Crime aid: Criminals’ fingerprints may soon be rendered in three dimensions by a scanner. Picture: FRETTIE
Crime aid: Criminals’ fingerprints may soon be rendered in three dimensions by a scanner. Picture: FRETTIE

At the continent’s largest research council, scientists have turned an office into a crime scene. Rethabile Khutlang, a Council for Scientific and Industrial Research (CSIR) biometric research engineer, is quick to reassure that it is not an actual crime scene.

They are performing a trial on their new technology — a three-dimensional fingerprint scanner that Khutlang hopes will find its way into the South African Police Service (SAPS) to help in identifying criminals.

"A crime scene is not a sterile lab … so we placed fingerprints all over the office on different substrates, and the idea was that these fingerprints were left by the burglar," he explains.

The police use fingerprints, DNA and dental imprints to identify people, but criminals are more likely to leave a fingerprint than a bite mark.

The first fingerprint was used to identify — and convict — a murderer in 1902. But even though the technology has been available for more than a century, it still has drawbacks.

Every person has a unique fingerprint. It is a pattern of friction ridges that is difficult to change and alters little with age. Glands in the skin secrete oil and perspiration, and this leaves an impression on surfaces in what are called "chance impressions" or "latent fingerprints". But distortion is a problem when trying to determine if a fingerprint belongs to someone.

There is smudging when a person touches a surface, and deformations occur when something that is three-dimensional (such as a fingerprint’s ridges and whorls) is transformed into two dimensions (such as a print left on a marble or glass surface).

The CSIR’s technology determines the "internal" fingerprint, using light to make a 3D reconstruction that captures details on deeper layers of skin. Known as optical coherence tomography, or optical ultrasound, the technique is used by researchers to create sub-surface images.

"The light source of the optical coherence tomography does not destroy DNA when extracting the latent fingerprints," says Ann Singh, a laser scientist at the council involved in the project.

"The differentiating factor of our technology is that the CSIR team has enabled higher-resolution fingerprint acquisition…. The result is a significantly more accurate and tamper-proof fingerprint representation."

There is more to a fingerprint than the ridges visible to the naked eye; there are more layers to it under the surface.

In a paper published in the scientific journal Biomedical Optics Express in 2016, French scientists suggested the optical coherence tomography technique would be useful for determining fingerprints.

The internal layer of a fingerprint, which this technology can read because it can see below the immediate surface of a fingertip, is a better identifier and less susceptible to distortion than an external fingerprint

"This internal layer, which we call here the internal fingerprint, serves as a ‘master template’ from which the external fingerprint regrows and is believed to be less affected by the damages sustained on the surface," the authors write.

"Imaging the internal fingerprint can be a more accurate way of fingerprint imaging."

The internal layer of a fingerprint, which this technology can read because it can see below the immediate surface of a fingertip, is a better identifier and less susceptible to distortion than an external fingerprint.

The aim is that the police will be able to use this technology to scan areas and lift fingerprints off surfaces without dusting and touching, which damages the integrity of the fingerprints.

But while catching criminals is a major focus of this technology, biometrics are also important for access control — and fingerprinting for access control suffers from many of the same problems as fingerprint lifting for forensics.

"The CSIR optical coherence tomography system is capable of acquiring both external and internal fingerprints," says Luke Darlow, CSIR biometrics and image-processing researcher.

"Combining these allows for a hybrid fingerprint that results in better biometric performance," says Darlow. "The acquisition of such detailed fingerprints means that fake prints can be easily detected."

The French scientists, based at the Langevin Institute in Paris, note that there are "not many different types of sensors on the market that are able to gather information from the inside of a finger". Their interest is more in using the technology to study skin conditions, but in SA, it could offer a way to fight crime and improve the country’s biometric data.

A major benefit of the technology is that fingers do not need to touch a surface, unlike the grimy glass of biometric scanners in home affairs offices and consulates.

"This solves the problem of elastic distortion in conventional ‘slap’-scan fingerprint acquisition devices, which is caused by the pressure of a finger on a surface," says Darlow.

"Since it is contactless, it is also hygienic, and residual fingerprints are not left on a scanner surface. In addition, unlike current technology, it is not influenced by moisture conditions or skin damage," he says.

However, a problem with the technology developed by the CSIR is that the scanner is large — about the height of a full arm — and heavy.

"The next step is to reduce the size of the device to a hand-held system and prepare for commercialisation," according to Delon Mudaly, the centre manager of the CSIR’s National Laser Centre.

The SAPS had not replied to requests for comment at the time of publication.

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