All About Glass

By Barry Santini, ABOM

Glass is a material possessing a deep and complex history. Stretching from the first use of volcanic obsidian as an arrowhead and cutting tool during the Stone Age, all the way to becoming the very foundation of today's worldwide telecommunication and Internet networks in the form of fiber optic cables, glass is arguably one of man's most important discoveries. In the year A.D. 1286, glass found itself paired in a new device, one now seen as worthy of inclusion in a list of the top 100 inventions of mankind, and one which will forever share its name: glasses.

But despite its long and storied history, today's eyecare professionals continue to see glass as a poor choice for spectacle lenses, citing weight, cost, delivery time, impact resistance and antiquated choices in design and treatments as its main detractors. Although glass currently makes up less than two percent of the $11.2 billion prescription market in the United States, advances in free-form processing, thinner and lighter material choices and the public's unquenchable desire for “scratch-proof” eyewear are fueling a growing demand for what glass lenses have always promised: superior optics, superior sun protection and unparalleled scratch resistance.

THE MYSTIQUE OF GLASS

The optical qualities of crown glass lenses (see side bar) are undisputed. Not only do glass surfaces highly resist the degrading scatter of fine scratches, they are also immune to the effects of contact and exposure to household solvents such as alcohol, acetone or the caustic pH of common cleaning agents, including oven and wheel cleaners. And no matter how you choose to clean them—from shirttail, sweater to sponge—lenses made of glass do not protest a careless approach. In fact, glass lenses stored without a case will, with just a few quick wipes, look like new. But there is always the elephant in the room with glass: the dilemma of weight: On one hand, who wants heavy glasses? On the other, the added heft in eyeglasses made of glass signals quality. Rather than light and cheap, “like plastic,” glass conveys a product of real substance and value.

THE MYTH OF GLASS

The myths of glass lenses are many, including that they are too heavy, not available in current lens designs and not pairable with a state-of-the art, anti-glare coating. But by far the biggest myth is that glass lenses have been outlawed by the U.S. government. The truth is actually a story about how a choice of words became less of a friend.

IMPACT RESISTANCE

In the United States, all prescription ophthalmic lenses must comply with the Federal Food, Drug and Cosmetic (FD&C) Act because being integral to a pair of prescription eyeglasses, they are covered as a component part of a medical device. Under section 201(h), Rx eyeglasses are classed as “... an instrument, apparatus... or accessory... intended to affect the structure or any function of the body of man..., which does not achieve its primary intended purposes through chemical action within or on the body of man..., and which is not dependent upon being metabolized for the achievement of any of its primary intended purposes.” Prescription eyeglasses are designated as a Class 1 medical device, which covers the lowest class of risk. Even though there is a low statistical risk of injury, the Food and Drug Administration (FDA) implemented an impact resistance standard for dress and nonoccupational eyeglasses. The intent of this standard is to safeguard the public from the potential compounding effects of eye or facial injuries if a glass lens should break when struck or otherwise involved in a concussive incident.

Under Title 21, Code of Federal Regulations, eyeglass lenses must comply with impact resistance standard 21 CFR 801.410(d)(2), which states that finished lenses—those having both surfacing and edging operations completed—must before being mounted in a frame, individually survive the impact of a 1-ounce steel ball, 5/8 inch in diameter, dropped from a height of 50 inches, without breaking or visibly fracturing. The regulation does not specify how the lens should be prepared to pass the test, nor does it specify a minimum thickness requirement be observed. However, professional wisdom about the “legality” of using or processing glass lenses has nonetheless become inaccurately colored as ECPs continue to confuse the best practices recommendations of the American National Standards Institute (ANSI), whose guidelines address minimum lens thicknesses, with the FDA's standard for impact resistance, which is law.

