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September 17, 2021

Lasers used to detect handprint.

It sure sounds like “laser speckle imaging” is the sort of thing you’d need grant money to experiment with, but as [anfractuosity] recently demonstrated, you can get some very impressive results with a relatively simple hardware setup and some common open source software packages. In fact, you might already have all the components required to pull this off in your own workshop right now and just not know it.

Anyone who’s ever played with a laser pointer is familiar with the sparkle effect observed when the beam shines on certain objects. That’s laser speckle, and it’s created by the beam reflecting off of microscopic variations in the surface texture and producing optical interference. While this phenomenon largely prevents laser beams from being effective direct lighting sources, it can be used as a way to measure extremely minute perturbations in what would appear to be an otherwise flat surface.

In this demonstration, [anfractuosity] has combined a simple red laser pointer with a microscope’s 25X objective lens to produce a wider and less intense beam. When this diffused beam is cast onto a wall, the speckle pattern generated by the surface texture can plainly be seen. What’s not obvious to the naked eye is that touching the wall with your hand actually produces a change in the speckle pattern. But if you take high-resolution before and after shots, the images can be run through OpenCV to highlight the differences and reveal a ghostly hand-print.

[anfractuosity] then uses the same technique on a calculator before and after some buttons have been pressed on it. Not only does the final cleaned up image clearly show the numbers on the display, but it highlights the individual buttons which were touched. Seeing this example, it’s not much of a stretch to think there could be some nefarious application for this technique. Could an attacker use laser speckle imaging to determine which buttons have been pressed on a lock keypad or alarm panel?


Luckily, it sounds like putting an attack like that into practice would be quite difficult. For one thing, the camera and laser need to be in exactly the same position when the before and after shots are taken, which would be all but impossible for a clandestine operation. Secondly, as evidenced in the video below, the imprints tend to decay fairly rapidly. The after shot has to be taken within a few minutes of the keypad being touched, making it even more difficult to pull off in the wild.

It’s not immediately obvious what practical applications this technique may have among the hacker and maker crowd, so we’d love to hear about any you might come up with. In any event, it’s an impressive accomplishment and an excellent example of what’s possible for the modern hobbyist. in digital cameras hacksLaser HacksTagged interference patternslaserlaser speckleopencv

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