Above: Zeiss' new Foundation scratch coating has a rather simple process-dipping, drying, and then on to AR coating

During the past five years, the United States has experienced growth in sales of anti-reflective coatings for ophthalmic lenses. The industry hasn't reached the point where AR coatings are routinely recommended to every patient, but current statistics reveal AR-coated sales far exceed the numbers dispensed even two years ago.

Increased sales of AR coatings have created some dilemmas for lens manufacturers, labs, and thin film coaters. Though eyecare providers, for the most part, are unaware of these issues, understanding them will help practitioners appreciate and cope with new technologies that are currently impacting lenses. This article explains current changes in this evolving technology.

In spite of the recent increase in AR sales, the United States still lags far behind Europe and the Far East. Two major issues were responsible for the eyecare professions' restrained interest in recommending AR to eyewear consumers during the 1990s:

Issue #1...Time. Traditionally, labs were not equipped to apply AR coatings and relied instead on outside facilities when customers ordered AR coatings. This added two or three days to the turnaround. Over time, delay for AR coatings became less of an issue, and many labs still use outside AR coating facilities, relying on airborne couriers for faster turn-around.

Though this system works well for many labs, others have opted to install their own in-house AR coating equipment. This is, in fact, a growing trend for larger laboratories. 

Issue 2...Quality. The second factor holding back greater use of AR in the 1990s was, though less apparent, probably a greater deterrent. Labs sending out lenses for AR coating sometimes found lenses coming back from coaters with minor hairline scratches or blemishes, just enough to reject the lens.

Labs argued that lenses sent to the coater had no blemishes, while coaters insisted the scratches had been on the lens, but only became visible when surface reflections were removed. In truth, both parties were correct. Labs have since learned that AR-coated lenses require greater care during surfacing.

The Quality Issue

Pre-conceived impressions of poor quality make it all the more important to take an in-depth look at quality issues in AR coating technology.

With glass lenses, AR quality is seldom an issue because it provides an exceptionally stable base for applying a thin film coating. Glass is non-porous and relatively resistant to heat-two factors that enable coaters to produce tightly bonded AR coatings that usually last the life of the lens.

When plastic lenses became popular, coaters lost that advantage, and a raft of new problems arose. For example, coating a plastic lens requires laying down an extremely fragile coating on a soft, porous and highly temperature-sensitive base.

As if this weren't enough of a challenge, two new concerns cropped up in AR coating of plastic lenses-scratch coatings and multiple indices.

Scratch coatings. Half or more of the plastic lenses dispensed in this country have anti-scratch coating, often applied by the lens manufacturer to at least the front surface. In addition, because most high index materials are softer than CR 39, they usually have scratch coating on the back surface. And, though backside coatings may be applied by the manufacturer, lenses that are surfaced by a lab will have back coating applied by that lab.

These variations on coating make-up create problems for the coater. Adding further confusion is the variety of methods used for applying the coatings. The manufacturer-applied front coatings are usually thermally-cured, but lab-applied backside scratch coatings are UV-cured.

Above: Essilor controls every aspect of its Crizal coating process at its own labs

Why the difference? One reason is that manufacturers have no time pressures and prefer thermal-cure scratch coating, which is considered to produce a better scratch coat. Time is essential to labs, so they cure backside coating with UV light-which takes about a minute.

But there are other issues involved with scratch coating, as well. For one, the scratch coat must have the ability to accept dye. In the case of poly, only the scratch coat will absorb color. Dye absorption requires a somewhat softer scratch coating and is an important consideration with backside coatings.

Factory-coated AR stock lenses generally have a superior AR coating for one simple reason. When factory AR is applied, the coater is dealing with thousands of lenses identical in every regard except power. As a result, factory-applied AR stock lenses are considered to have a more durable AR coating than custom-coated prescription lenses.

Varying indices. This problem arose as high index became popular.

An ideal AR coating will have the same refractive index as the lens to which it is applied. Today, custom coaters coat eight variations of high index plastic lenses, varying from 1.54 to 1.71.

Varying index is not a major issue, but it does complicate things for custom coaters. A typical Rx batch will have an assortment of high index lenses, and coaters will match the coating index to the average of the lens batch.

Of all these challenges, the greatest problem comes from the variation of scratch coatings because that directly affects the durability of the AR coating. If, for example, the coater is able to fine-tune the AR process specifically to the substrate and scratch coating of each lens, AR coatings will produce the maximum in adhesion and durability. Controlling all those variables of plastic lenses has much to do with the durability of the AR coating. For prescription lenses, it seems to be an insurmountable problem but here again, ingenuity is solving the problem.

Controlled Environment Coatings

The AR process in the lab: coating...

Two major lens manufacturers (Essilor and Sola) have established AR coating systems that produce superior coatings on a custom-coating basis. They do this by tightly controlling every aspect of the process. Essilor coatings produced in this way are called Crizal, and can only be ordered from an Essilor laboratory.

Essilor will only apply the coating on their own lenses. Sola has no laboratories, but has established relationships with a number of authorized labs that follow special coating procedures mandated by Sola. The Sola coating, called UTMC, chemically fuses AR with scratch coat and substrate material.

New Developments in Technology

A new generation of scratch coating will be released this fall. The coating technology was created by a cooperative venture between Carl Zeiss Optical and nanoFILM, a U.S. chemical firm. Called Foundation(tm), the new scratch coating is unique in how it is used. Applied by dipping a lens in the solution, it can be applied to any non-glass lens, whether the lens is already scratch coated or not.

NanoFILM calls its coating technology Glastomer(tm). It was developed initially as a coating for protecting metal parts. From an AR standpoint, what makes the new scratch coating most interesting is that, when applied to any plastic lens, coated or uncoated, it reportedly will eliminate inequities coming from chemical and physical differences between factory-applied scratch coating on the front and lab-applied coating on the back. The new scratch coating will be available only from Zeiss labs.

SDC Coatings has developed a somewhat similar dip scratch coating that is available to all labs. More and more labs are beginning to switch to thermal-cure scratch coatings, finding they offer better scratch protection. These dip thermal cure coatings require a three-hour cure. Some labs use an infrared flash to pre-cure lenses before putting them in the oven. Another new coating development coming out of SDC involves a cushion-coat technology.

Several lens manufacturers produce AR-coated high index stock lenses with 1.0mm centers. AR coated high index lenses can be made this thin because the lenses have a special "cushioned" scratch coating that absorbs shock, permitting the lenses to pass FDA drop ball testing. These super-thin lenses have only been available in stock form because labs don't have access to cushioned coating technology.

However, SDC has developed a new two-stage scratch coating process enabling labs to apply a cushioned primer in combination with a super tough topcoat, so labs will be able to apply cushioned coatings to surfaced lenses.

...and rinsing. Photos courtesy of AR Council

Therapeutic Thin Film Coatings

A relatively new use for thin film coating involves color blindness. The application of corrective optical aids-colored filters or colored light-can improve the ability of color-deficient persons to distinguish differences between colors. Thin film coating technology is now being used in therapeutic aids for deficient color vision.

ColorMax Technologies, a Tustin, Calif.-based company, has developed multi-layer, thin-film coated lenses that can be used as a corrective aid for congenital red-green color deficiencies. The lenses recently received an FDA clearance and use a thin film coating because it offers precision of spectral filtering that usually cannot be achieved with tint or dye methods.

The Future of Coatings

In many ways, the future for lens coatings is here now. New and improved scratch protection and improved AR coatings are already impacting the industry, and eyewear consumers will be the ultimate beneficiaries. Con-sumers are going to benefit from improved eyewear. And, the professions will gain improved profits, while experiencing fewer complaints. EB