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Fingerprint biometrics get a 3D upgrade from digital holographic imaging

University of Dayton’s Partha Banerjee has developed a way to capture fine detail
Fingerprint biometrics get a 3D upgrade from digital holographic imaging

At a microscopic level, a fingerprint is like a mountain range; so is its forensic biometric mark, which is made of oil accumulated in ridges that have depth and fine detail beyond what a standard 2D fingermark reveals – even if those ridges are only microns high.

Professor Partha Banerjee, director of the Holography and Metamaterials Laboratory at the University of Dayton, believes that with only two dimensions to a fingerprint, too much is missing from the complete biometric picture. In an article for The Conversation, he describes a process for using digital holography to map and visualize fingermarks in 360-degree 3D.

“Biometric identifiers record fingermarks only as 2D pictures,” writes the professor. “A 2D fingerprint neglects the depth of the fingermark, including pores and scars buried in the ridges of fingers that are difficult to see.”

Banerjee defines the three types of fingermarks based on visibility: patent marks are the most visible (like bloody prints at a crime scene), plastic are embedded in soft surfaces like Play-Doh or silicon, and latent are the barely visible kind that detectives have to dust for. Similarly, there are three levels of geometric detail in a fingermark. Level 1 covers visible ridge patterns such as loops, whorls and arches. Level 2 applies to minute details such as bifurcations, endings, eyes and hooks. Features at Level 3, which are not visible to the naked human eye, include pores, scars and creases.

Digital holography works on a scale that can accommodate Level 3 details, and is therefore able to show all of the 3D topological characteristics of a fingermark. In collaboration with Akhlesh Lakhtakia, a professor at Penn State, Banerjee’s lab developed a technique for preserving fingermarks using a layer of nanoscale columnar thin film (CTF). To achieve this, fingermarks on glass, wood and plastic were aged in different environments, at various temperatures and humidity levels, then harvested with a coating of CTF. Described as “pillars of glassy material that uniformly cover the fingermark, like a dense growth of identical trees in a forest,” CTF conforms to the topology of the fingermark in the same way a Pin Art toy captures the impression of a hand.

The next step is to create a hologram from the preserved mark. This is achieved by splitting green and blue light wavelengths from a laser, so that light reflected from the fingermark is also superimposed onto a reference wave directed into a digital camera. The resulting interference pattern creates what is called a hologram: a 2D image recorded on the digital camera.

“Researchers then import the hologram to a computer, where they can use the physical laws of wave propagation to figure out where the light waves from the laser bounced off different parts of the object,” explains Banerjee. They can then reconstruct the fingerprint as a 3D image that can be viewed from any perspective on a digital screen.

The Miami Valley Regional Crime Lab in Dayton, Ohio, has developed grading systems for the collection process performed by Lakhtakia and his team at Penn State, and is working toward a similar grading system for Banerjee’s digital holographic images, to measure features such as clarity.

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