Novel fingerprint collection technique could help forensic investigators preserve evidence

KelvinProbe
(Kelvin probe in action. Source: KP Technology)

Forensic technology for fingerprint biometrics used by law enforcement still looks quite similar to what was used generations ago, before computing and digital imaging technology overhauled most of the field of biometrics. Now, a new fingerprint image capturing technique that relies on Kelvin probe technology and preserves the evidence for other investigative techniques has been developed.

The field was dramatically changed by the advent of automated fingerprint identification systems (AFIS), but has remained largely unchanged beyond that, particularly compared with the rapid advances in other biometric modalities.

Susanna Challinger began working as a research assistant at KP Technology just as a forensics project began which had been initiated when Police Scotland contacted the company for its technical expertise, she tells Biometric Update in an interview. The Royal Commission for the Exhibition of 1851 provided funding for the research, and Challinger set out to develop a new technique for fingerprint imaging. Her research into biometric fingerprint recovery from metal surfaces like gun cartridges shows that Kelvin probe technology can be used to non-destructively process fingerprints for identification.

A Kelvin probe is a tool which Challinger compares in form factor to a needle, which measures the electronic energy level of materials through ‘atomic force microscopy.’ It is used primarily in the semiconductor and electronics industries, for research on materials from solar cells to phone screens.

“Most fingerprint techniques focus on looking at the components of a fingerprint’s deposits to identify where the fingerprint is from that side, but the Kelvin probe looks at the problem the other way around,” Challinger explains.

The changes in energy level left on conductive surfaces from fingerprints, which Challinger describes by comparison to the contour map of a hill, persists for more than three years in most cases, as it turns out.

This means that forensic fingerprint gathering can be carried out on any crime scene’s conductive surfaces, even three years later, with a non-contact and non-destructive technique that then permits other types of analysis on the same material or piece of evidence. The powder that is typically used can contaminate evidence, eliminating the opportunity to gather hidden prints or additional information from it.

“You’ve got a limited amount of evidence, you have to make a decision what you’re going to analyze and how you’re going to analyze and what you’re going to do first,” Challinger says. “The potential of the Kelvin probe is that it wouldn’t limit or restrict any subsequent DNA or fingerprint techniques after you’ve done that initial process.”

The technique developed by Challinger is slow, she says, but police feedback is that it fits within their processes.

Challinger worked with forensics investigators from the University of Dundee, who provided fingerprint samples. KP imaging was tested against visible prints on an iron surface initially, and then with latent prints and on brass, with up to ten donors. Further testing was conducted with on Nickel and on gold from one donor, and in all cases the Kelvin probe successfully captured fingerprint data to “first and second-level identification detail.”

The completed images can have background effects from metal alloys like brass, but the identification experts consulted said it did not affect forensic matching at all.

“Certainly, the images it can produce, the identification experts were delighted when I showed them some examples,” Challinger reports. “They said they’d love to have images of that quality to work on.”

As a sophisticated piece of equipment, a Kelvin probe is fairly expensive, however unlike traditional fingerprinting techniques, it does not require consumables and the operational cost that goes along with them, so over time police departments and law enforcement should find the cost of the technology similar to what they already pay for fingerprinting. It is also not overly difficult to use, according to Challinger.

The Kelvin probe technique’s high sensitivity means that the operator must be careful of the surface being tested, as for example breathing on it could affect the image. Taking care not to disturb a crime scene is however a normal consideration in forensics, Challinger notes.

Challinger’s research also included attempts to increase image clarity with vacuum metal deposition, but it turned out that unprocessed prints worked came through just as clearly.

Ultimately, the operator must carefully set the distance to ensure the image being collected is clear, but after that can press start and walk away.

The next step in the development of this novel technique would be an operational trial to see if the results in the lab can be duplicated in the field. If successful, this non-contact, non-destructive technique could potentially be a useful initial approach to gathering data on serious crime from metal objects and surfaces, while preserving the forensic integrity of those objects or surfaces for further investigation.

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