Hotter objects tend to glow more brightly than cooler ones. Turn up the heat on an electric stove, for example, and the black coils turn bright orange. But a new coating bucks the rule that hotter equals brighter. Its brightness doesn’t always change as it warms.
At least, that’s true for a certain type of light called infrared (IN-frah-red). It is invisible to our eyes, although special cameras can see it. The new coating might one day prove handy for fooling those cameras. Infrared cameras essentially picture something’s temperature. These cameras measure how much infrared light objects give off. But over a particular temperature range, the coating doesn’t emit more infrared light when it gets hotter. In that sense, it acts as a temperature camouflage. It hides thermal information from the cameras, explains Mikhail Kats. He is an applied physicist who works at the University of Wisconsin–Madison.
In experiments, his team heated a piece of sapphire. It was coated with a material called samarium nickel oxide. The researchers imaged the process with an infrared camera. It images showed little change as the coated sapphire warmed from around 105° to 135° Celsius (221° to 275° Fahrenheit).
As temperatures rise, the material would normally get brighter. But at a certain point, it switches from an insulator to a metal. At that point, the material’s brightness stays the same even as it continues heating up.
Scientists knew of other substances that could confuse infrared cameras. These materials appear cooler as their temperatures climb. But the new material achieves a perfect balance. It can warm up without a change showing up in infrared images.
Kats and his colleagues reported their findings in the December 26, 2019 Proceedings of the National Academy of Sciences.
The coating could act as a privacy shield, Kats says. It could hide people or objects from heat scanners, which are becoming cheaper and more common. Heat-sensing cameras can even be used to scan people for medical conditions without their knowledge. Those cameras also can detect sources of heat (such as people) behind thin walls.
Physicist Karl Joulain works at the University of Poitiers in France. He finds the new material interesting. But current uses for it are limited, he adds. The camouflage effect applies only to certain wavelengths of infrared light. Detectors that look at other wavelengths could still spot something heating up.
For now, the coating isn’t useful for hiding people. The temperatures at which the camouflage effect occurs is too high. But Kats thinks he can shift the temperature range. Mixing other substances with samarium nickel oxide may allow the effect to work at lower temperatures.