Anti-Fog and Anti-Reflective Optical Coating

Optical Coating Prevents Fogging and Reflections Researchers have developed an optical coating system that combines antifogging and antireflective properties. The new technology can help improve the performance of lidar systems and cameras. Anne Gärtner, research team leader from the Fraunhofer Institute for Applied Optics and Precision Engineering in Jena, Germany, and Jena Friedrich Schiller University, said: "Moving from the cold outdoors into a warm room causes glasses to fog up and obstruct vision. The same can apply to sensors like lidar systems used in autonomous vehicles. Even if fogging occurs, it is important that surfaces remain extremely transparent so that functionality is preserved." In Applied Optics, Gärtner and colleagues describe how they combined an antifogging polymer coating with porous silicon dioxide nanostructures that reduce reflections. Although the coatings described in the article were designed specifically for lidar systems, the technology developed can be adapted for many different applications. Gärtner said: "In our coating system, antifogging and antireflective properties are combined perfectly, which was not previously possible. Samples produced with this new coating technology have been successfully used for a year in various airborne lidar prototypes operating in diverse climate conditions worldwide." The coating system described in the article was developed in response to a need identified by Leica Geosystems in Heerbrugg, Switzerland. Leica Geosystems develops airborne lidar measurement systems used for land and urban mapping. When there are extreme temperature differences between the environment and the measurement system, fogging sometimes occurs on optical surfaces and compromises functionality. Gärtner's team collaborated with Leica Geosystems to develop a solution that controls unwanted light reflections as well as fogging. Gärtner said: "We used a polymer that acts as a water reservoir to prevent fogging on the optical surface. However, differences in the refractive indices of the polymer material and the surrounding air lead to unwanted reflections and ghost light. To prevent these reflections, we combined the antifogging film with very small structures up to 320 nm in height, creating an antireflective effect along with water permeability." To create the multifunctional coating system, researchers applied the AR-plas2 technology developed at the Fraunhofer Institute for Applied Optics and Precision Engineering. This technology allows various nanostructures to be built up in layers. The process involved etching a nanostructure into the antifogging coating and then producing a second nanostructure on top of it. With this technology, it is possible to adjust the refractive indices of the nanostructures to adapt the design of the dual nanostructure to achieve very low reflection across a broad spectral range. Researchers tested the antifogging and antireflective effects of the coating systems using reflection measurements obtained with a spectrophotometer and fogging measurements obtained after holding the optic with the antireflective/antifogging side over heated water. These laboratory tests demonstrated that the multilayer system exhibits very low reflection across a broad spectral range, which is impossible with a single nanostructure. Additionally, the nanostructures did not affect the antifogging properties of the coating. Since the structures are produced in a standard plasma-ion-assisted coating machine, the newly developed technology can be easily incorporated into commercial production processes. This coating technology is currently being applied in several lidar prototype systems as well as being used in state-of-the-art smartphone cameras. Researchers are now investigating how the coating system can be transferred to other fields, such as adaptive lighting systems in the automotive sector or the development of quantum computers. Gärtner said: "Optical systems are becoming increasingly complex, and therefore demands on image quality are rising. With nanostructures, antireflective properties can be achieved with impressive results that are often not possible with conventional coatings. With the fundamental understanding we have gained in recent years, we are confident that we can transfer nanostructured coatings into real-world applications."   Academic Reference: Anne Gärtner et al, Combined antifogging and antireflective double nanostructured coatings for LiDAR applications, Applied Optics (2022). DOI: 10.1364/AO.476974 Source