PHOTOGRAPH BY NED MATURA; From top: Vuarnet VL006 from Vuarnet; Derek Lam 256 from Modo
By Barry Santini
If it’s been a while since you dispensed a pair of glass lenses, you’re not alone. Most ECPs regard glass as a poor choice for spectacle lenses, citing weight, cost, delivery time, impact resistance and antiquated choices in designs and treatments as its main detractors. Glass lenses—which only 60 years ago were once the only lenses you could get—today account for only a few percent of total lens sales in the U.S.
But after years of declining sales, interest in glass lenses is starting to grow anew.
Advances in free-form processing, thinner and lighter material choices together with the public’s unquenchable desire for “scratch-proof” eyewear are fueling a steadily growing demand for what glass lenses have always promised: superior optics, superior sun protection and unparalleled scratch resistance. To satisfy that demand, several U.S. independent labs, including FEA Industries, Luzerne Optical, Quest Labs, Optical Dynamics and Aura Optics have invested in the latest technology to process glass lens prescriptions. Sunglass companies such as Vuarnet, Serengeti and Costa are expanding their branded lens offerings in both plano and authentic prescription form. Manufacturers such as Barberini, Corning and Schott are continuing to produce glass lens blanks.
It’s beginning to look like a new dawn for glass lenses.
THE MYSTIQUE OF GLASS
The optical qualities of crown glass lenses (see sidebar) 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 to sweater or sponge—lenses made of glass do not protest a less than careful approach. In fact, glass lenses stored without a case will, with just a few quick wipes, look like a million dollars when the return to duty calls. But there is always the gorilla in the room with glass: the dilemma of its weight. On one hand, who wants heavy glasses? On the other, the added heft of glass lenses sends subtle signals of quality. Rather than light and cheap—“like plastic”—glass subliminally conveys substance and value.
THE MYTH OF GLASS
The myths of glass lenses are many, including that they are uncomfortably heavy, not available in current lens designs, and not pair-able 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.
In the U.S., 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 is the lowest class of risk. Even in view of the low risk of injury, the Food and Drug Administration (FDA) has implemented an impact resistance standard for dress and non-occupational eyeglasses, thereby safeguarding the public from the potential compounding effects of eye and facial injury if glass lenses break while being struck or involved in a concussive incident.
Under Title 21, Code of Federal Regulations, eyeglass lenses must comply with the impact resistance standard 21 CFR 801.410(d)(2), which states that finished lenses—those having both their surfacing and edging operations completed—before being mounted in a frame, must individually survive the impact of a 1-ounce steel ball, 1 inch in diameter, dropped from a height of 50 inches without breaking or visibly fracturing. The regulation does not, however, specify how the lens is to be prepared to pass the test, nor does it specify a minimum thickness that must be used. Professional wisdom about the “legality” of glass lenses has become inaccurately colored because ECPs have confused the best practices recommendations of the American National Standards Institute (ANSI), which states guidelines regarding minimum lens thickness, with the FDA’s standard for impact resistance, which is the law.
Lenses can be made compliant by several methods, including heat, chemical tempering or lamination. The careful wording of the standard allows for future developments that may advance the state of the art. One company already moving in this direction is Corning Glass Works, the developer of the ion exchange method of chemical tempering glass, which will soon launch a high-index of refraction, ophthalmic derivative of their Gorilla glass technology. This 1.6 index material promises to allow a 1.5-mm center or edge thickness lenses that will pass the impact resistance test with proper chemical tempering. For most prescriptions, including those that are plus or minus 3.00 diopters and below, glass is poised to help resurrect consumer demand for glass lenses. And remember that while plastic and other ophthalmic materials are allowed 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 an age of multimillion unit, worldwide air bag recalls, this fact can be quite attractive to the eyeglass wearer seeking the best quality.
