Hotter objects typically glow brighter
than cooler ones, making them stand out in infrared images. But a newly designed
coating bucks the rule that hotter equals brighter. For certain wavelengths of infrared
light, the material’s brightness doesn’t change as it warms, researchers report December 17 in Proceedings of the National Academy of
Sciences
.

Made of samarium nickel oxide, the thin
coating “hides temperature information of surfaces from infrared cameras,” and
could therefore be used as a privacy shield, says applied physicist Mikhail
Kats of the University of Wisconsin–Madison.

A basic rule of physics known as the Stefan-Boltzmann
law states that the brightness of the thermal radiation emitted by an object grows
rapidly with increasing temperature. Turn up the heat on an electric stove, for
example, and the coils get brighter. The same trend goes for invisible
wavelengths of light, such as infrared. Infrared cameras measure how much
thermal radiation objects emit in infrared wavelengths to estimate their
temperatures. So if the normal link between temperature and radiation is
broken, the camera can be fooled.

Samarium nickel oxide doesn’t flout the
Stefan-Boltzmann law. Instead, the material’s increasing brightness at higher
temperatures is counteracted by a decrease in its emissivity — the tendency to
emit thermal radiation. That decrease occurs due to the material switching from
an insulator to a metal. The two effects balance out so that, for certain infrared
wavelengths, the material’s brightness remains constant as its temperature
changes, Kats and his colleagues found.

infrared image of researchers
Hotter objects like people typically appear brighter in infrared images. A team of researchers (some members shown) has developed a coating that may work as camouflage from infrared cameras, although it doesn’t yet operate at temperatures relevant for hiding humans.Courtesy of the Kats group

Earlier research has pinpointed substances
that could confuse infrared cameras by appearing cooler as their temperatures rise (SN: 10/25/13). But the new material achieves
a sweet spot where the material’s temperature can rise or fall with little sign
of the temperature difference showing up in infrared images. In experiments,
the researchers heated a sapphire sample coated with the material, and its
temperature in infrared images appeared mostly unchanged from around 105° to
135° Celsius.

“The phenomenon is quite interesting”
says physicist Karl Joulain of the University of Poitiers in France. But current
applications of the material are “quite limited,” he says. The effect applies
only to certain infrared wavelengths. Detectors that look at other wavelengths
could still spot an object’s change in temperature.

Still, with infrared devices becoming cheaper and more common, “that comes with quite a bit of privacy implications,” Kats says. The cameras can be used to search unwitting subjects for medical conditions, for instance, or to detect sources of heat behind thin walls.

For now, the high temperatures at which
the camouflage effect occurs means it wouldn’t be useful for concealing people.
However, Kats thinks the temperature range can be changed by working with
alloys of samarium nickel oxide, which may have different properties.