responded with targeted products. Mostly, however, these researchers borrow what they need from other scientific disciplines.

Many new capabilities combined with digital photography now allow forensic scientists to capture and store evidence that can be shared collaboratively with colleagues and experts. “Increasingly, image database capability is becoming a requirement because the rules regarding evidence processing are highly defined and the amount of forensic evidence is increasingly large,” says Schwartz. “Database software, like that included with Nikon products, can document and store bullet casing images and automobile VIN numbers.”

Researchers at the McCrone Group, Westmont, Ill., are also heeding the call for collaborative science with the creation of an Internet database of microscopy images from around the world, called the McCrone Atlas of Microscopic Particles. “This online reference site will take advantage of the Internet and digital imaging to provide an extremely valuable resource to microscopists who routinely need to identify unknown substances,” says Fester.

But not all the needs of forensic microscopists and their growing community are being met. “One place where the user’s needs are not always met is in academic education and training in microscopy, particularly PLM,” says Zona. One place that targets PLM is the College of Microscopy, in Westmont, Ill, where Zona is the VP and director of education.

The goal of the college, according to Zona, is to advance the knowledge and understanding of light and electron microscopy for materials analysis, using state-of-the-art light and electron microscopes andaccessories.

The CSI effect
While more time-consuming and less glamorous than what is portrayed on television, forensic microscopy remains an extremely powerful tool in solving crimes. “Much of what is seen on CSI and other related TV shows is fictional,” explains Zona. “Nevertheless, they more often than not just get the images right—close-up views, flashing lights, and printouts. Real crime labs seldom can do what is done in a few minutes on TV.” For example, the exact time of death and the exact sequence of events leading up to that death are extremely difficult to pin down. “When forensic science cannot produce what is expected, the public and investigators are disappointed and lose faith. It is ironic that the CSI effect is good for forensic science by making this work very public, but bad for forensic science because of the disappointment of reality,” says Zona.

—Martha Walz


Advertisement
Advertisement