AR: How coatings increase lens efficiency
Eyecare Business's technology update discusses recent advances in AR and what they mean to you
By Joseph L. Bruneni

Anti-reflection coated lenses currently represent the fastest-growing premium lens category in this country. Currently at 18 percent, according to the AR Council, it was reported at Essilor's national sales meeting last month that AR growth is exactly paralleling where PAL sales were a decade ago. One reason for this growth has been a wave of advancements in AR technology. This, in turn, is prompting a wide array of improvements in both the AR coating process and AR coating equipment.

Many laboratories are making major investments in equipment. The largest outlay is usually for the AR coating equipment itself (up to $500,000). But, as AR technology grows more sophisticated, labs are learning that the ideal AR coating requires more than just coating equipment. It also requires a faultless surface beneath the AR to obtain maximum efficiency from the coating. An effort to meet this need is leading to quantum advances in the equipment used for surfacing and for applying the scratch coating lying under the AR.

To better understand AR technology and the information in this story, readers should be familiar with certain terms used in the coating industry. See the glossary sidebar.

Though we in optical tend to think of anti-reflective coatings as primarily for spectacle lenses, AR for ophthalmic lenses grew popular long after it was widely used in other industries. AR coatings have long been a staple in photography and have been integral to the aerospace industry from its earliest days.

Thin-film coatings used in those fields far surpass the level of AR coatings used for spectacles. Optical could adapt some of the far-out coating techniques developed for the aerospace industry, but the requirements for ophthalmic lenses add complications that other thin-film applications never face.

For instance: The surfaces we coat are complex and curved, and it's always easier to AR coat flat surfaces. The second issue is more troublesome and comes from the wide variations in the substances we coat. The curved surfaces vary, the substrates vary, the indices vary, and the chemical variations introduced by a variety of scratch coatings vary. All in all, AR coating of ophthalmic lenses is one of the most complex applications for thin-film coatings.

This example of multiple-layer AR coating is courtesy of Satis

IMPORTANCE OF SCRATCH COATING

A major issue comes from the wide use of scratch coating on ophthalmic plastic lenses. Coaters readily admit that coating glass lenses is a snap because glass is glass, while plastic lenses vary in virtually every component.

Applying multi-layer AR coatings to lenses involves the most complex computer-controlled equipment used in the ophthalmic industry--yet the overall performance of these sophisticated thin-film coatings depends more on what lies beneath the AR. No single element has a greater effect on the efficiency of an AR coating than the scratch coating under it. If the scratch coating fails to adhere to the lens, the AR coating on top is directly affected.

Much of the dissatisfaction with early AR coatings resulted from poor performing scratch coating beneath the AR. When a scratch coating sloughs or peels off the lens surface, so does the AR coating. Coaters know this, but it is difficult to control when custom coating a variety of ophthalmic lenses.

FACTORY VS. CUSTOM COATING

Factory-coated AR stock lenses generally provide superior performance because the coating has been applied to large batches of identical lenses. The lens manufacturers usually coat thousands of lenses at a time.

The lenses may vary in foci power, but all share common properties, permitting the manufacturer to fine-tune the AR process to the individual characteristics of the lenses. They have matched substrates--all poly, all CR 39, for example--and all share the same index. They have identical scratch coating and the chemical consistency and formulation of both substrate and scratch coating is controlled and identified to the coater. As a result, AR coatings are formulated specifically to the lenses being coated.

This is more difficult in custom coating. A batch of lenses for AR coating in a laboratory can vary widely. The refractive index will vary. Each batch often includes a variety of lens brands.

Another variation comes from the chemistry of the scratch coating. Some have no scratch coating on the backside, some have factory-applied coating on one or both sides (stock lenses), while others have scratch coating applied by a lab on the backside. Also, lens manufacturers use thermal-cure coatings while most labs use UV-cure coatings.

As a result, custom coaters usually formulate their AR coatings to match the average of what is in the chamber. Lens manufacturers and labs are finding ways to overcome this inconsistency in custom coating. The following list does not include every example, but provides a sense of the improvements taking place.

	Edged lenses are mounted in a rack and ready for insertion into an AR chamber.

AR COATING ADVANCES

Here's a look at how several companies are dealing with this issue.

Essilor. Market leader Essilor continues to make investments in AR. It says the best way to produce the ultimate AR coating is to maintain control over every facet of lens processing, from casting to final product.

Essilor begins with uncoated lenses and closely monitors the composition of the lens substrate (monomer), the casting process (injection molding for polycarbonate), and scratch coating application (both sides), culminating in total control of the AR application itself. The result is Crizal, first introduced in Europe and Canada and now available in the U.S.

Sola also recognized the need for full control of both manufacturing and processing of AR-coated lenses, but chose to form close alliances with selected wholesale laboratories. The result is their UTMC® high-performance AR coating, available from authorized Sola labs that follow processing controls established and monitored by Sola.

