Stealth Sheet
A newly developed stealth sheet can hide hot objects like human bodies or military vehicles from infrared camerasHongrui Jiang via University of Wisconsin-Madison

Ever since Sridevi-Anil Kapoor starrer 'Mr India' science-fiction thriller in 1987 went on to become a big hit in Bollywood, many people are awaiting an invisibility cloak the way the hero gets to use it. Not exactly a cloak or a wrist watch, but this new stealth technology claims to be able to make humans completely invisible to Infrared (IR) cameras by absorbing light.

Security cameras, surveillance systems as well as drones make use of infrared-based night vision and this new "stealth sheet" could be one way to effectively hide from them, especially for the army to plan surgical strikes across the borders.

Infrared cameras can not only see at night, they can even see through heavy fog at night and this makes them invaluable in a war zone or in regions that require heavy security like a military outpost. The stealth sheet that researchers from the University of Wisconsin-Madison have created masks light leaving the body in the IR spectrum. Any hot object like a vehicle moving in the dark, will light up in an IR camera and developers of this sheet claim to make them practically invisible, says a report by the university.

The sheet itself is only about one mm thick—about the thickness of 10 sheets of white paper—so it is easy to use and comparatively lightweight to existing masking tech. It works by absorbing infrared light instead of trying to mask the heat itself. That makes the new sheet a lot more capable of hiding the IR light signatures from cameras, says the report. Developers of this tech claim that it can absorb up to 94 percent of the IR light that it encounters. Because it traps all that light behind itself, cameras looking for IR cannot see any traces of the source of heat.

"What we have shown is an ultrathin stealth 'sheet.' Right now, what people have is much heavier metal armor or thermal blankets," says Hongrui Jiang, of the University of Wisconsin-Madison. Heat from a body or a vehicle emits radiation and infrared light that is invisible to the human eye, but is visible to the right kind of camera. This sheet can absorb this light much better than existing technologies, says the report.

This stealth sheet, says the report strongly absorbs light in the mid- and long-wavelength IR range and it is within this range that the human body typically glows with infrared light.

Also, the stealth sheet can be incorporated with heating elements that can project a complete image on its outside so that it can effectively fool any camera pointed directly at it. "You can intentionally deceive an infrared detector by presenting a false heat signature," says Jiang. "It could conceal a tank by presenting what looks like a simple highway guardrail."

How the 'stealth sheet' works

The material that was used to trap IR is something called Black Silicon, says the report and it is something that is used in solar cells to trap light. Black Silicon has many millions of microscopic needle like projections on its surface called nanowires that cn trap light efficiently. They are all packed in tight and point upwards. When light reaches it, it simply bounces around and gets trapped within itself between all those vertical needles. This works well with visible light.

To trap IR, which is not visble to the human eye, the team tweaked the way it works. "We didn't completely reinvent the whole process, but we did extend the process to much taller nanowires," says Jiang.

The study was first published in the research journal Advanced Engineering Materials.