GLASS
THE (PRISTINE) WINDOW TO THE EYES
When optics matter, glass has been—and continues to be—the premium lens material of choice
BY SUSAN TARRANT
When you’re talking pure optical clarity, the original spectacle lens material remains king today. Though other materials have since come along, glass remains the material of choice when “seeing clearly” is the overriding consideration.
In fact, when superior optics and durability are desired—in spectacle lenses and even in non-optical product—glass is, and always has been, the go-to material.
Among the many advantages of glass as an ophthalmic material is its structure, which enables lenses to be produced in a range of refractive indices without significantly altering glass’ intrinsic optical, physical, and chemical properties.
New material substrates, such as Corning® Clear 15, and free-form processing technologies have created the ability to process glass lenses thinner (and, therefore, lighter).
Did you know that glass lenses must adhere to stricter drop-ball testing than other lens materials?
All dress eyewear ophthalmic lenses must pass drop-ball testing. But non-glass material (CR39, polycarbonate, plastic resins, etc.) can be batch-tested—meaning a statistically significant sample out of every batch of lenses is given the test.
For glass lenses, it’s different. Per FDA regulations, each and every glass lens must pass a drop-ball test.
It’s all to ensure eyecare professionals and patients alike are reassured of the safety and wearability of the lenses. Every single one.
WHEN CLARITY COUNTS
When excellent optical clarity is the most important element of a product, which material is chosen?
Think cameras. And, the Hubble telescope lens. Or, think about the lens in your phoropter and the display of your high-definition iPhone. It’s glass—universally accepted to provide the sharpest, clearest optics of all lens materials.
Why?
Though it will vary depending on the index and makeup of the glass, glass lenses have the highest Abbe value (crown glass has an Abbe value of 59). The higher the Abbe value, the less dispersion of light occurs when passing through the lens and the less chromatic aberration is experienced by the wearer. That equates to pristine optics.
Because of today’s glass lenses’ sharp optics, lighter and thinner profile, and better-than-ever strength and safety, they are making a strong comeback as an option in the premium product arena for prescription eyewear and beyond.
In fact, glass lenses are in high demand in the premium sunglass market, where superior optics is a coveted feature. Performance-focused brands, such as Serengeti and others, are expanding their offerings of glass sun lenses (in both plano and Rx lenses) due to consumer demand.
UNPARALLELED TOUGHNESS
There is a reason that other lens materials and treatments compare their own features (such as scratch resistance) to those of glass lenses. The scratch resistance of glass is unparalleled in the optical marketplace. It’s glass—and, it’s naturally scratch-resistant.
Glass, by its very nature, is also extremely resistant to chemicals, acids, alkalis, and other abrasives. Windex, acetone, and other everyday solvents simply do not damage glass lenses.
All of these features make it a very popular choice for those working in the elements or for whom toughness is desired.
Glass also provides a very stable base for AR and other treatments. Though not tintable after manufacture, its rigid substrate (versus more flexible substrates of other lens materials) provides an excellent surface for coatings.
Today’s glass lens material also helps to promote excellent, stable photochromic performance, including color consistency and temperature sensitivity.
Choosing today’s new glass lenses for the right patients in the right situation is a win-win for both eyecare professionals and their customers.
LOOKING TO THE FUTURE
Even in non-optical applications, we find ourselves in what’s being referred to as the Glass Age—where science is creating new possibilities for glass-enabled technology and design. These possibilities have resulted in the ubiquitous optical fiber and Gorilla Glass, which evolved from a method of strengthening glass developed by Corning years ago and is now used in smart phone and other digital device display screens. Gorilla Glass is always evolving, as is a newer innovation called Willow Glass, a very thin, very bendable (yes, bendable!) glass.
Glass pioneers like Corning are getting closer and closer to what many consider the Holy Grail of materials—very thin and nearly unbreakable glass.
GLASS—THE PIONEER MATERIAL
Glass is primarily made of silica, the main ingredient of beach sand. Silica, mixed with some other ingredients and treated, becomes glass.
Though the earliest glasses were made from natural crystal, they were too heavy to wear on the face and were still quite blurry.
Glass lenses were lighter to wear, but chromatic aberration was a problem. In 1730, English barrister and amateur optician Chester Moore put two glass lenses together (one made from crown glass and one from a newer “flint glass”) to correct the problem. It worked, and glass spectacle lenses were in demand from the mid-1700s on.
(Source: American Academy of Ophthalmology Museum of Vision)
It wasn’t until around the mid-1900s that other lens materials were developed—and became popular as fashion considerations (such as oversized frames) made the weight of glass lenses undesirable.
Today, new glass lens technologies have answered such issues, however, with glass substrates that can be ground thinner (and lighter) than ever before.
Corning glass lenses are produced in the company’s facility in France
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