You and your optician spent some time finding that perfect frame that reflects your personality, your lifestyle, your image. You have also determined what lens “style” best suits your lifestyle and visual needs. Here we are going to provide information on the various lens materials that are available to you and the benefits each has to offer. Lens material selection influences many factors including: appearance, comfort, clarity and safety.
Lens Materials Features and Benefits
In the early days of vision correction, all eyeglass lenses were made of glass. If you find you leave your glasses in your car and expose them to extreme heat, glass lenses retain their shape whereas plastic lenses may warp. Glass lenses are also more resistant to scratching than plastic. Although glass lenses offer exceptional optics in addition to the above benefits, they are heavy and can break easily, potentially causing serious harm to the eye. While glass lenses offer certain specialty benefits they have become the least requested material for eyeglasses.
In 1947, the Armorlite Lens Company in California introduced the first lightweight plastic eyeglass lenses. The lenses were made of a plastic polymer called CR-39, an abbreviation for “Columbia Resin 39,” because it was the 39th formulation of a thermal-cured plastic developed by PPG Industries in the early 1940’s. Virtually all lens styles are available in the CR-39 material however, plastic lenses are more prone to scratching. In order to prolong the life of plastic lenses it is recommended to add a premium scratch resistant coating. While CR-39 absorbs some UV Rays it needs to have a high quality UV filter added for further protection. Because of its light weight (about half the weight of glass), low cost and excellent optical qualities, CR-39 plastic remains a popular material for eyeglass lenses.
In the early 1970’s, Gentex Corporation introduced the first polycarbonate lenses for safety glasses. Originally developed for helmet visors for the Air Force, for “bulletproof glass” for banks and other safety applications, polycarbonate is lighter and significantly more impact-resistant than CR-39 plastic, making it a preferred material for children’s eyewear, safety glasses and sports eyewear.
Glasses with polycarbonate lenses offer the highest degree of protection to keep your child’s eyes out of harm’s way while providing clear, comfortable vision. Polycarbonate eyeglass lenses are 10 times more impact-resistant than glass or regular plastic lenses, and they exceed the FDA’s impact resistance requirements by more than 40 times. Anyone with permanent vision loss in one eye due to injury or disease should definitely wear glasses with polycarbonate lenses at all times — even if they don’t require prescription lenses to see clearly — to protect the vision in their “good” eye and prevent an injury that could be potentially blinding. Polycarbonate lenses are also about 20 percent thinner than standard plastic or glass lenses, so they are a good choice for anyone who wants slimmer, more attractive lenses.that is light weight.
Though polycarbonate is an extremely impact-resistant material, polycarbonate eyeglass lenses can be easily scratched without a protective scratch-resistant coating. Most polycarbonate eyeglass lenses come with a factory-applied, scratch-resistant coating to keep the lenses clear for as long as possible, even when worn by children. Also, polycarbonate lenses typically are sold with a warranty against scratches for a specified period of time. Ask your optician for details.
A newer lightweight eyeglass lens material with similar impact-resistant properties as polycarbonate is called Trivex, which was introduced for eyewear in 2001. A potential visual advantage of Trivex is its higher Abbe value (see below).
High Index Lenses
High-index eyeglass lenses are the right choice if you want thinner, lighter lenses and eyeglasses that are as attractive and comfortable as possible. Thinner, lighter high-index lenses are especially recommended if you have a strong eyeglass prescription for nearsightedness, farsightedness or astigmatism. But high-index lenses can make your eyeglasses noticeably slimmer, lighter and more attractive regardless of how strong your prescription is.
Lightweight lenses are even more of a benefit for farsighted prescriptions, which can make conventional lenses very heavy. And most high-index lenses also have an aspheric design, which gives them a slimmer, more attractive profile and reduces the magnified “bug-eye” look that conventional lenses cause in strong farsighted prescriptions
There is a wide variety of thin, lightweight high-index eyeglass lenses, based on how efficiently they bend light. The light-bending ability of eyeglass lenses is determined by the “index of refraction” of the lens material. This refractive index is the ratio of the speed of light when it travels through air to the speed of light when it passes through the lens material. The speed of light is reduced the more it is refracted as it passes through a lens material. Therefore, lenses that bend light more efficiently have a higher index of refraction than those that bend light less efficiently, and lenses with a higher refractive index are thinner than lenses of the same power made of materials of a lower refractive index.
In short: the higher the refractive index, the thinner the lens.