Reviewed by Lexie CornerDec 11 2024
A group of researchers from Rice University has addressed a long-standing issue in thermal imaging, making it possible to capture clear images of objects through heated windows. The research, published in Communications Engineering, could have applications in security, surveillance, industrial research, and diagnostics.
Say you want to use thermal imaging to monitor chemical reactions in a high-temperature reactor chamber. The problem you would be facing is that the thermal radiation emitted by the window itself overwhelms the camera, obscuring the view of objects on the other side.
Gururaj Naik, Study Corresponding Author and Associate Professor, Rice University
One potential solution is to coat the window with a substance that inhibits thermal light emission toward the camera, but doing so would make the window opaque. To overcome this problem, the researchers created a coating that doubles the contrast of thermal imaging over traditional techniques by filtering out the thermal noise of a hot window using an engineered asymmetry.
The design of nanoscale resonators, which act as tiny tuning forks, is at the heart of this innovation. They trap and enhance electromagnetic waves within particular frequencies. The resonators are crafted from silicon and arranged in a meticulously designed array, enabling precise control over the emission and transmission of thermal radiation through the window.
The intriguing question for us was whether it would be possible to suppress the window’s thermal emission toward the camera while maintaining good transmission from the side of the object to be visualized. Information theory dictates a ‘no’ for an answer in any passive system. However, there is a loophole ⎯ in actuality, the camera operates in a finite bandwidth. We took advantage of this loophole and created a coating that suppresses thermal emission from the window toward the camera in a broadband but only diminishes transmission from the imaged object in a narrow band.
Gururaj Naik, Study Corresponding Author and Associate Professor, Rice University
This was accomplished by creating a metamaterial with two layers of various resonator types divided by a spacer layer. The design keeps the coating transparent enough to absorb thermal radiation from objects behind the window while suppressing thermal emissions aimed at the camera.
Our solution to the problem takes inspiration from quantum mechanics and non-Hermitian optics.
Ciril Samuel Prasad, Doctoral Engineering Alum and Study First Author, Rice University
The end product is a ground-breaking asymmetric metawindow that can image heat at temperatures up to 873 K (about 600 ºC).
This breakthrough has important ramifications. An immediate application is in chemical processing, where monitoring reactions within high-temperature chambers is crucial.
By resolving the well-known “Narcissus effect,” in which thermal emissions from the camera itself obstruct imaging, this method has the potential to transform hyperspectral thermal imaging beyond industrial applications.
According to researchers, accurate thermal imaging is crucial for energy conservation, radiative cooling, and defense systems.
The researchers noted, “This is a disruptive innovation. We have not only solved a long-standing problem but opened new doors for imaging in extreme conditions. The use of metasurfaces and resonators as design tools will likely transform many fields beyond thermal imaging from energy harvesting to advanced sensing technologies.”
Henry Everitt, an Adjunct Faculty Member at Rice University and Senior Scientist at the US Army Research Laboratory, contributed equally to the research.
The United States Army Research Office supported the research under cooperative agreement number W911NF2120031.
Journal Reference:
Prasad, C. S., et al. (2024) Thermal imaging through hot emissive windows. Communications Engineering. doi.org/10.1038/s44172-024-00316-y.