Oct 5 2016
Injection-molded optical lens made from transparent plastic demonstrating the nanostructured antireflective coating. Only the coated right half shows the effect of the antireflection system. The uncoated left half reflects the light. CREDIT: © Photo Fraunhofer IOF
It is possible to manufacture transparent plastic optical lenses in different shapes in a cost-effective manner, although there is a disadvantage as they reflect light just as much as glass. Fraunhofer researchers will be launching a new type of antireflective coating at the K trade fair in Düsseldorf.
This new coating will be capable of reducing reflections from plastic lenses and stray light. This enhances the performance of headlights and cameras, and is beneficial for virtual reality technologies and Industrie 4.0.
The key task of optical lenses is to focus light. These lenses allow LED headlamps to flexibly adjust their beam and shine brightly, and help cameras take photos with sharp focus. Complex lens systems are needed as they allow optimum control over the path of a ray of light.
These contain a number of lenses that are of varied shapes. Transparent plastic is considered to be the ideal material for lenses, as the cost-effective process of plastic injection molding can be applied to develop different shaped lenses.
However, one main problem is that light reflects from the surface of the lenses. Plastic has a refractive index (a measure of how much light is reflected) of almost 1.5, and the refractive index of air is 1. This suggests that plastic lenses reflect around 8% of the incoming light.
Curved surfaces intensify this effect when light strikes them at an oblique angle of incidence.
Dr. Ulrike Schulz, Fraunhofer Institute for Applied Optics and Precision Engineering IOF
Refractive Index Greatly Reduced
The K 2016 trade fair, in Düsseldorf, is a globally significant trade fair for the rubber and plastics industry. Fraunhofer IOF scientists will be attending this fair scheduled from October 19-26 (Hall 7, Booth SC01).
The scientists aim to exhibit a new type of antireflective coating suitable for curved plastic lenses. The coating decreases the refractive index at the surface of plastic optics to approximately 1.1, providing a near-perfect transition to air.
Recently, Fraunhofer IOF tested prototypes of the coating in varied lens systems, in close collaboration with industry partners. The results highlight that the method noticeably reduces stray light, which refers to the reflected light that spreads through the lens systems, for instance in cameras, and interferes with how they focus light rays.
The experiments performed by the team also prove that lenses treated with the antireflective coating produced at Fraunhofer IOF allow an increasing amount of light when compared to the standard lenses. This latest technology is ideal and beneficial for car headlamps, camera optics, and also a number of growth areas such as virtual reality or gesture controlled devices for Industrie 4.0.
Imaging systems are growing ever more important as a data gathering tool – and they will need ever more powerful optical lenses.
Dr. Ulrike Schulz, Fraunhofer Institute for Applied Optics and Precision Engineering IOF
New Multilayer Nanomaterial
A number of innovative nanostructured film layers with standard homogeneous oxide layers have been sandwiched by the antireflection system developed by the Fraunhofer IOF researchers. In successive layers, the team dilutes the plastic with more air until the refractive index at the surface is close to being equal to that of air.
This was achieved using new nanomaterials capable of being used for lenses with complex shapes. The stacking of several layers permits them to double the antireflective coating’s thickness relative to earlier solutions.
It is possible to distribute the standard antireflective coatings unevenly on curved lenses, such that the coating continues to be thinner at the edge rather than on the convex center.
This physical thinness translates into optical thinness: the thinner layer only prevents reflections of short-wave light. By contrast, several layers of nanostructured film can cover a wider wavelength spectrum, while at the same time reducing reflections of light at oblique angles.
Dr. Ulrike Schulz, Fraunhofer Institute for Applied Optics and Precision Engineering IOF
Plastic optics is best suited for this process. It is also possible to directly integrate the antireflective coating’s bottom layer into the plastic using plasma etching. “In this way, we can apply antireflective coating to a wide range of plastics,” Dr. Schulz says.