Ophthalmic lenses: Progress and promise

Key Points

Ophthalmic lenses have come a long way! The state of lenses today is quite different from yesterday, and options continue to expand in materials, technology, and design at a faster rate than ever. This rate of change requires change in the way of thinking in regard to dispensing ophthalmic lenses and prescription eyewear. Gone are the days of glass, plastic, or polycarbonate.

One needs to have an understanding of materials, designs, and changes in processes of manufacture to appreciate today's high-tech quality options. Knowing where materials and designs originated helps in seeing the road from which we traveled, as well as gaining insight into where we are going in the world of ophthalmic lenses.

The original materials for ophthalmic lenses were mainly quartz crystal and crown glass. Many of us remember the days of crown glass and plastic, when polycarbonate had not been developed yet. We did the best we could to improve vision with these two choices. Soon, polycarbonate starting making its way into the market, and it has been a busy highway ever since.

Crown glass: While crown glass lenses are rarely dispensed in the United States, it continues to provide the best-quality optics. The index of refraction (n) of crown glass is n = 1.523, and it is often used as the standard when comparing different ophthalmic materials. The Abbe value of glass is high (58), so chromatic aberration is hardly an issue. The problem, though, is that it is extremely heavy, and not as safe as plastic-based materials. Crown glass remains popular in different parts of the world, given its durability against scratching and its consistent optical properties.

CR-39 plastic: Named after PPG's subsidiary, "Columbia Resin's 39th Formula," CR-39 has maintained its place in optics and continues to be a favorite among dispensers. CR-39 has many optical properties similar to glass; the Abbe value is excellent (58), the optics are consistent, and it is light-weight and comfortable. Its index of refraction is n = 1.498.

Polycarbonate: Made of polycarbonate granules (similar to plastic BBs, and fairly clear), this material has improved in optics and clarity over the years. Early polycarbonate lenses were flawed; however, the CD-ROM market forced manufacturers to improve quality. Its index of refraction is n = 1.586, and it has safety properties six times those of the FDA standards. With an Abbe value of 30, chromatic aberration-sensitive patients/clients may not be as happy as they would with a material with a higher Abbe value.

Trivex: Invented by PPG, Trivex is the first material invented specifically for visual purposes. While other lens materials made their way to consumers from other areas like the military and space travel, Trivex was invented to deliver the best of three worlds: quality optics, safety, and light-weight comfort. Its index of refraction is 1.530, and its Abbe value is 42 to 45, depending on manufacturer. Like polycarbonate, its safety properties exceed six times those of FDA safety standards, it has a high Abbe value, and it is a great option for drill-mounts. Keep an eye on this material-it will likely carve out a niche in the optical materials marketplace.

High-index materials: With all of the various indices of refractive materials offered (n = 1.6, 1.66, 1.67, 1.7, 1.72, 1.8), it is best to check individual manufacturers in regard to specifics like Abbe value and specific gravity. High-index materials are excellent, because they refract light more, so one does not need as much of them. This yields the same quality optics with less material, resulting in thinner, flatter lenses.

A word of caution, though: a high-index lens will reflect more light than the other materials. As the index of refraction of the material increases, so does the reflectance factor-the amount of light loss due to reflecting off multiple surfaces. When dispensing high-index lenses, it is a must to add anti-reflective coating to bring back the light lost due to reflectance.