Commercial Coating / Standard Anti-Reflective Coating

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Advances in Anti-Reflection Coating Technology
By Edwin Ellefsen, President of Opticote, Inc.

Anti-Reflection coatings (AR) have improved significantly over the last five years. Three events stand out as major reasons for the improvement. These are the widespread use and improvements of ion guns, hardcoats and topcoats.

Ion guns were originally conceived by NASA as a means of space propulsion for satellites. In concept the ion gun functions by using plasma and an electron source to create ions and accelerates them by means of electrical bias. The output is a stream of ions (which can be varied in force and number) that is directed towards the lenses. The ion stream can be compared to sandblasting. Ions hitting the lenses knock away loose contaminants. They can even etch the surface slightly. This promotes adhesion of the subsequent coating to the lens, similar to the way one might lightly sand a shiny surface before painting.

The ion gun is used in the vacuum chamber just before coating. Adding a final cleaning in the vacuum chamber is an improvement over past practices. Initially lenses were cleaned outside the vacuum chamber and transported through the "room air" before loading in the vacuum-coating chamber. Exposure to the air alone is enough to contaminate the lenses. By definition, the vacuum is substantially void of materials that can re-contaminate the lenses.

There are different types of ions and the type of ion has a big influence how effectively and efficiently the lenses are cleaned. Argon ions are heavy so they can easily etch away contaminates. If oxygen is added, ozone is created. Ozone is highly reactive and attacks hydrocarbon contamination such as finger oils that can be very difficult to remove.

Organic hardcoats (lacquer) have been used almost as long as plastic lenses. It was initially thought they had little value when used with AR coated lenses. It is now well known that a good hardcoat (particularly if it is non-tintable) is an indispensable part of a quality AR coated lens. Hardcoats serve two important functions. First, to provide a strong backing to support the AR coating. Secondly, to create a barrier that slows the migration of water from the lens to the backside of the AR coating.

AR coatings are hard and brittle. By comparison, plastic lenses are soft and spongy. Think of a single paper tissue (representing the AR coating) lying on a soft feather pillow (representing the lens). If you poke your finger at the tissue it easily rips. If you place a single tissue on a hard desk and try poking it with your finger, the tissue remains intact and undamaged. The analogy holds with AR coated lenses. The organic hardcoat (over the lens and under the AR coating) supports the thin brittle AR coating much as the hard desk supports the paper tissue.

Water attacks AR coatings and accelerates adhesion failure. Organic hardcoats act as a barrier and slow the migration of water from the lens to the coating. This protects the AR from the bottom up.

The final advancement is hydrophobic topcoat. It helps protect the coating from the outside, encapsulating the AR coating in a manner that promotes a longer AR life.

Besides protecting the lens from water, the topcoat makes the lens easier to clean. The topcoat is slick so dirt wipes off more easily without heavy pressure or excess rubbing both of which can accelerate scratching. New chemical compositions for topcoats are tougher and last longer. Topcoats are now applied in the vacuum chamber. They used to be applied after the coating process, by dipping. Just as using the ion gun to clean lenses under vacuum promotes a better AR/lens bond by not exposing the clean lens to "room-air", so applying topcoat under vacuum promotes a better, longer-lasting topcoat.

Although not a technological advance, there is a final factor that has contributed to the growth and use of AR coatings in the USA. It is the increasing sophistication of suppliers and customers and the realization that "one size does not fit all". Today there are numerous types of substrate and hardcoat materials. The same coating process and materials do not work equally well on all lenses. By tailoring the preparation, process and materials to each type of lens, optimal performance is achieved. This is a vast improvement over the past practice of using one coating formula for all lenses.
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