Have doctors and dispensers finally written off glass as a viable material for modern eyeglasses? Sales of glass lenses have continued going steadily downhill for the last 20 years, and floor surveys taken during recent optical trade shows indicate many younger eyecare professionals dispense virtually no glass lenses. Of greater concern to glass manufacturers is the fact that the only time glass is discussed in many offices is when the doctor or dispenser is trying to convince a die hard glass-wearing patient to switch from glass to plastic.

This leads us to ask the question: Are glass lenses so outmoded that they should be eliminated from the eyecare professional's armament? Let's examine the current lens market so you can decide for yourself.

Three Reasons

Glass advocates believe three current factors point to a re-emergence of glass as a viable lens material in the United States. It's important to add those words "in the United States" as a reminder that, in the rest of the world, glass still outsells all other lens materials. Only in the United States have sales of glass lenses fallen to a single-digit market share.

End of the decline. Sales of glass lenses have continued to decline during the past 12 months, but glass manufacturers report that this long-term erosion of the glass market definitely slowed in 1999 and that glass sales may even be static. In a lens market that, by all reports, has been flat for a number of months, it is significant that one glass manufacturer (Corning) is reporting sales increases in all glass products. They believe increased sales are evidence that there is still a firm base of consumers who want the benefits that come with glass.

Reawakened awareness. Frame sizes have come down dramatically in the last few years, and this has an immediate and direct influence on the weight issue.

With smaller frames, corrections as high as 4.00D are viable in glass, and weight differences between glass and plastic are negligible -- less than a couple of grams. Many dispensers, however, haven't considered how much frame size relates to the weight of eyewear. When they do, an awakened awareness of glass often results.

New products. New glass materials are encouraging manufacturers to broaden their glass lines. For example, one popular aspheric line of lenses was recently expanded by duplicating the series in high index glass. Made of Clears 16� high index glass, the lenses are available in both plus and minus and, when chemtempered, can be surfaced to 1.5 centers.

World Market

The optical industry has become a world market, and some prominent foreign lens manufacturers have become increasingly active in the U.S. market. These companies produce extensive ranges of glass lens products for their home markets. This wider range of products will help glass become more viable in this country.

Much like single vision plastic lenses, many glass lenses currently sold in the United States are produced offshore. Glass, particularly in multifocal form, is very labor-intensive, and this factor has accelerated the move to foreign production lines.

Producing lenses in glass does have certain advantages over plastic lens production. It's easier (and less costly) to manufacture special-order multifocals in glass. Glass production is component-based rather than tool- or mold-based as plastic lenses are. For example, to produce a special size bifocal or trifocal segment in glass is relatively easy. To produce the same blank in plastic requires special molds and would be prohibitively expensive.

Legal Issues

Legal issues have long influenced the use of glass. In fact, adoption of the FDA ruling that glass lenses must be made to a minimum thickness effectively doomed glass lenses in the United States. At the time of the ruling in 1972, the only effective way to increase impact resistance of glass lenses was heat treating. In this process (which takes just minutes), lenses are heated almost to the melting point and then quenched with a blast of cold air. This compresses surface molecules and substantially increases resistance to breakage. Scratch a heat-treated lens, however, and this compression is dissipated and impact resistance is substantially reduced.

Some time after the FDA ruling, an improved method of increasing the impact resistance of glass lenses was developed. In this process, lenses are submerged in a bath of molten salts for a period of time. During submersion, an ion exchange of surface molecules takes place, creating a more effective compressed surface for the lens.

Today, chemical tempering, or "chemtempering" of glass lenses is the preferred method and produces a considerably safer lens. Unfortunately, the process requires submersion in the salt bath for 16 hours. Because of the time requirement, labs must use heat-treating rather than chemtempering when a rush job is required.

A minimum thickness requirement for glass lenses has been dropped, and current FDA regulations are now performance-based. As a result, any chemtempered glass lens that passes a drop ball test is a legal lens, no matter what thickness it is. For Z87 industrial safety use, however, 3mm is still required, and lenses heat-treated in an oven must still follow the 2mm minimum thickness rule.

Corning produces two modified types of glass that can be routinely processed to 1.5 center thickness. The first, Thin & Dark�, darkens and lightens faster than standard photochromic glass. The second is a high index (1.60) clear glass called Clear 16�. When lenses made of either new glass are chemtempered, they can be processed to 1.5 centers.

Since there is no minimum thickness requirement for dress lenses, can a 1.0mm glass lens pass the drop ball test? In point of fact, it's likely that a properly surfaced and tempered glass lens might pass the test, but most labs will refuse such an order. The reason is simple. Labs must replace any lens that breaks during the drop ball test. Labs have learned to err on the side of caution.

Glass and Sunglasses

Many of the world's best-selling sunglasses are still made with glass lenses. World-renown names such as Ray-Ban, Maui Jim, Revo, and Serengeti are examples. There are compelling reasons why these companies' designers prefer glass for their sunglasses. For one thing, weight is seldom a major factor, since the lenses are plano. Glass provides consistent performance. They also like glass because it has built-in scratch resistance that lasts for the life of the lenses, and because it lends itself to thin film coating -- an important component in sun lens design.

Benefits of Glass

All told, there are several important benefits to glass.

Industrial. In spite of polycarbonate, glass is still widely preferred by safety engineers for certain industrial occupations. In many industrial environments, plastic or poly safety lenses last only a few days before becoming too scratched or pitted for use.

High index. The recent announcement of a 1.70 high index plastic was treated as a major industry event, while high index glass in 1.70 has been available for three decades and 1.80 and 1.90 index in glass has been available for 10 years.

Tints. Colors in glass are very stable. Replacing one lens in a 30-year-old pair of sunglasses will reveal the color of the old lens exactly matches the new lens.

Niche products. Many special-application lenses are best supplied in glass, including x-ray lenses, radiation-blocking lenses, laser protection glasses, and a wide range of special sunglass applications.

AR coatings. Glass provides the most stable base for AR coatings. With plastic lenses there can be incredible variations between the available substrates, factory-applied front side hard coats, and lab-applied backside hard coats, as well as between thermal-cure coatings and UV-cure coatings. With a glass lens, there are no variations.

Knock-about glasses. Occasional-wear eyewear like computer glasses and reading glasses usually receive rough treatment and are ideal candidates for the superior scratch protection offered by glass. Weight is less of a factor because these glasses are seldom worn for long periods of time.

The Future for Glass

There has been a Glass Council for a number of years. Among recent Council projects was the creation of an ABO-approved seminar on glass lenses titled, "The Original Premium Lens." As one might expect, the chairman of the Council, Corning's Cathy Lehnhausen, is upbeat on the future for glass. "Independent eyecare professionals and wholesale laboratories are looking for premium products and programs which will distinguish them as the specialist," she says. "That's our target audience and is where premium glass materials will enjoy great success."

The new premium glass materials are not commodity materials, and offer enhanced profits, improved performance, and better distribution opportunities for both independent practitioners and laboratories. So, practitioners can use glass to distance themselves from those resin products that have become commodities.

Renewed Lab Interest

Many labs today are not equipped to process glass. They find it more expedient to farm out glass orders to labs that specialize in fabricating this material. There are a number of laboratories that are skilled in processing glass and want this business. In fact, 33 laboratories have joined forces in the The Corning Connection -- a group created to help labs promote the concept of using premium glass to differentiate a professional eyecare practice.

For a practitioner, as well as the lab, glass represents a point of differentiation. The fact that many doctors and dispensers ignore glass creates a great opportunity for those who do take the time. Understanding all this, even a true skeptic would probably agree that it's not yet time to write off glass. EB