A group of researchers from Rice University has cracked a longstanding challenge in thermal imaging, enabling the capture of clear pictures of objects seen through warm windows. Findings from their research, published in the journal Communications Engineering, could have significant applications in various areas such as security, surveillance, and industrial diagnostics.
“Imagine wanting to use thermal imaging to observe chemical reactions inside a very hot reactor chamber,” explained Gururaj Naik, an associate professor of electrical and computer engineering at Rice and the study’s lead author. “The issue you would encounter is that the thermal radiation emitted by the window itself can drown out the camera’s ability to see objects beyond it.”
One way to tackle this issue could be to coat the window with a material that minimizes thermal light emission towards the camera, but this would make the window opaque. To circumvent this challenge, the team designed a coating that cleverly utilizes engineered asymmetry to filter out the thermal interference from a hot window, resulting in twice the contrast in thermal imaging compared to existing techniques.
The foundation of this innovation lies in the creation of nanoscale resonators, which act like tiny tuning forks that trap and amplify electromagnetic waves at specific frequencies. These resonators, crafted from silicon and arranged in a precise formation, enable fine-tuned control over the window’s thermal radiation properties.
“We were curious to see if we could diminish the window’s thermal emissions towards the camera while ensuring that transmission from the object being visualized remains strong,” Naik stated. “While information theory suggests that this isn’t feasible in passive systems, we discovered a workaround. The camera operates within a limited bandwidth, allowing us to create a coating that can suppress thermal emissions broadly directed towards the camera, while only reducing the transmission from the viewed object in a narrow spectrum.”
This was accomplished by crafting a metamaterial made of two different layers of resonators separated by a spacer layer. This innovative design permits the coating to minimize thermal emissions aimed at the camera while remaining sufficiently transparent to detect thermal radiation from objects located behind the window.
“Our approach draws inspiration from quantum mechanics and non-Hermitian optics,” remarked Ciril Samuel Prasad, a doctoral engineering graduate from Rice and the primary author of the study.
The outcome is a groundbreaking asymmetric metawindow, capable of delivering clear thermal images even at temperatures reaching 873 K (around 600 °C).
The potential impacts of this advancement are considerable. One immediate use is in the chemical processing sector, where monitoring reactions within high-temperature environments is vital. Moreover, this technique could transform hyperspectral thermal imaging by mitigating the persistent “Narcissus effect,” which arises when thermal emissions from the camera itself disrupt imaging. The researchers foresee applications in energy efficiency, radiative cooling, and even defense operations where precise thermal imaging is crucial.
“This represents a groundbreaking innovation,” the researchers emphasized. “Not only have we resolved a historical issue, but we’ve also paved the way for new imaging possibilities under extreme conditions. Our use of metasurfaces and resonators as design frameworks is likely to influence a variety of fields beyond thermal imaging, from energy harvesting to advanced sensing technologies.”
Henry Everitt, a senior scientist at the United States Army Research Laboratory and part-time faculty at Rice, is also listed as a co-author of the study.
This research received funding from the United States Army Research Office under cooperative agreement number W911NF2120031.
