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UNDERSTANDING AR

A look at the complex processes behind this important lens treatment. Part 1 of our two-part series on lens coatings

BY JENEAN CARLTON, ABOC, NCLC

Anti-reflective lenses nearly eliminate reflection, reducing it to less than 1% of the incident light. This benefits wearers by allowing more light to enter the eye, thus improving visual acuity.

But there’s even more benefits. AR lenses help in diminishing the ghost images and halos around lights that can cause night driving difficulties. Therefore, they make low-light driving much safer for drivers, passengers, and pedestrians alike. Another plus, a cosmetic benefit, is that people are able to see your eyes more clearly when you wear them, without the distracting reflections of non-AR lenses.

The inside view of an AR chamber (photo courtesy of Essilor)

THE CHALLENGE OF AR

One would think that manufacturing anti-reflective lenses would be as simple as dipping them into a chemical bath or spraying the lenses with some sort of solvent. This simple description of the AR process understates the complexities of manufacturing these lenses. In fact, fabricating anti-reflective lenses involves a high level of chemistry and physics knowledge in order to produce a quality thin film that is transparent and long lasting.

Ask lens manufacturers the most difficult part of fabricating ophthalmic lenses and they will tell you coatings and thin films present the most challenging aspect of the production process. To shine some light on the level of difficulty in manufacturing AR lenses, consider how lens materials react to temperature variations.

To some degree, all lens materials expand and contract based on the temperature in the environment. Resin lenses expand in warm temperatures and contract in cooler temperatures. This presents a significant challenge when designing optical coatings and thin films. Coatings and thin films (including AR) must expand and contract with the lens substrate underneath.

If the films, coatings, treatments, and lens material don’t expand and contract at the same rate, the coatings and/or films may separate from the lens surface. This problem makes lenses appear cloudy or display a “crazing” effect. “Crazing” is a term used to describe a coating that has separated from the base lens material. It looks somewhat like a spider-web effect across the lens surface. Both of these problems are telltale signs of a coating or thin film application that has separated from the lens surface.

The curvature of spectacle lenses adds even more complexity to AR. This problem is not experienced in other products that carry AR coatings—as TV monitors, smartphones, tablets, and other electronic devices involve AR treatments on flat surfaces.

STACKING THE LENS

Anti-reflective lenses consist of multiple layers of thin films, with the combined treatment being referred to as an AR “stack.” Though manufacturers have proprietary formulas for their AR lenses, most anti-reflective stacks are composed of adhesions layers, silica hard coatings, low refractive index layers, high refractive index layers, a hydrophobic layer, an oleophobic layer, buffer layers, lacquer hard coats, and more.

Hydrophobic and oleophobic layers make the lens surface slippery, easier to clean, and less likely to get oily and attract dust. Some companies also apply an anti-slip coating layer to help prevent lens slippage during the edging process.

The scratch-resistant hard lacquer coat is underneath the AR stack coating. You might think it makes more sense to have the hard coating as the outermost layer to keep the lens from scratching. However, the hard lacquer coat is actually placed underneath the AR stack because anti-reflective thin films are brittle and ultra thin. Having the hard coat underneath the other layers acts as a supportive structure for the multi-layers of the AR stack (see sidebar, page 68).

APPLICATION METHOD

Thin films like anti-reflective and mirror coatings are applied to lenses in most cases by means of vacuum deposition (though some methods may vary). This is a process employed to deposit layers of materials—molecule-by-molecule or atom-by-atom—on solid surfaces. Vacuum deposition equipment is expensive but necessary to uniformly deposit the metal oxides (typically silica, silicon dioxide, and titanium dioxide) used in the AR process. These materials are applied in ultra-thin layers; so thin they are measured in angstroms, a unit of length equal to one-tenth of a millimicron, primarily used to express electromagnetic wavelengths.

An example of a multi-layer AR stack (photo courtesy of K-MARS Optical)

To apply the coatings, AR laboratories have “clean room” facilities. This means the application process takes place in an environment where dust and other contaminants are reduced to a very low level thanks to special machinery and procedures designed to make sure the coating process is performed effectively.

AR chambers are usually kept in “clean rooms” (photo courtesy of K-MARS Optical)

Any damage, debris, or moisture on the lens surface will cause the coating to be defective by not adhering properly to the lens. Therefore, the lenses must be meticulously cleaned. Manufacturers use ultrasonic cleaners to remove any surface contaminants from the lenses such as oil from fingertips. The lenses are placed in a rack and then loaded into the vacuum chamber.

