Disclaimer: This post originally appeared on a tech blog. It is reused with permission.
In the world of technology, biometric identification has been slowly creeping from the world of science fiction to fact. On the most far-fetched front, we have iris scanners, predicted by movies such as Minority Report. On the less fantastic front, voice recognition systems have become a more common technology, though we’re still working out the kinks. Facial recognition is also getting good enough to prompt Jamie Zawinski into being fascinated by “dazzle” methods to defeat facial scanning. But the most reliable form of biometric identification has actually been the fingerprint reader.
History of fingerprints
Fingerprints (technically “friction ridge skin impressions”) themselves have been in use for identification purposes for longer than most people realize. The custom extends to ninth century China, where merchants applied their fingerprints to documents as a means of signature. The fourteenth century Persians also used fingerprints as a last means of identification.
Surprisingly, law enforcement was late to the game in using fingerprints to confirm a person’s identity. As late as 1880, one Dr Henry Faulds published a paper on fingerprint classification in the science journal “Nature” and offered the use of his technique to London’s Metropolitan Police, but was turned down.
This conflicts with the popular idea of the fictional Sherlock Holmes, which first saw publication in 1887. Many people picture the famous detective with a magnifying glass searching for fingerprints. In fact, Holmes was depicted gathering clues from hoofprints and footprints, as well as other marks such as tire tracks from a bicycle, but rarely fingerprints. The first story in the Holmes canon using fingerprints as a clue was “The Adventure of the Norwood Builder”, published 1903.
Fingerprinting and law enforcement
The first use of fingerprints in a police investigation was in 1892 in Argentina, when a bloody fingerprint was used to convict a murderer in a trial. The event was inspired by the investigating officer read the work of Sir Francis Galton’s pioneering research on the subject. This broke the ground for the world’s first use of fingerprints in law enforcement, and eventually in the security sector. In 1897, the world’s first fingerprint bureau opened in Calcutta, India, and in 1901 Scotland Yard of the United Kingdom opened a fingerprint bureau. The United States adopted fingerprints to identify criminals in 1906, starting in New York City.
Before fingerprints, identification of suspects was made through other physical means, in a clumsy system known as anthropometry. This system took measurements of the length and width of various parts of the body, from fingers to feet to the head, and once the record was stored, a suspects description could be matched with a handful of sorted descriptions – provided the person had been measured from early adulthood. Anthropometry is now a very lost science. Fingerprint identification has proven to be a far more convenient and reliable replacement.
Geeky facts about fingerprints
Fingerprints come in a series of four distinct general patterns, these being “arch”, “loop”, “whorl”, and “tented arch”. The loop pattern can also face either left or right, giving five distinct patterns. These patterns, when distributed across all ten fingers, already serve to narrow down identification before the details of an individual print are examined.
When a single print is compared to a known record, you need a highly accurate capture of both prints. Sloppy smeared prints introduce doubt. After that, a sampling frequency of 20 points per millimeter is high enough to visualize a fingerprint in sufficient detail for identification purposes. The fingerprints are compared by examining these points, and only a certain number of points matching is necessary to produce a confident match.
Here’s a bonus bit of knowledge for you: Hollywood films typically show someone wiping a metal surface, such as a gun, with a tissue or cloth to clean off their fingerprints. In fact, this doesn’t work! The interaction of the body’s natural oil, which leaves the fingerprints, and inorganic salts, found in untreated metal surfaces such as gun handles and bullet casings, interact to form a bit of corrosion which is capable of preserving the fingerprint. This isn’t visible to the naked eye, but it leaves a trace pattern in metal–ion complexes that can be recovered in a forensic lab.
By the way, the same action explains why handling of coins causes them to wear down fast. That’s why coin collectors prize an uncirculated specimen wrapped in protective gear. Human fingerprints corrode metal, we’re like acidic monsters wandering around leaving our toxic prints on everything!
However, crime detection is now embracing DNA matching more that fingerprint matching, while fingerprint identification moves into the private sector. Sticking your thumb on a reader to unlock your office is one thing, but nobody’s prepared to give a blood sample and wait two days for routine purposes.
