Anti-reflective coatings do not just significantly improve comfort for glasses-wearers, they also contribute to the wearer's safety. Without this precaution, they would have to contend with strong reflections. At night, for example, the headlights of a vehicle approaching from behind could dangerously dazzle the wearer.
A requirement for good images
Microscopes, telescopes and camera lenses also rely on anti-reflective coatings. Without anti-reflective coatings, zoom lenses with ten or more elements would make no sense: A proportion of the reflected light would always be bounced back to the next lens element. Because this back-and-forth reflection grows exponentially with the number of lens elements, this results in extremely disruptive light spots on the image or creates a blurry veil over the top.
Reflection occurs at the barrier between air and glass. An anti-reflective coating disrupts this effect by creating additional barriers: An extremely thin transparent layer made from a different substance is applied to the surface of the glass. This means that the light falls on two reflective barriers – between the air and the coating and between the coating and the glass.
So, two reflections take place and the separate rays each travel different distances. The waves no longer oscillate synchronously on their return journey. If the trough of one wave now meets the peak of another, these two waves will cancel each other out. Optometrists call this destructive interference. And if you apply multiple layers to create multiple barriers, the reflective effect can be more or less completely eliminated.
The individual layers are just a few nanometers thick. To produce such delicate layers, a process called physical vapor deposition (PVD) is used. The solid coating material is evaporated using a variety of methods or atomized by means of ion bombardment and then applied to the lenses. All of these methods have one thing in common: They require a vacuum chamber because this process only works under high vacuum. The Busch Group offers a wide range of vacuum pumps for this task.
Transparent materials that have a different reflective index to glass are used to create anti-reflective coatings. A high-quality anti-reflective effect is generated by stacking several layers of these materials. The quality of the lens depends heavily on the evenness of these layers. The same principle applies to other functional layers applied to modern glasses lenses.
A polarization layer, also known as a polarizing filter, makes sure that reflected vertical light waves do not make it through the glass and into the eye. Light waves like this are mainly created by reflection, for instance when sunlight bounces off a wet street. The polarizing filter minimizes dazzle and also facilitates vision that is richer in contrast while improving the perception of color.
An antistatic layer prevents the build-up of static charge and unwanted forces of attraction. A layer with a lotus effect makes it more difficult for dirt and oil from the skin to stick to the lens; it also ensures that droplets of water can drip off. A hard elastic layer protects the lens – which tends to be made from relatively soft plastic – from small scratches and other mechanical effects. For functional reasons, the hardened layer has to be applied directly to the lens. The anti-reflective coating and other special finishes are then applied on top. Once additional adhesive layers are applied, there may be ten or more layers on one side of a lens.