Optical coating approach prevents fogging and unwanted reflections

19 Jan 2023

Technology helps sensor and camera systems perform optimally by keeping optics transparent.

Researchers in Germany have developed an optical coating system that combines antifogging and antireflective properties. The new technology could help boost the performance of lidar systems and cameras.

“Walking into a warm room from the cold outside can cause glasses to fog up, blinding the user,” said research team leader Anne Gärtner from Fraunhofer Institute for Applied Optics and Precision Engineering (IOF) and Friedrich Schiller University Jena, both in Jena, Germany.

She added, “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.”

Gärtner and colleagues describe in Applied Optics, how they combined a polymer coating that prevents fogging with porous silicon dioxide nanostructures that reduce reflections. Although the coatings described in the paper were designed specifically for lidar systems, the technology can be tailored for many different applications.

“In our coating system the anti-fogging and anti-reflective properties are excellently combined, something which has not been previously feasible,” said Gärtner. “Samples manufactured with this new coating technology have already been used successfully for a year in several airborne lidar prototypes operating in various climatic conditions around the world.”

Seeing more clearly

The coating system described in the paper was developed in response to a need identified by Leica Geosystems, Heerbrugg, Switzerland. Leica Geosystems develops airborne lidar measurement systems that are used for terrain and city mapping.

When there are extreme temperature differences between the environment and the measuring system, fogging sometimes occurs on the optical surfaces, impairing functionality. Gärtner’s team collaborated with Leica Geosystems to develop a solution that managed the fogging as well as undesirable light reflections.

“We used a polymer that prevents fogging on an optical surface by acting as a water reservoir,” said Gärtner. “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, we combined the antifog film with very small structures — up to 320 nm high — to create an anti-reflective effect together with water permeability.”

To make the multifunctional coating system, the researchers applied “AR-plas2” technology, developed at the Fraunhofer IOF. It allows several nanostructures to be created on top of each other. The process involved etching a nanostructure into the antifog coating and then fabricating a second nanostructure on top.

With this technology, it is possible to adjust the refractive indices of the nanostructures to tailor the design of the double nanostructure to achieve very low reflection over a wide spectral range.

Laboratory tests showed that the resulting multi-layer system exhibited very low reflection over a wide spectral range, which would be impossible with a single nanostructure. Additionally, the nanostructures did not affect the coating’s antifog properties.

Real-world applications

Because the structures are generated in a standard plasma-ion-assisted coating machine, the new approach can be easily incorporated into commercial manufacturing processes. In addition to being applied in several lidar prototype systems, the coating technology is already being used in cutting-edge smart phone cameras.

The researchers are now exploring how the coating system could be transferred to other areas such as adaptive lighting systems in the automotive sector or the development of quantum computers.