DUTY TO ADVISE
Because of legal precedents in the last 35 years, presenting the choice of glass lenses has been thrown into an undeserved, negative light. After assessing a client’s visual needs, hobbies and lifestyle choices, eyecare professionals have been guided by rulings in tort law and product liability cases to recognize they have a responsibility to present to every consumer the full range of lens choices within a hierarchy of impact resistance—which has unfortunately become termed the “duty to warn.” A further deterrent is the requirement for the buyer to sign an informed consent statement, to be kept on file for seven years, detailing both the lens variety and final choice selected. This represents a serious obstacle to prescribing or recommending glass lenses. In the end, it really boils down to whether glass lenses are presented in a positive, benefit-filled manner, allowing the consumer make their preferred choice, or to engage in negative selling. Remember: Today, the sale of plano eyewear and sunwear in non-optical environments does not have to meet the current duty to warn and record-keeping requirements. Only businesses with doctors or licensed eyecare professionals present must comply with choice disclosure. Instead of duty to warn, perhaps it would be better to re-characterize this lens advisory process, renaming it instead to “duty to advise.”
FRAME CHOICE 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-fitting frame possible. To help distribute the weight properly, plastic frames featured 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 age of ultra-lightweight lenses, frames and spring hinges, to appreciate how important structural frame quality is to comfortable-fitting eyewear. Having to master this often deters younger opticians from foraging into the forest of fitting glass lenses. However, making a comfortable pair of eyewear containing glass lenses will pay big dividends for all your adjusting and fitting skills.
The basic fitting principles for eyewear with glass lenses include:
Maintaining 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. The use of super lightweight frames may appear to be a wise choice in offsetting the additional weight of glass lenses, but these types of frames should be avoided. Nylon frame materials, which are highly resistant to heat-induced spread, are recommended.
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.
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 of glasses slipping. Proper wrapping of the tip will help make this bony process an anchor for the glasses, ensuring a nearly slip-free fit.
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, and the use of these frames will help ensure a more comfortable fit with glass lenses.
Optimal bridge fit—Last but not least, be sure to carefully choose the proper size, angle and splay of the bridge pads. While sometimes overlooked when fitting lighter weight resin lenses, it is absolutely essential to success when fitting glass lenses. Mastery here represents one of the highest skills of the true optician’s art.
LEAVE EDGING TO THE EXPERTS
Even if you grew up in the age of glass lenses and presently have an in-office finishing system, you probably will find glass lens finishing capability absent. And even if you have in-office glass finishing, it is likely that your edger is not capable of executing all the latest beveling profiles necessary for today’s frames. With a number of labs investing in glass lens processing, it is best to find one to work with and to farm out the finishing, tempering and impact-testing compliance of your glass jobs. If the lab is a true specialist in glass, they’ll probably be offering free-form lens processing as well. And that is the most important reason for today’s renewed interest in glass lenses.
FREEDOM IN FREE-FORM PROCESSING
No other single element in the resurgence of glass lenses is more important than how free-form technology is allowing a large 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 actually go down in glass progressives made with free-form processing, because it is far cheaper to manufacture a lens digitally than conventionally. For example, using a semi-finished blank of Corning’s PhotoGray Extra glass, savings as high as 50 percent are achievable when compared to surfacing a molded progressive blank with conventional technology.
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, is finally able to offer the same optimization available for resin single-vision when made of glass. Glass is more stable and resistant to the blurring effects of vibration that occur when the free-form cutter transitions from the reading to the distance curve.
DOLLARS 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 most consumers will continue to pay for premium eyewear as long as they can see true product value being offered. Therefore, it makes sense for practices that pride themselves on quality to position glass as a true luxury material, one with a long and prestigious history.
As eyewear fashion will forever include smaller, iconic shapes, it is always the perfect time to present glass as an attractive lens choice. With glasses again being made of glass, perhaps eyewear itself will move up again in perceived quality, reversing some of its recent descent into commodity. Let’s celebrate a new dawn for a material whose historical significance and contribution to human advancement cannot be easily overstated. Glass: the finest optic available for your eyes.
