Glass

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.

MAKING GLASS

The technique of glass blowing was developed in Egypt around 50 B.C. … 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, like a balloon. … as the glass cools, the high surface tension of its outer molecules align themselves into a microscopically smooth layer. … 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 imageforming capabilities of glass.

CROWN GLASS

Early glass was made primarily from silica, the primary ingredient in sand. … 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, … Carl Zeiss and mathematician Ernst Abbé, together laid down the basic theory … 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. … 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.

SCRATCH AND ABRASION RESISTANCE

… 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 Glass The history of glass 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.

… Glass lenses have always remained pure to the highest standards of optical quality …

​​​​​​​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.

… 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.

Santini, Barry. “Optics Fit for a King.” 20/20 Aug.2015 Online Version