Many temperature sensors such as thermal-imaging cameras measure thermal radiation emanating from the surface of an object, of which most falls within the infrared spectrum. Hotter objects emit higher-frequency radiation, which can edge into the visible light spectrum and cause an object to glow visibly.

The technique developed by the University of Wisconsin-Madison engineers makes it possible to look beneath the surface of some materials via “depth thermography”. It works with a class of materials which are partially transparent to infrared radiation.

“We can measure the spectrum of thermal radiation emitted from the object and use a sophisticated algorithm to infer the temperature not just on the surface, but also underneath the surface, tens to hundreds of microns in,” said Professor Mikhail Kats, an electrical and computer engineer at the university. “We’re able to do that precisely and accurately, at least in some instances.”

Kats and his colleagues heated a piece of glass and analysed it using a spectrometer. They then calculated the temperature at various depths of the glass using an algorithm, which was based on many measurements of thermal radiation emitted from objects of various materials. The algorithm determines the temperature gradient which best fits their observations.

Kats said that this experiment is a proof of concept for depth thermography and he hopes to apply the same technique in future to more complex multilayer materials. He also intends to apply machine learning techniques to improve the process.

Eventually, he hopes that depth thermography could be used to measure semiconductor devices to gain insights into their temperature distributions while in operation. The technique could be used in various applications which do not permit the use of conventional temperature probes, such as in extremely hot gases and liquids, and in applications for which measurements must be taken remotely.

“We anticipate relevance to molten-salt nuclear reactors, where you want to know what’s going on in terms of temperature of the salt throughout the volume,” he continued. “You want to do it without sticking in temperature probes that may not survive at 700°C for very long.”