Your Fingerprint's Real 3D Form: Research on Holograms to Enhance Forensic Fingerprint Analysis

 When you unlock your smartphone with your fingerprint, your phone checks a flat pattern to make sure it's the right fingerprint before letting you in. However, the mark your finger leaves on the button is more than just a flat print – it's a 3D structure called a fingermark.

Fingermarks are made of tiny ridges of oil from your skin, each ridge only a few microns tall, which is really thin, like a tiny part of a human hair.

Biometric identifiers, like fingerprint scans, only capture fingermarks as 2D images. This means they miss out on a lot of details. A 2D fingerprint doesn't show the depth, like pores and scars in the ridges, which can be hard to see.

I'm an educator and scientist studying holography – a way of showing 3D information. My team has figured out how to map and visualize fingermarks in 3D on a computer using digital holography.

There are three types of fingermarks: patent, plastic, and latent. Patent ones are visible, like bloody fingerprints at a crime scene. Plastic ones are on soft stuff like clay or chocolate. Latent ones are the least visible and need special methods, like using powder or gas from super glue, to be seen.

Fingermarks have three levels of details. Level 1 is the visible ridge patterns – loops, whorls, and arches. Level 2 is smaller details like bifurcations and endings. Level 3 is tiny things like pores and scars that you can't see with your eyes.

Since fingermarks are usually 2D pictures, my team worked with another group to develop a technique that shows all the 3D details. They used a thin film layer on top of the fingermark and created holograms with lasers to capture the 3D structure.

This hologram-making process involves splitting laser light into two parts. One part shines on a camera directly, while the other shines on the fingermark. When the reflected light from the fingermark combines with the direct light, it creates a hologram, which is like a 2D picture capturing the 3D details.

We've already made 3D pictures of latent fingermarks using this technique. By using different light wavelengths, we can even see tiny details like pores in the 3D reconstructions.

Our collaboration with a crime lab in Dayton, Ohio, involves grading the quality of fingermarks and developing a new method for grading 3D reconstructions. We're working to enhance the analysis of fingermarks for crime investigations, using modern technology to build on the long history of using fingerprints as unique identifiers since ancient times.

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