Lenses can be made compliant by several methods, including heat or chemical tempering or lamination. The careful wording of the impact standard allows for future developments that may further the state of the art. Soon, Corning Glass Works, the developer of the ion exchange method of chemical tempering glass, will launch a high-index, ophthalmic derivative of their Gorilla glass technology. This 1.6 index material promises to allow for processing lenses with 1.5 mm center or edge thickness that will pass the impact resistance test with appropriate chemical tempering. For prescriptions that are plus or minus 3.00 diopters and below, a range which covers 85 percent of Rx lenses dispensed, this new glass is poised to help resurrect consumer demand for glass lenses. And while plastic and other ophthalmic materials can employ batch testing for impact standard compliance, lenses made of glass must be individually tested, further assuring the ultimate in quality control and compliance for the end consumer. In today's age of multimillion unit worldwide air bag recalls, this fact alone could be quite attractive to the eyeglass wearer seeking the ultimate in quality.

DUTY TO ADVISE

Most eyecare professionals today do not realize that after assessing a client's visual needs, hobbies and lifestyle choices, they have a responsibility to present to consumers lens materials in a hierarchy of impact resistance. Unfortunately, guided by 35 years of rulings in tort law and product liability cases, this became known as the “duty to warn,” and the recommendation of glass lenses has been subsequently thrown into an undeserved and negative light. A further deterrent to the choice of glass lenses is the requirement for the buyer to sign an informed consent statement, to be kept on file for seven years, detailing the variety and final lens choice selected. This has represented a serious obstacle to doctors and staff prescribing or recommending glass lenses. But in the final analysis, the sale of glass lenses really boils down to whether they are presented in a positive, benefit-filled manner alongside the other material choices. Remember: The sale of plano eyewear and sunwear in non-optical environments is not required to meet the same advisory and record-keeping requirements that businesses who employ doctors or licensed eyecare professionals presently do. Instead of duty to warn, perhaps it's more accurate to re-characterize the lens advisory process as “duty to advise.”

THE FREEDOM IN FREE-FORM PROCESSING

No other single element in the resurgence of glass lenses is more important than how the advent of free-form lens processing technology is facilitating an increasing variety of glass lens designs to be made at reasonable cost. Through the use of digital surfacing, glass lenses are finally able to offer the same optimization available for resin single vision and progressive designs. Further, most traditional and specialty glass designs, including bifocals, trifocals, occupational multifocals and task-specific tints, including didymium, are now available with digital optimization. Even better, costs can actually go down in glass progressives made with free-form processing, because it is far cheaper to manufacture a lens digitally than to mold its front surface. For example, using a semi-finished puck of photo gray extra glass, savings as high as 50 percent are achievable compared to traditionally surfacing a molded progressive blank.

In addition, the full spectrum of sophisticated glass sun filters becomes instantly available with the application of free-form surfacing technology. Even digitally-based, lenticular edge-thinning techniques are now possible in glass free-form manufacture. And the holy grail of digital bifocal manufacture, the inside segment round bifocal—so difficult to produce with consistent quality in resin—actually becomes easier when made of glass. Glass is more stable and resistant to the blurring effects of vibration that can occur as the free-form cutter transitions from the reading to the distance curve.

OPTICS FIT FOR A KING

Glass has universally been considered the highest standard for optical quality, with “optics as good as glass” being an oft heard marketing comparative. Let's review the vari ous qualities that have earned glass its enviable reputation as the finest material for optical lenses.

MAKING GLASS

In Egypt around 50 B.C., the technique of glass blowing was developed. Unlike cast and mold-forming techniques used in metalworking, glass has material qualities that allow it to be formed using a blowing technique. In glass blowing, glass is heated into liquid state and gathered at the end of a blowpipe. While air is blown through, the gathered glass stretches and inflates, similar to a balloon. As the thinner regions cool but remain viscous, expansion continues without the surface puncturing. As the glass further cools, the high surface tension of its outer molecules aligns themselves into a microscopically smooth layer. This is one part of the reason that glass can be made transparent. The other part is that electrons within clear glass atoms do not easily absorb the energy of visible light photons, allowing them instead to pass through without being deviated or scattered. Note that higher energy, non-visible UVB photons are absorbed, which is why glass can inherently filter these shorter UV wavelengths. These two characteristics form the foundation of the optical image-forming capabilities of glass.

