GLASS LENSES
Glass�New Twists On An Old Theme
It's not the lens your father wore. In fact, advances in glass are creating new niches for this old workhorse
By Joseph L. Bruneni
Photography by Peter Baker

Glass has been the primary material for making ophthalmic lenses since early in the 13th century. The only competitor to glass for centuries was natural rock crystal, a material often preferred by wealthy patrons because of the material's greater resistance to scratching and breaking. Called "Pebble" lenses, these rock crystal lenses were sturdy and long lasting, often passed from one generation to the next. 

Until the advent of World War I, all lenses in the United States were made from imported glass (primarily from Germany). The outbreak of war in Europe made imports from Germany impossible, and the U.S. government funded a program by Bausch & Lomb to create a glass-making plant here.

Glass lenses: Clear 16, courtesy of Corning

Those were the glory days for glass, and it took a second World War to produce a material that eventually would replace glass for eyewear in this country-CR 39�. Except for strong corrections, the weight of glass had not been a major annoyance for eyeglass consumers. Their greatest aggravation came from broken lenses. Eighty percent of the orders received by labs on a Monday morning were replacing broken lenses. However, the weight issue became important as frame sizes grew larger in the years following World War II. 

A second influence, of course, was an FDA ruling in 1971 that U.S. glass lenses must be hardened or tempered. The rule included a requirement that lenses had to be 2.2mm thick at their thinnest point. For many corrections, that ruling virtually doubled lens thickness (and weight). 

That event marked the start of a trend from glass to CR 39. Glass usage in the United States started dropping until about two years ago, when glass sales hit five percent, an all-time low. Some glass producers now say glass usage has increased during the past 18 months. Usage has certainly leveled out and may be increasing slightly.

Photochromic Glass

One thing sustaining the level of glass sales has been photochromic glass. Anyone who was in the industry during the mid-1960s when photochromic glass lenses were first introduced will remember the excitement that accompanied this event. For the next 30 years, anyone wanting lenses that darkened in the sun could only get them in glass. In fact, fully half of all glass sales in this country over the last 15 years have been photochromic. Even when a viable plastic photochromic lens became available in 1990, glass photochromic lenses continued to sell.

A New Lease on Life

The glass market began to change in the mid-1990s, due to a number of influences. As a result, some laboratories are now actively promoting glass lenses. 

Here are some of the reasons given for this change of heart regarding a lens material many had written off.

Frame Styles. Frame designs have steadily grown smaller since the mid-1990s. As a result of smaller frame sizes, the weight of glass lenses is not the issue it was a few years ago. 

New Materials. Another issue is the development of new glass substrates that can be processed thinner.

Two new substrates, both from Corning, have this property. The first, Clear 16�, carries a higher refractive index than crown glass (1.60) and the material can be surfaced to 1.5 center thickness and still pass the FDA drop ball test. The second, Thin & Dark�, carries the same index as conventional crown glass (1.52). Like Clear 16, the material can be surfaced to 1.5mm center thickness, producing lighter lenses.

Premium Lenses. With the development of these new materials, glass lenses now fall into the premium category. 

Increased AR Sales. Glass lenses have always been the preferred substrate for AR coatings. Glass is more stable, less temperature sensitive, and has none of the chemical inconsistencies common to coated and uncoated plastic lenses. As consumers learn to appreciate the value and advantage of AR, glass usage is expected to rise.

Superior Photochromics. Unlike plastic photochromics, glass' ability to darken and lighten lasts forever. That advantage should be explained when discussing glass photochromics with patients. 

Lens Designs. Glass multifocals are available in more segment styles and sizes than any other substrate. A number of useful occupational and special use multifocals are only produced in glass.

Consumer Awareness. Consumers generally know what glass is and understand many of its advantages. They do, however, voice a common concern, asking: "Isn't glass terribly heavy?" 

After explaining how the weight issue has changed, dispensers usually experience a positive response. 

Greater Lab Involvement. For labs, glass has been something of a dilemma. Some lab owners observed the gradual decline of glass orders and decided to simply abandon the market. Other labs took the opposite approach and joined forces to go out and actively promote glass. 

No Longer a Stepchild

One issue contributing to the general perception that glass was a dying category for labs came from the way glass is processed. Glass and plastic lenses are never produced on the same equipment partly because minute ground glass particles coming from processing glass will inevitably scratch plastic lenses produced on the same equipment. Much like a stepchild, the glass production line always seems to end up with equipment discarded by the plastic line. 

In some labs that is changing. As glass has become more profitable, some labs have invested in new equipment ordered specifically for processing glass. 

With all the advancements in lens technology that have taken place during the past 10 years, both eyewear consumers and eyecare professionals have come to expect the best vision possible. It's interesting to note that in the scientific field, the most precise optics are always made of glass. It's possible that this total acceptance of glass in the field of precision optics may ultimately impact ophthalmic optics as well. 

There is a major hurdle, however, facing anyone going out to promote glass lenses. Many of today's dispensers have virtually no experience with glass. For most of them, the only time they ever order glass is when a patient comes in wearing glass and refuses anything else. These dispensers were taught that glass was an old-fashioned material that should be discouraged. It is a mindset not easily changed.

For those who are wise enough to change that attitude, offering glass broadens the services of a practice and will be viewed as a positive advancement by patients. That's particularly true when the doctor or dispenser takes time to explain the benefits of this new niche material.