LENS COATINGS...a growing role
What you should know about the importance of the new anti-scratch and AR coatings with today's high-tech lenses
BY JOSEPH L. BRUNENI

Coatings have also become an acknowledged component of modern lenses, and uses for lens coatings are expected to continue growing. The industry certainly has come a long way from those early attempts to find a way to eliminate hairline scratches on CR 39 lenses.

THEN AND NOW

Many of today's high-tech lenses now involve some type of lens coating. Before plastic lenses, however, lens coatings were unknown. The introduction of lightweight plastic lenses more than 50 years ago started slowly. People were attracted by the fact that their glasses could now be half the weight of glass lenses, but they found lightweight plastic came with a new hazard--hairline scratches. It took more than 20 years for plastic lenses to become popular, and when they did, it was primarily because of an FDA ruling that glass lenses had to be tempered, making them thicker and heavier than people were used to.

Despite the weight issue, plastic lenses needed to stand up to the wear and tear of daily use to be truly embraced. There were a number of attempts to develop a scratch coating for plastic lenses. One of the most effective scratch coatings was developed by 3M. The company was so enthused about the effectiveness of its scratch coating, it bought Armorlite, the first company to manufacture plastic lenses (today's Signet Armorlite) in 1978. Before long, lens manufacturers were offering coated and uncoated CR 39 lenses.

As high index lenses became popular, they brought other issues with them. For the most part, high index substrates are softer than CR 39 and, as a result, scratch coatings became essential for these softer, more expensive lenses. As the use of high index lenses increased, scratch coating became more of a necessity. Today, most high index lenses have a factory-applied scratch coating on at least the front surface.

PHOTO COURTESY OF SATIS VACUUM An interior view of an AR chamber. Lens manufacturers typically coat thousands of lenses at a time, providing consistency

SCRATCHING THE SURFACE

Scratch coatings fall into two basic categories and it's important to understand the differences between the two.

Thermal-cure scratch coatings. Most scratch coatings applied by a lens manufacturer are thermal-cure. The lenses are dip coated (covering both sides) and cured in large ovens for some 16 hours. Thermal-cure coatings tend to be non-tintable. Manufacturers individualize the thermal cure coating to the specific index of the lenses being coated. Thermal-cure coatings require highly trained operators and are considered the most efficient scratch coating.

UV-cure scratch coatings. These are typically applied in a spin technique, and the coating is applied only to the backside. The coating is tintable and can be applied to any index. The primary advantages are a fast cure cycle and the fact that little operator training is required.

	PHOTO COURTESY OF VISION CONSULTANTS The entire AR coating process is guided and controlled by computer

THE POLY REVOLUTION

The next step in the evolution of lens coatings came with the introduction of lenses made of polycarbonate. Poly is so soft that lenses must always have scratch coating. During the first few years of poly, lenses were produced only in stock lens form, but when semi-finished poly became available, labs encountered a major problem. Surfacing a polycarbonate lens removes the factory-applied scratch coating on the backside. This meant labs had to apply scratch coating to the backside of surfaced poly lenses. At that time, the late 1980s, labs weren't equipped to apply scratch coating.

Traditional scratch coatings required curing lenses in an oven for 16 hours, something totally impractical in a lab where lenses must be delivered fast.

The single-most significant event launching polycarbonate lenses into the mainstream was the development of coating equipment for applying a fast-cure scratch coating on the backs of surfaced poly lenses. UV-cure scratch coatings didn't require 16 hours in an oven. They were cured by exposure to ultraviolet light. Labs could now coat and cure lenses in minutes instead of overnight. The development of UV-cure coatings for labs marked the start of the age of polycarbonate. Today, any surfacing laboratory processing poly is applying scratch coating in-house. Labs don't apply backside scratch coating on all high index lenses, but most quality labs will coat the back of high index lenses of 1.60 or higher.

As seen in this example, AR coatings prepared by manufacturers have layers to ensure adhesion and optical quality

AR AWAKENING

As the use of anti-reflective coatings grew in this country, lens coatings entered a new phase. When labs began to install in-house AR coating equipment, they found the efficiency of AR coatings was largely influenced by what was under the AR. AR coaters have always preferred to apply AR coatings to scratch-coated lenses. Uncoated lenses have a soft, porous surface that doesn't lend itself to the application of AR. The growing use of higher index lenses, usually with scratch coating on at least the front surface, seemed to be exactly what AR coaters wanted. Instead, a whole new can of worms opened up.

Adherence challenges. Labs discovered that when the scratch coating under AR fails to stick to the lens substrate, both scratch coating and AR will slough off the lens. Often, the AR coating gets the blame for failure when the actual cause was a poorly adhering scratch coating.

Uniformity troubles. Front side scratch coatings are applied by the manufacturer and thermally cured in an oven. Lenses surfaced in a laboratory either have no backside scratch coating or have a UV-cured coating. The chemistry in UV-cured coatings differs from thermal-cure coatings. Coaters now recognize that all these variations under AR make it difficult to produce uniform AR coatings that perform equally well on every lens.

	PHOTO COURTESY OF VISION CONSULTANTS Coating performance relies on thorough cleaning before application

TAKING STOCK

Manufacturers typically coat thousands of lenses that are identical in every respect except for lens power. This gives them two advantages

Total control, total success. Manufacturers know the chemistry of both substrate and scratch coating on the lenses, and they can fine-tune the AR process to those lenses. As a result, factory-coated AR stock lenses generally have superb AR coatings that perform exactly as they should.

Branding and building consistency. Recognizing the advantages of total control of everything under the AR led to the development of branded AR coatings. They start with uncoated lenses and apply thermal-cure scratch coatings on both sides. These AR coatings aren't available for all lenses, they take longer to process, and they tend to be more expensive, but they produce superb AR coatings.

Controlling every facet of the lens being coated is difficult, if not impossible for independent laboratories. Most labs process lenses from a variety of manufacturers and deal with substrates of differing indices. Research has gone into developing a primer coat which labs could dip lenses in before AR-coating them. The primer coat covers any scratch coatings on the lens and equalizes all the lens variations labs must deal with. The primer coat also provides the AR coating with a firm, consistent base to adhere to. These primer coats have been developed and may prove to be the best way for labs to deal with the many lens variations .

PHOTO COURTESY OF VISION CONSULTANTS Edged and uncut lenses are loaded into the chamber for AR coating

LENSES RULE

What does the creation of so many coating options mean to your dispensary? A definite shift can be seen in the bottom line for eyewear sales. For the first time, the total dollars generated by the sale of lenses outperformed dollars generated by frame sales. That trend, spotted for the first time about three years ago, is a bellwether event for both the industry and the eyecare professions.

It certainly doesn't mean that frame fashions have become unimportant. On the contrary, fashion remains a vital ingredient in eyecare. In fact, most eyecare practitioners acknowledge that half of the people coming in for eye exams today do so because they want a new look, not because they aren't seeing properly. Fashion in eyewear continues to be a major economic force for the industry.

The significance of lens sales surpassing frame sales is simply this: A wealth of new technology has transformed ophthalmic lenses from simple visual devices into high- performance appliances that enhance vision in ways never before possible.

That being said, however, the real reason lenses now rule the roost is because of the way their cost has soared. Technology has its price, and it's not unusual today for these high-performance lenses to cost far more than the frame, quite a change just from a few years ago.

How AR WORKS
Anti-reflective coatings let in light to cancel reflective light. In the charts below, the process is explained.