CROWN GLASS

Early glass was made primarily from silica, the primary ingredient in sand. As sand is made up of ground up pieces of rock and quartz, it is referred to as mineral glass. The name crown glass comes from the crown shape taken by molten glass as it is spun and blown. In the early days of glass blowing, if a window was to be made, the spinning would continue, expanding and flattening this crown into a round disk, suitable for glazing. Over centuries, the term “crown glass” has remained with us, even as the manufacturing methods evolved beyond the early hand techniques used in glass blowing.

CHOOSING A FRAME WITH GLASS LENSES: CHOOSE WISELY In the 1950s, as frame fashion began to blossom and lens sizes increased beyond the simple formula of Lens size = Pupillary Distance - Bridge Size, attention began to be paid to the size and shape of the nosepads and bridge area. In an era where the only choice in lens materials was glass, European and Guild-trained opticians prided themselves on helping people to select the best possible fitting frame. To help distribute the weight properly, plastic frames began to feature bridge pads with increased front-to-rear dimensions. Additionally, the hinges and the thickness of the plastic used were engineered to maintain structural rigidity, shape retention and adequate temple clamping force. It can be hard for opticians today, born into the golden age of ultra-lightweight lenses, frames and spring hinges, to appreciate the how important structural frame quality is to comfortable-fitting eyewear. Having to master this can deter younger opticians from foraging into the forest of fitting glass lenses. However, learning to make a comfortable pair of eyewear containing glass lenses will pay back big dividends in all your adjusting and fitting skill sets. The basic fitting principles for eyewear with glass lenses include: 1. Maintains good temple clamping force—Although some of the stronger spring hinges may be employed here, most frame choices should feature stiffer materials, quality construction and solid hinges. Although the use of super lightweight frames may appear to be a wise choice in offsetting the additional weight of glass lenses, these types of frame construction should be avoided. Nylon frame materials, which are highly resistant to heat-induced spread, are recommended. 2. Adequate temple length—When fitting glass lenses, too-short temple lengths must be avoided. Proper occipital tension and around the ear fitting is essential to help keep glass-lensed eyewear from constant slipping. 3. Fitting the mastoid bend—Some individuals will present a rounded bony extrusion behind the ear called the mastoid process. This high ground can easily become your best ally or your nemesis. Disregard contouring the temple around this protrusion, and your client will quickly become an adversary, with constant complaints that their glasses are slipping. Proper wrapping of the tip will help make this bony process an anchor for the glasses, ensuring a practically slip-free fit. 4. Make friction your friend—With satin and brushed finished plastics becoming a trend, use these styles to increase the friction between skin, hair and frame, and help offset the additional weight of glass. Further, some plastic styles feature rubberized coatings and rubber nosepad inserts. The use of these frames will help ensure a more comfortable fit with glass lenses. 5. Optimal bridge fit—Last but most assuredly not least, be sure to carefully choose the proper size, angle and splay of bridge and nosepads. While sometimes forgivable when fitting lighter weight resin lenses, it is essential to success when fitting glass lenses. And mastery here represents one of the highest skills of the true optician's art.

THE IMPORTANCE OF ABBE

In the middle of the 19th century, prompted in part by the desire to improve the performance of optical microscopes, the son of a window glass manufacturer, Otto Schott, along with instrument maker Carl Zeiss and mathematician Ernst Abbe, together laid down the basic theory, glass specifications and production techniques required for the production of high quality optical-grade glass and the manufacture of precision optics. Amongst defining desirable glass qualities, including uniform refractive index and freedom from inclusions and waves, Dr. Abbe created an equation to describe how light is dispersed or spread into its constituent colors as it is refracted, which is today called its Abbe value. Low dispersion Abbe values for single elements of optical grade glass range from 50 to 85, with crown glass having a nominal value of approximately 60. Over time, the family of crown glasses came to be considered as any glass having a low refractive index and high Abbe value, which includes ophthalmic crown glass, with its 1.530 index and 60 Abbe value. Even today, of all the classic optical aberrations, only the image-inducing blur of poor Abbe value cannot be offset or eliminated by design optimization in eyeglass lenses.

