It is often alleged that spectacle lenses with an AR coating attract more dirt than uncoated lenses. This is not correct: both become equally dirty. However, dirt or marks such as fingerprints or water stains are more noticeable on coated lenses, as they impair the antireflection properties of the coating. The better the AR coating, i. e. the better the reduction of reflections, the more evident this effect becomes.
It was for this reason that a hydrophobic (water-repellent) coating was developed for lenses with a multilayer (Super ET) coating to make it more difficult for dust and dirt particles to adhere to the lens. This markedly simplifies lens care.
Clean Coat is applied to the AR coating and smoothes the lens surface so that dust and dirt particles have greater difficulty adhering to it.
Clean Coat has also better "wetting" properties than AR coatings. This term is used to describe the behaviour of water on the lens surface.
A drop of water displays the property of contracting and adopting the shape of a sphere. This is the result of the cohesion forces which act between the water molecules and hold them together.
If the force of adhesion between the lens material and a drop of water is greater than the force of cohesion between the water molecules, the individual water molecules will try to deposit themselves on the lens. The greater the force of adhesion, the more direct the contact between the water molecules and the lens surface becomes. The drop of water loses its spherical shape when it touches the lens and spreads over its surface.
The application of Clean Coat to the lens surface reduces the adhesion between the lens material and the drop of water to such an extent that the cohesion of the water molecules is greater than the adhesion. When the drop touches the lens surface it retains its spherical shape and drips off in much the same way as water drips off the surface of a freshly polished car.
This hydrophobic behaviour is attributable to so-called alkylsilanes which consist of a chain of hydrocarbons and at least one SiOH group.
Due to its strong affinity to the base material, the SiOH group ensures good adhesion of the hydrophobic coating. The water-repellent or hydrophobic effect is created by the hydrocarbon chains.
The application of Clean Coat to lenses with a multilayer Super ET coating is the latest innovation to be implemented in the field of coating technology at ZEISS. The extremely thin (5 – 7 nm), optically ineffective coating was introduced for all Super ET products in 1995.
Like an AR coating, Clean Coat is applied to the lenses in vacuum installations. Substances are deposited which make the lens hydrophobic. Separate processes are used for plastic and glass lenses. While Clean Coat and the AR coating can be applied to plastic lenses in a single coating installation, glass lenses receive the coatings in two separate installations.
Clean Coat is standard with the following AR coatings: