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How a Fingerprint Is Traced to the Person Who Made It – Interactive Microscope System Facilitates Dactyloscopist Training

“We have to take your fingerprints.” This sentence is spoken in nearly every TV crime drama to a suspect sitting in the interrogation room. But what exactly is it that makes a fingerprint so valuable for detectives in real life? How do fingerprint experts, known as dactyloscopists, perform their jobs? Dieter Kauer has trained dactyloscopy experts at the Federal Office of Criminal Investigation in Wiesbaden, Germany for over 20 years. Here, he explains from his decades of experience how a dactyloscopist works, the value of his reports as court evidence, and how the Leica IMS500 interactive microscope system is used in combination with Leica stereomicroscopes to train these experts.


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What type of work do you and your colleagues do at the Forensic Science Institute of the Federal Office of Criminal Investigation?

"The Forensic Science Institute is a center for training colleagues from all over Germany as experts in various areas. The course participants can choose from different specializations, including dactyloscopy, ammunition, tool traces, handwriting, and certificates. As head of dactyloscopy training, I am in charge of the three-year training of fingerprint experts. I started at the Federal Office of Criminal Investigation (BKA) in 1974 as a trainee inspector and have trained dactyloscopists since 1989. I learned about microscopy in courses at the universities of Tübingen, Germany, and Lausanne, Switzerland.

The students in this area have normally had a few years of hands-on experience before they come to us. The modular training consists of practical and theoretical elements and ends with an examination in which the prospective dactyloscopists prepare three written reports. They then defend one of these reports as part of a final examination. On average, we train 14 dactyloscopists a year.   

Of course, we also work in an advisory capacity. My colleagues and I are often consulted at the request of the state offices, which enables me to keep in regular contact with my former course participants all over Germany."

What exactly do dactyloscopists examine?

“A dactyloscopic clue is formed when hydrolipid film, a mixture of water and fat on top of the skin, is transferred to a surface. When an object is touched with the hands or feet, this hydrolipid film that accumulates on the friction ridges of the skin is transferred to the object. Friction ridges are the raised lines on the palm of the hand and the sole of the foot, also called hairless skin. We usually work with the trace material found at crime scenes and the reference material supplied by the digital fingerprint collection or analog fingerprint card. Our objective is to identify people in general, to prove or rule out the possibility that a set of fingerprints belongs to a particular person, and recognize connections between crimes. The first step is to visualize the fingerprints, i.e., to create an optical contrast to the fingerprint substrate. We use physical or chemical techniques, depending on the surface properties.

We examine the friction ridges, which remain unchanged for life. In Germany we basically divide them into three different patterns in the fingertips: whorls, loops, and arches. These are group-specific, i.e., we can only use them for classification and preselection. But within these patterns there are more individual differences, the anatomic features. These are missing lines in the general ridge pattern or extra lines – for instance a bifurcation. Anatomic features are individual characteristics that vary from person to person and even from finger to finger. Even identical twins are born with different fingerprints, because random processes already influence the formation of the friction ridges in the womb. This makes dactyloscopy different from DNA analysis, for example.”  

What’s the procedure for analyzing trace material?

"We work from the general to the specific. For the first phase, in which we identify the pattern and look at the anatomic features, we only need a magnifier with up to 7x magnification. If we want to examine the fingerprint in more detail, we use a stereomicroscope, which provides up to 50x magnification. This enables us to detect tiny pores and their relative positions, which are again quite unique.  The stereomicroscope also reveals the contours of the friction ridges and intermediate lines more clearly. However, not everybody has these incomplete friction ridges.  

This is followed by the comparison phase in which we look for matches with the reference material that we usually get from a database. The Automated Fingerprint Identification System (AFIS) currently contains over 3 million fingerprint cards. It makes work easier, but it doesn’t do the work for us. If we enter a fingerprint into our system, the system offers us a hit list. We then work through this list manually, i.e., with our eyes.  

The third phase is evaluation. Patterns are frequent, so they are not as "valuable" as anatomic features. Also, some anatomic features are more common than others.

The next step is verification according to the four-eyes principle. A second colleague checks everything the expert has done in the first three steps. In dactyloscopy, we don’t normally talk in terms of probabilities, but of “yes or no.” At the moment, we have a twelve-point system in Germany. If there are twelve corresponding anatomic features, we assume that the prints are identical.  Dactyloscopy therefore has high evidential value in court."

Why do you use an interactive microscope system?

"In the past, it was extremely difficult to discuss a microscope image with course participants. Everyone who looked through the microscope had to adjust the eyepieces for his or her visual field first. This used to take a lot of time for a group of up to nine participants.  

With an interactive microscope system, each participant has a small screen on the desk and can individually adjust the definition and image section. Course participants can consult with the people sitting next to them whenever they like. We can also use a 55 inch monitor to show individual screen images to the whole class and discuss them as a group. These are important didactic advantages for the lecturer."

What are your most memorable cases?

"I’ll never forget the day I started work after qualifying as an inspector in 1977. I had just had a quick opportunity to introduce myself at my new workplace before my colleagues whisked me off to a crime scene in Oberursel. It was the villa of the murdered banker Jürgen Ponto, who had been shot by Red Army Faction (RAF) terrorists. In the fall of the same year I was also in the squad investigating the scene of the kidnapping and murder of the President of the German Employer Association, Hanns Martin Schleyer. This crime was also committed by RAF terrorists. These cases were not only spectacular, but also extremely challenging from a technical point of view. Many investigation methods were still at the development stage in those days. It was an extremely interesting time in forensics and a time that has stuck in my mind.   

However, I also remember other cases that were maybe not as spectacular but especially interesting for other reasons – cases, for example, that involved the analysis of bloody fingerprints and the question: what was there first, the fingerprints or the blood? Let’s assume somebody is on a visit, goes to the restroom, and leaves fingerprints. Two days later, a murder is committed in this house, and these fingerprints are covered in blood. If this person is one of the suspects, it’s vital to find out whether the fingerprints were there before the blood or vice versa – a tricky undertaking."

What is the appeal of your work?

"I’ve worked with fingerprints for just over 35 years now, from securing evidence via analysis to teaching, and I still really enjoy my job. No two cases are the same, because no two fingerprints are the same – the material is always new. Another very interesting aspect is that I keep meeting new colleagues and constantly changing audiences.  Because of my experience, I have gradually come to know all the dactyloscopy experts in Germany. We are a closely-knit network – rather than regarding each other as competitors, we support and advise each other.

I also find my work on international dactyloscopy panels exciting. I’m involved with Interpol and the European Network of Forensic Sciences (ENFSI). The objective of our study group is to develop dactyloscopy standards for the whole of Europe. But we have not reached our goal yet, we still have a way to go." 

How do you think your field of work will change in future?

"More and more of our work will be automated in the future, and technology will be more and more convenient. However, the final decision will always be made by a human being.  A lot has changed in the past few years. For nearly 100 years, police officials stained each finger of the individual with printer’s ink and rolled it on a sheet of paper – three times in all. One version stayed at the police station, one went to the State Office of Criminal Investigation, and one to the Federal Office of Criminal Investigation. Today, many large police stations have live scan systems. The suspect puts his finger on the glass plate and the system scans his fingerprints. The scanned image is transmitted to the computer, which automatically classifies the fingerprint except for the final stage of manual checking.  It sometimes used to take over an hour just to classify one ten-fingerprint card, whereas nowadays it can be done in a matter of minutes. However, there is less scope than before for dactyloscopic empiric values and dactyloscopic vision. This will be the challenge for dactyloscopic training in the next few years."