SCRATCH AND ABRASION RESISTANCE

Nothing points up the superiority of glass to resist abrasion and scratching better than the marketing comparisons found in the latest resin coatings, which cite how well scientific testing reveals these coatings are compared to glass. The results of the Bayer test, where abrasion resistance is measured in haze gain after the back and forth exposure to sand particles, and the tumble test, which attempts to simulate real-world scratch and abrasion exposure by tumbling lenses in a proprietary recipe of assorted sharp or abrasive substances, continue to leave both professionals and consumers to doubt how accurately they represent their personal experience with scratching and resin eyeglasses.

Using the Mohs scale of mineral hardness, a test of comparative scratch resistance, fingernail rates 2.0 to 2.5, diamond rates 10.0 and ophthalmic crown glass rates between 5.5 and 6.0, comparable to the hardness of a common pocketknife. Yet consumers, notoriously poor observers of proper lens care and perpetrators of routinely storing glasses on their head or hanging from their shirt, still have unmet expectations for the marketing claims of scratch resistant coatings that they hear as promising to be “scratch-proof.” Glass lenses to the rescue!

OPTICS GOOD AS GLASS

With a 700-year legacy, it's no wonder that glass has earned its reputation as the standard against which all other lenses are measured. But beyond its superior scratch resistance, low chromatic aberration and excellent sun filtering, there is another reason for the optical reputation of a glass lens. In the early 20th century, glass was the sole material choice present at the birth of mathematically-based lens design. Starting with the custom-made Zeiss Punktal lenses, through the manufacturing efficiencies made possible by corrective curve design; the optical performance of glass eyeglass lenses had never been less than paramount. Before the shift from function to fashion arrived with the oversized styles of the late 1970s, the need for flatter or aspheric base curves that would yield thinner and lighter lenses was not an overriding concern for opticians or consumers. Glass lenses were therefore never subject to the pressures that caused labs and dispensers to set aside optics in favor of fashion, which is what happened to polycarbonate lenses in the early years of their manufacture. Glass lenses have always remained pure to the standards of optical quality. But with the explosion of eyewear fashion in the 1980s, resin began its unstoppable rise to market penetration because there was always one glass characteristic that could not be overlooked when mounted in large frames: weight.

WORTH THE WEIGHT

Today, as the pendulum of eyewear fashion begins its swing back to smaller eye sizes, iconic shapes and vintage styles, the perfect opportunity to differentiate your practice by specializing in glass lenses is presenting itself. With new thinner, high-impact materials, free-form processing and the application of the latest coatings technology, glass lenses are now offering choices on par with the technology of resin alternatives, but with the renowned optics of glass.

Many independent and OEM labs now exist that can speedily deliver the quality of glass lenses at reasonable prices. Premium sunglass companies, such as Vuarnet and Costa, are offering the benefits of their branded sun styles in both plano and authentic prescription form. There's no longer any reason to “weight” before you offer patients the quality of glass lenses.

DOLLAR AND SENSE

Today more than ever, consumers expect full bang for their buck. For many reasons, many have felt that their prescription eyewear has been overpriced. But eyewear consumers will willingly continue to pay for premium eye-wear as long as they can see true product value being offered. So it makes sense for practices that pride themselves on quality to see glass not as a material in sunset, but as a true luxury material, one with a long and prestigious history.

As today's eyewear fashion pendulum begins its move back to iconic shapes and smaller eye sizes, perhaps it's the perfect time to present glass as an attractive lens choice. With glasses again made of glass, perhaps eyewear itself will move up in perceived quality, reversing some of its recent descent into commodity. Glass is a material whose historical significance and contribution to human advancement cannot be easily overstated. Glass: The finest optic available for your eyes.