Tracking a mystery

The Institut Fresnel has the privilege of being able to choose from various technologies to find those best suited to the respective project. In so doing, Lumeau works very closely with the experts from Bühler. “We propose new concepts, and then we develop a proof-of-concept with Bühler,” says Lumeau. The filter for the Taranis project was produced on the Bühler Helios coating machine.

The machine works under vacuum. The coating material is sputtered when bombarded with ions, and the knocked-out atoms condense on the glass substrate. Layer by layer, the different materials are applied. About 150 layers are required for a Taranis filter. Compared with antireflection coatings that are applied on prescription lenses, for which filters are made of only a few layers, this is quite a lot. The total coating time is therefore significantly longer, too. While this takes a few minutes for lense coating, for the Taranis filter it took more than 10 hours to fabricate. “For the light to be filtered at the exact wavelength, it is essential that every layer be applied at exactly the right thickness,” explains Lumeau. “This means in the nano range. The individual layers are much thinner than 1 micrometer. They are typically less than one-thousandth of the diameter of a strand of hair.”

With such dimensions in the nano range, it is no surprise that measurability is an important issue. “The technology has developed rapidly in the last few years. Today, coatings that looked impossible 15 years ago are now commonly fabricated,” says Lumeau. “The challenge is to further improve the precision of the layer thickness and reach atomic precision for each layer.” For this reason, the Institut Fresnel and Bühler are planning to work together on improving optical measuring systems (OMS). The goal is to create new methods for OMS that automatically perform the evaluations and are therefore much more precise than today’s systems.

The new OMS technique is not the only project that could be of interest to the optical industry. Institut Fresnel and Bühler are also working on developing a variable filter. This should allow several color values to be visible at the same time. “People no longer want to be able to capture only one spectral value with one filter, but instead several at the same time,” says Lumeau.

To make this possible, it is necessary for the individual layers of a filter to be of various thicknesses in different spots. “Bühler offers us the best technology on the market for manufacturing homogeneous complex filters,” Lumeau says. “Using this as the basis, we are developing a prototype process for variable filters.” Institut Fresnel, in turn, is making the results of its research available to Bühler. “We are developing a standard process from this and will offer it to our customers,” says Yvonne Bonnin-Degner, Area Sales and Service Manager for Bühler. “This is the perfect merging of research and industry,” adds Lumeau.