SPECTACLE LENSES

Patient satisfaction begins with a quality product. It's as simple as that. But some things are not that simple, especially when it comes to AR coating. Getting a quality AR coating requires prepping of the lens, proper coating materials, and--most importantly--proper communication with a lab.

AR coatings themselves have never been better. Multiple layer coatings are now the standard of the industry. They address not only reflections of the most obvious single color wavelength, but also cancel out all primary color wavelengths by reflecting exactly back upon themselves. Lenses today can be so invisible that people want to put their finger where the lens should be.

Despite these quantum leaps in AR quality, there is still trepidation on the part of both patient and dispenser to commit to AR coatings. This is mainly due to past breakdowns in coated lenses caused by several factors.

Everyone involved with AR coated lenses--from the lens manufacturer to the wearer--plays a key role in a successful AR experience. Ignoring any stage in between could spell failure.

Those who routinely recommend AR coatings understand the value for the patient. They are giving them at least six percent clearer vision, depending on lens material and coating, and are building their business in the process. Aspects of AR technology are just getting better!

The technological boom continues with concept lenses and companies have attempted to eliminate any breakdown by tightly controlling the substrate materials used. They plan to use their own base lenses, to which their own anti-scratch coating should adhere superbly, then top it off with a multi-layer AR coating that is specifically formulated for toughness and adhesion to the Scratch Resistant Coating (SRC). Since all of the components of the lens are proprietary to the manufacturer, the finished product should be very durable.

These lenses are considered a premium product and can be sold at a premium price. Yet, should we ignore the mainstream custom coatings? By no means. We can still fabricate lenses from our own stock, then use in-house scratch resistant coatings as a sound substrate for an out sourced AR coating. Interviews with several independent coating labs netted good advice for creating a lens worthy of coating--yielding a lasting lens and a satisfied patient.

First and foremost in building a quality AR coated lens is knowledge. This knowledge is obtained through testing and meticulous record keeping. AR coating labs that are successful have often spent hundreds of thousands of dollars in testing materials and in research to determine how to best coat their customers' lenses. Fortunately, they are happy to give their customers the benefit of this expensive knowledge. 

PREVENTING COATING FAILURE

There are many factors that play an important part in preventing AR coating failure. You should ask your coating lab if there are any particular lens materials that need your special attention to ensure quality. Generally, coating labs prefer to have both sides coated with a scratch resistant coating, which is more stable than the lens itself. Some lens materials may be AR coated directly, but you should check with the coating lab to be certain.

Here are some specific areas you need to consider when ordering AR coated lenses.

Thermal vs. UV. The SRC can have a profound effect on the finished product. Some coating labs achieve better adhesion with thermal cure coatings which are somewhat impractical for a small lab or retail operation.

Most smaller concerns will opt for UV-cured scratch resistant coatings that are much faster and certainly provide a durable SRC. Most coating labs have coatings that are optimized for UV-cured scratch resistant coatings, as well as for those that are thermal cured.

Hard vs. soft. An SRC coating that is too soft can allow the AR coating to flex too much, causing the glass-like layers to fracture. On the other hand, an ultra-hard SRC coating, though a solid foundation, may have adhesion problems.

Cushion coats. If you are having frequent premature ARC breakdown, take the time to ask the coating lab about their experience with the type of coating you use. Some coating labs overcome this problem with a cushion coat. This allows some shift between the substrate and the ARC. It also improves impact resistance allowing for thinner center thickness on certain materials.

Front and back side coats. Most lenses today are scratch resistant coated on the front side, from the factory. These coatings are very good quality as a rule.

Jack Jue, with I-Coat, suggests selling patients on the value of good quality hard coats for the back surface. This will minimize product returns.

Back-surface processing. How can back-surface processing be improved? First, when grinding a lens, the goal is to get the smoothest surface possible.

According to Ed Ellefsen of Opticote, normal polishing is done with 1.5 to 2 micron polish. This produces billions of tiny scratches that the naked eye perceives as a nice polish. Under extreme magnification, however, these scratches look like a mountain range.

Sub-micron polish. As a result, sub-micron polish is preferred. This reduces the peaks and valleys that the AR coating covers. This is important since the higher peaks will wear down more quickly and create areas where the environment can breach the thin film which is only about .25 micron thick and undermine the AR coating.

Although scratch resistant coatings are not perfectly smooth, they tend to fill in most of the valleys, giving the AR coat a more supportive base. Ellefsen made this analogy: Put a tissue on a pillow and poke through it--it will tear with ease. But, if you put it on a hard table and poke it--it will stay intact.

Avoiding slick chemicals. Another factor in maintaining good adhesion is to avoid using slick chemicals, especially silicone, in any stage of production. This would include some mold release agents at the manufacturer's level, anti-foaming polish additives with silicone at the lab, and even some cleaners used before sending the lenses to be coated. Silicone is very difficult to remove and will cause a poor bond between AR coat and substrate.

Tinting and UV inhibitors. These can be another factor affecting AR adhesion because these dyes are absorbed into the surface of the lens and leave a residue that can prevent adhesion.

It is generally true, with a few exceptions, that the less tintable an SRC is, the better substrate it provides for the AR coat. This creates some difficulty in tinting because, typically, the scratch resistant coating itself absorbs most of the tint. Most lens materials can be tinted before the SRC is applied, but they must be tinted in their uncut form. It can be tricky to get gradient tints in the right place.

The advantage, related by Jack Jue, is that you lock in the color. In accelerated UV testing at I-Coat, there was noticeably less fading. The SRC now provides a sealing layer for the AR coating from the dye. If a tintable coating must be used (for example, on polycarbonate) it is advisable to immerse the tinted lens in hot water to remove the surface residue.

Cleaning lenses. Pay attention to details like cleaning lenses meticulously before sending for ARC. This cleaning can reduce scratches significantly. Ellefsen also recommends cleaning progressive lenses with alcohol before sending, to remove ink residue that may not be visible to the eye. The coating can fail where the ink is not removed.

Communicating with labs. Your coating lab can tell you how they prefer to 
work. Pay heed to what they tell you because this advice will be the culmination of all their experience.

Pleasing your customer may mean that you will buy a premium base lens on which to produce an AR coating. You may want to change your scratch resistant coating, or make changes in the curing time. Solvent based SRCs can evaporate making the SRC thicker, requiring more curing time.

Keeping records. It is also a good idea to do your own research over time. If you have had frustrations with AR coatings, you may be able to determine the cause with a little careful record keeping. Keep track of the brand and material of lens used, SRC curing types and times, and how the lenses were tinted. Query the patient to ascertain what part their treatment of the lenses had to play.

Using finished lenses. Finally, consider using finished ARC lenses whenever possible. Jue says many coating labs are working with manufacturers to stock finished lenses, not normally available with ARC, to speed delivery. Such business practices could account for around 30 percent of all lenses produced.

These lenses have factory applied, non-tintable SRC that makes them ideally suited for AR. The coating labs then develop the most compatible ARC for these lenses.

This type of innovation will surely increase growth in the AR market and encourage consumers to try AR.

And, by taking a few steps before ordering AR coated lenses, you'll see your own success rate soar. EB