Some manufacturers use an ion process to clean the lenses in the chamber. The ion “guns” generate ions and accelerate them via an electrical source. The result is an ion stream directed against the lenses that knocks away any loose contaminants or debris. This process also aids with the adhesion of the AR materials.

VACUUM DEPOSITION

The steps involved in the vacuum deposition process are precisely dictated by formulas uniquely developed by each coating company. Once the lenses are in the chamber, an electron “gun” emits a narrow beam of electrons that vaporize the materials used to create the multi-layer AR thin film. The materials are vaporized in a given order with each manufacturer having their own proprietary formula for their coating.

The vaporized materials are evenly deposited on the lenses as they spin inside the vacuum chamber. The lens rack spins inside the chamber so that the lenses are perfectly coated with metal oxides and other materials that create the multilayer AR stack.

Each layer of the coating stack is cured after each step of deposition. Called thermal curing, it is a process that toughens or hardens a material and is achieved with heat.

PROVEN BENEFITS

The Illinois College of Optometry recently conducted a study of both AR and non-AR lens performance, which was supported by a grant from The Vision Council. The study’s goal was to determine the perceived effect of anti-reflective coatings on various daily activities including tasks like driving and using a computer as well as using hand-held electronic devices such as tablets and smartphones.

The majority of participants stated a preference for AR lenses over non-AR lenses, and reported that AR-coated lenses provided better clarity and comfort when performing normal daily activities including driving, working at a computer, and using hand-held devices. When compared to non-AR lenses, contrast sensitivity was also improved with AR lenses. The participants said they were likely to recommend AR lenses to others and would continue wearing them in the future.

Optical Coatings, Part 1

JENEAN CARLTON, ABOC, NCLC

Anti-reflective lenses enhance the performance and aesthetics of spectacle lenses. Try to find all of the terms in this puzzle that relate to these thin films and coatings. Check back in the March issue of Eyecare Business for the answers.

ACROSS

5. Coatings and _____ _____ must expand and contract with the lens substrate underneath. (2 words)

6. Lenses must be _____ cleaned before the coating process can begin.

8. _____ and thin films present the most challenging aspect of the lens manufacturing process.

10. Anti-reflective lenses consist of multiple layers of thin films with the combined treatment being referred to as an AR _____.

13. Having the hard coat underneath the other layers acts as a _____ structure for the multilayers of the stack.

14. The hard lacquer coat is placed _____ the AR stack.

16. AR lenses make low-light driving much _____ for drivers, passengers and pedestrians alike.

19. The lens rack spins inside the chamber so lenses are perfectly coated with the _____ _____ and other materials that create the AR stack. (2 words)

24. All lens materials expand and contract based on the _____ in the environment.

25. An electron “gun” emits a narrow beam of electrons that _____ the materials used to create the multi-layer thin film.

26. Vacuum deposition equipment uniformly deposits the metal oxides such as ______ _____ used in the AR process. (2 words)

27. AR coating laboratories have _____ _____ facilities. (2 words)

28. Anti-reflective lenses nearly _____ _____ on the lens surfaces, thereby increasing the amount of light entering the eye. (2 words)

29. Study participants stated they were likely to _____ AR lenses to other and would continue wearing them in the future.

DOWN

1. Anti-reflective lenses nearly _____ reflections on the lens surfaces thereby increasing the amount of light entering the eye.

2. Resin lenses_____ in warm temperatures and contract in cooler temperatures.

3. Some manufacturers use an_____ _____ to clean the lenses in the chamber. (2 words)

4. Angstrom unit measurements are primarily used to express _____ _____. (2 words)

7. _____ is a term used to describe a coating that has separated from the base lens material.

8. The _____ of spectacle lenses adds more complexity to the fabricating process of AR lenses.

9. Fabricating anti-reflective lenses involves a high level of _____ and physics knowledge.

11. Each layer of the ______ stack is cured after each step of deposition.

12. Manufacturers have _____ formulas for their AR lenses.

15. _____ _____ is a process that toughens or hardens a material and is achieved with heat. (2 words)

17. Thin films such as anti-reflective and mirror coatings are applied to lenses by means of _____ _____. (2 words)

18. AR lenses help in _____ the ghost images and halos around lights while night driving.

20. AR materials are applied in ultra-thin layers, so thin they are measured in _____.

21. _____ and oleophobic layers make the lens surface slippery, easier to clean and less likely to get oily and attract dust.

22. The scratch-resistant hard _____ _____ is underneath the AR stack. (2 words)

23. Study participants stated a _____ for AR-coated lenses over non-AR lenses.