Optical Coating Improves Performance of Cameras, Sensors

Researchers have developed a coating for optical systems that is both antifogging and anti-reflective, two properties for these systems that so far haven't been possible for one material to provide reliably, they said.

Scientists from German institutes Fraunhofer Institute for Applied Optics and Precision Engineering and Friedrich Schiller University Jena created the polymer coating, which uses porous silicon dioxide nanostructures to prevent fogging as well reduce reflections, researchers said.

The team—led by research team leader Anne Gärtner—specifically developed the coating to be used in the lidar sensing systems used to drive autonomous vehicles. However, the coating—which can help optical surfaces keep their crucial state of transparency—can be used in other applications, such as cameras and displays, researchers said.

“Walking into a warm room from the cold outside can cause glasses to fog up, blinding the user,” Gärtner explained. “The same can happen to sensors such as the lidar systems used in autonomous cars. It is important that surfaces remain highly transparent, even if fogging occurs, so that functionality is maintained.”

The team was inspired by a need identified by Leica Geosystems in Heerbrugg, Switzerland, to develop the coating system, researchers said. The company develops airborne lidar measurement systems that are used for terrain and city mapping.

Scientists at Leica Geosystems noticed that sometimes fogging occurs on optical surfaces when there are extreme temperature differences between the environment and the measuring system, impairing their functionality. In the case of a lidar system guiding a self-driving vehicle, anything that hinders functionality can potentially be life-threatening, making it important for scientists to keep the surfaces clear and transparent, researchers said.

How the Optical Coating Was Developed

The optical coating developed by the team is a multi-layered system of nanostructures created on top of each other. The process involved etching a nanostructure into the antifog coating and then fabricating a second nanostructure on top, researchers said.

The resulting technology allows for the adjustment of refractive indices of the nanostructures to tailor the design of the double nanostructure; this achieves very low reflection over a wide spectral range, Gärtner explained.

“We used a polymer that prevents fogging on an optical surface by acting as a water reservoir,” she said. “However, differences in the refractive indices of the polymer material and the surrounding air leads to unwanted reflections and ghost light."

To prevent these reflections, researchers combined the antifog film with very small structures—up to 320 nanometers high—to create an anti-reflective effect together with water permeability, she said.

“In our coating system the anti-fogging and anti-reflective properties are excellently combined, something which has not been previously feasible,” Gärtner said.

The researchers tested the anti-reflection and antifog effects of their coating system using various measurements—including ones for reflectance acquired with a spectrophotometer, and ones for fogging obtained after holding the antireflection/antifogging side of the optic over heated water, they said.

In these tests, the multi-layer system demonstrated very low reflection over a wide spectral range, which researchers said would be impossible with a single nanostructure. Additionally, the nanostructures did not affect the coating’s antifog properties, they added.

Applications of Optical Coatings and Future Design

Researchers published a paper on their work in the journal Applied Optics.

In their design process, researchers generated the nanostructures in a standard plasma-ion-assisted coating machine, which means the fabrication of the coating can easily be incorporated into commercial manufacturing processes, they said. The coating already is being applied to several prototype lidar systems, as well as in state-of-the-art smartphone cameras, researchers added.

Next steps for the technology are an exploration of how the coating system could be transferred to other areas of application, such as adaptive lighting systems in the automotive sector or the development of quantum computers, they said.

“Optical systems are becoming more and more complex and thus the demands on image quality are also increasing,” Gärtner said. “With nanostructures, anti-reflective properties can be achieved with impressive results that are often not feasible with conventional coatings."

The understanding that the team gained from their work with the coating for lidar systems can inform future development of nanostructured coatings for other real-world applications as well, she said.