Optics Fit for A King
Glass has been universally hailed as the highest standard for optical quality, with “optics as good as glass” being an oft heard marketing comparative. Let’s chart the various qualities that have earned glass its enviable reputation as the finest material for optical lenses.
The technique of glass blowing was developed in Egypt around 50 B.C. Unlike cast and mold-forming techniques used in metal working, 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 blow pipe. While air is blown through, the gathered glass stretches and inflates, like a balloon. As the thinner regions cool but remain viscous, expansion can continue without the surface puncturing. As the glass further cools, the high surface tension of its outer molecules align themselves into a microscopically smooth layer. This is 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 light is absorbed, which is why glass inherently filters these UV wavelengths. These two characteristics together form the foundation of the optical image-forming capabilities of 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. If a window was to be made, the spinning would continue, expanding and further flattening the crown into a round disk suitable for glazing. Over centuries, the name “crown glass” has remained, even as the manufacturing methods evolved beyond the early hand techniques used in glass blowing.
THE IMPORTANCE OF ABBÉ
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 Abbé, together laid down the basic theory, specifications and production techniques required for the production of high quality optical-grade glass and the manufacture of precision optics. While defining the desirable qualities of optical grade glass, including uniform refractive index and freedom from inclusions and waves, Dr. Abbé created an equation to describe how light is dispersed or spread into its constituent colors as it is refracted, which today is called its abbé value. Low dispersion abbé values for single elements of optical grade glass range from 50 to 85, with crown glass having a value of approximately 60. Over time, the family of crown glasses came to be defined as any glass having a low refractive index and high abbé value, which includes the 1.53 index and 60 abbé of ophthalmic crown glass. Even today, of all the classic optical aberrations, only the image degrading blur of low abbé value cannot be offset or eliminated through 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 claims for the latest resin coatings, which consist of test results showing how closely their performance compares to that of glass. However, even good results on both the Bayer test where abrasion resistance is measured in the haze gain observed 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, still leave some professionals and consumers doubting how accurately they correspond to their personal experience with resin lenses.
Using the Mohs scale of mineral hardness, a test of comparative scratch resistance, where a fingernail rates 2.0-2.5 and a diamond rates 10.0, crown glass rates between 5.5 and 6.0, approximately the hardness of a common pocket knife. Yet consumers continue to be problematic for eyecare professionals. Notoriously poor observers of proper lens care, routinely storing glasses on their head or hanging from their shirt, eyeglass wearers continue to be upset from unmet expectations. For example, they expect the benefit of scratch resistant coatings to actually 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 only material choice present at the birth of mathematically-based lens design. Starting with the custom-made Zeiss Punktal lenses and through the manufacturing efficiencies made possible by corrective curve design, the target optical performance of glass eyeglass lenses has always been 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, which would yield thinner and lighter lenses, was not an overriding concern for either 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, as befell polycarbonate lenses in the early years of their manufacture. Glass lenses have always remained pure to the highest standards of optical quality. But with the explosion of eyewear fashion in the 1980s, resin materials began their rise to market dominance because there was one glass characteristic that could not be overcome when mounted in large frames: weight.
WORTH THE WEIGHT
Today, as the pendulum of eyewear fashion beginning its swing back to smaller eye sizes, iconic shapes and vintage styles, specializing in glass lenses can yet be another differentiator for an eyecare practice. With new thinner, impact-resistant materials, free-form processing and the application of the latest coatings technology, glass lenses now offer 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 also 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.
Contributing editor Barry Santini is a New York State-licensed optician from Seaford, N.Y.
He thanks the following companies for their help with this article:
Prescription labs: FEA Industries, Aura Optics, Luzerne Optical, Optical Dynamics, Quest Labs
Glass lens manufacturers: Barberini, Corning, Schott
OEM glass Rx sunglass manufacturers: Vuarnet, Serengeti, Costa