Fingerprint Security: Facts About Fingerprints

A general overview of how fingerprints are formed, how people leave them behind, and what technologies exist to detect and examine them.

By Patrick Lawless

Fingerprints have fascinated people for centuries. They have been used as a method of personal identification (to sign contracts for instance) since ancient times. But where do fingerprints come from? Is it true that they are all different?

Fingerprints are formed before birth, during the development of the hands. Fingerprints aren't actually formed in the skin, but are caused by ridges in the flesh underneath the skin. Their development is partially random, and can be affected by health issues, sometimes distorting them or eliminating them altogether. Genetics plays some part in their formation, but even identical twins (who have identical DNA) have different fingerprints. Fingerprints fall into a set number of patterns, which allows us to catalogue them and perform fingerprint searches more easily. The most common fingerprint pattern is the "Loop", in which the fingerprint ridges start at one side of the finger, loop around at the tip of the finger, and come back to the same side they started on. If you look at your own hands, the chances are you have at least one fingerprint of that type. Over 60% of all fingerprints are loops. Some other common pattern types are Whorls, Arches, and Accidentals. There are many sub-categories as well. And of course, the same ridges cover your palms and the soles of your feet. Both of these have been used to identify people in the past.

Well then, if over 60% of fingerprints have the same pattern, how can they all be different? That is because fingerprints are examined using two different sets of criteria. One way of looking at fingerprints is using their "Class Characteristics". These are general features about the fingerprints, which apply to all fingerprints of that type ("Loop" is a class characteristic). For instance, to say that a car is a "Four-Door Sedan" is to talk about a Class Characteristic of that car. No one would suggest that all four-door sedans are exactly alike. If however, you were to say that the car had a New York State license plate number of X3D4R5, then that is what's called an "Individual Characteristic". No two cars have a New York plate number of X3D 4R5. So fingerprints are identified to a particular person using their individual characteristics, not their class characteristics. If you look very closely at a fingerprint, you will notice that it is made up of ridges on your skin. These ridges aren't always continuous; they stop, split into two, form little pockets (called "lakes") and even appear to cross each other at times. It's these individual features that are different between one fingerprint and the next. By examining and counting these individual features, a fingerprint expert can come to some opinion about whether a particular person made a particular fingerprint. If he or she finds characteristics that don't match, and can't be explained due to pressure distortion or other common problems, then they are obliged to state that the fingerprint did not come from that person. If they find a sufficient number of similarities however, and there are no unexplainable differences, then they may give the investigators and the court their expert opinion that the fingerprints were made by the same person. How many similarities do they need to find? That varies by jurisdiction, from as few as 8 to as many as 16. Ultimately, the fingerprint examiner must state his opinion, and the court must decide how much weight to put on it.

So now that we know what fingerprints are, how do people leave them behind at a crime scene? Since your fingers are covered in skin, they are also covered in skin pores. Skin pores produce oils and sweat, which are distributed on your fingers. When you touch something, those liquids are left on the surface, in the shape of your fingerprints (just like a rubber stamp with ink on it). It's those compounds that can be detected by forensic technicians, using a number of methods. The oily compounds left behind are often detected by good old fingerprint powder, just as you see on any TV detective movie. Like everything however, science has advanced fingerprint powders quite a lot since the early days. Modern fingerprint powders contain various compounds that did not even exist when fingerprint powders were invented (making them far more sensitive), and are usually applied using special soft fiberglass brushes, which do less damage to the fingerprints. When a fingerprint has not yet been detected, it's known as a "latent print" (latent for short). Modern science has found many other ways to detect latents, however. When you touch an object, the chemicals in your sweat may be absorbed into that object (paper is a good example of this), and there are now chemicals which can develop these latents quite nicely. Even paper, which has been soaking wet, can now be successfully examined for fingerprints, using advanced chemicals. The Laser has been a great boon to law enforcement officials as well, due to its ability to detect certain fingerprints. Some types of Lasers cause chemicals in some fingerprints to "fluoresce" or glow, which allows them to be photographed as evidence. There are also dyes that can be sprayed on pieces of evidence to help fingerprints to be more visible to the Laser. The Laser has been extremely useful in obtaining fingerprints from human bodies, a feat that was nearly impossible only a few years ago. Many surfaces, which were difficult to obtain useable fingerprint from in the past, are now routinely examined successfully due to all of these scientific advances.

One of the most common uses of fingerprints is of course, the identification of criminals. When a person is charged with a serious offence, they are fingerprinted using either the traditional ink pad and paper form, or in some cases, with a modern electronic scanner. Their prints are then sent to a fingerprint-cataloguing computer, where they are scanned in and all of the individual characteristics are mapped out for the computer's database. When a fingerprint is found at a crime scene, the forensic technician sends a copy to the computer agency, where it too, is entered. The computer supplies a list of possible matches, and an expert fingerprint technician examines the two prints by eye, to determine if they are from the same person. These computers have revolutionized the searching of fingerprints all around the world. All in all, the world of fingerprints is an exciting one, and there are always new advances furthering the methods of detecting them.

© High Speed Ventures 2011