Satis Vacuum. The company's Endura Performance AR is a customized process that enhances AR coating by hardening and improving the AR stack. The total process takes place in-chamber with cushioning layers added to provide flexibility to the AR coating. Available only from authorized labs, the layers in the Satis process include an adhesion layer and are topped off by a proprietary easy-clean sealant layer. The Endura AR stack consists of 11 layers.

Seiko Optical. Its new 16-Layer High Impact Process (HIP) for its AR coating improves adhesion while significantly reducing crazing, peeling, and scratching. Its coatings are index matched to each material, and the 16-layer process includes a primer, hard coating, eight layers of AR, and a hydrophobic top. The uniform index of lens material and coating is reported to eliminate rainbow reflections and includes a hydro sealer.

Hoya utilizes SMP (Substrate Matching Properties) on all of its HiVision AR coatings. By matching to the substrate, the coatings react the same optically and mechanically. Its coatings are mixed to match each substrate, providing long wear and durability while eliminating birefringence--a commonly observed Newton ring or rainbow effect seen on coated lenses, which can result when the index of the scratch coating does not exactly match the substrate index.

For Hoya's lenses, only one material is run through the vacuum chamber at a time, and a specific coating formula is designed for each material. Additionally, the front and backside of the lenses have different coating constructions.

Integrating benefits such as scratch resistance with AR is important to all coating companies.

Pentax, for instance, has developed its own integrated AR process called Surpass™.

Zeiss says its new Foundation™ undercoating chemistry provides superior scratch resistance for both front and back surfaces and can be provided as a stand-alone scratch coating or combined with Zeiss Super ET® or Gold ET AR coatings. As a stand-alone scratch coating, Foundation includes the Zeiss Clean Coat hydrophobic coating that chemically bonds to any other coating present on the lens. The process requires dedicated equipment for application. Currently, 20 labs have committed to the new technology that currently can only be applied to CR 39. Foundation technology for other substrates is planned.

CrystalCoat™. SDC Coatings is a coating specialist operating primarily as a supplier to lens manufacturers. CrystalCoat is designed to help manufacturers provide scratch coatings that more closely match the index of the substrate as a way to eliminate birefringence. To do so, each CrystalCoat IM coating (IM stands for index matching) is designed for a specific monomer index. SDC produces IM coatings with indices running from 1.49 to 1.67.

Stratum™ and Stratum™ 2D Lens Coating System. The coating chemicals developed for this advanced coating system for plastic and polycarbonate lenses were developed jointly by Lens Technology and Gerber Coburn. The equipment makes it possible for labs to apply both highly tintable coatings and non-tintable coatings in the same Stratum equipment.

Such advancements are ongoing with lens suppliers and coaters.

Optima. This firm, for example, is currently installing new state-of-the-art AR coating equipment at its dedicated facility.

MAINTENANCE OF AR

Just as the substrate and scratch coating beneath AR play important roles in AR performance, what lies on top of the AR stack is equally important. That's why so many companies have developed and utilize hydrophobic top coatings.

These additional issues influencing the efficiency and effectiveness of AR coatings have prompted a multitude of product launches. Nanofilm, for example, produces a hydrophobic top coating called PermaSeal, used by many leading AR coaters.

Some major coaters have coined their own trade names for the Nanofilm hydrophobic coating, while others use the name PermaSeal. Unlike other hydrophobic coatings, PermaSeal hydrophobic coating is applied outside the chamber in special equipment.

The company says this process produces less contamination, leading to better AR performance.

Another issue affecting AR performance is how patients care for their AR coated lenses. Just as an automobile requires regular maintenance for maximum performance, so does AR. To that end, a variety of companies have produced lens cleaning products.

Encouraging proper maintenance of AR coated lenses and providing products to help patients care for their lenses is an important part of the eyecare professional's role.

Surveys have proven that dissatisfied AR wearers usually have used soap and water with improper cleaning cloths (often clothing), but those who properly care for their AR lenses seldom have complaints about AR coatings.

The technical advancements in this complex field of lens coatings may seem somewhat overwhelming to eyecare professionals.

Understanding the complexities of coatings is a challenge for the eyecare professional. That's partly because these coatings fall somewhere between the worlds of chemistry and physics, two fields with which many of us are unfamiliar or uncomfortable.

Fortunately for many of us, dealing with these complex issues is not the responsibility of the eyecare professions.

Lens manufacturers and laboratories, on the other hand, are deeply involved in this new world, and the result of their efforts is that eyewear consumers have access to substantially improved lenses with wearing benefits never before possible.

There is little doubt that lenses made of plastic are here to stay. With the new generations of scratch coatings now available, plastic lens performance is approaching that of glass, but without the inherent problems of glass.

Add in the benefits of a tough, long-life AR coating to the equation, and the result is that consumers are enjoying benefits which have never before been possible.

One of the speakers at Vision Expo several years ago made a prediction that lens coatings would eventually impact ophthalmic lenses more than any other factor.

Looking at the state of the market today, and the field's potential, it's beginning to look like he was right.