Engineers have remotely determined the temperature beneath the surface of certain materials using a new technique called depth thermography. The method may be useful in applications where traditional temperature probes won't work, like monitoring semiconductor performance or next-generation nuclear reactors.
Many temperature sensors measure thermal radiation, most of which is in the infrared spectrum, coming off the surface of an object. The hotter the object, the more radiation it emits, which is the basis for devices like thermal imaging cameras. Depth thermography, however, goes beyond the surface and works with a certain class of materials that are partially transparent to infrared radiation.
The researchers are able to 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 within.
For the project, the team heated a piece of fused silica (a type of glass) and analyzed it using a spectrometer. They then measured temperature readings from various depths of the sample using computational tools previously they previously developed in which the thermal radiation given off from objects composed of multiple materials was measured. Working backward, they used the algorithm to determine the temperature gradient that best fit the experimental results.
This particular effort was a proof of concept. In future work, the team hopes to apply the technique to more complicated multilayer materials and to apply machine learning techniques to improve the process. Eventually, they want to use depth thermography to measure semiconductor devices to gain insights into their temperature distributions as they operate.
This type of 3D temperature profiling could also be used to measure and map clouds of high-temperature gases and liquids; for example, in molten-salt nuclear reactors where it's important to know the temperature of the salt throughout the volume without having to use temperature probes that may not survive at 700 °C.