Hi-index lenses are made of a special plastic material that refracts light in a different way than regular plastic lenses. With hi-index lenses, vision can be corrected with less material, making the lens much thinner. Plastics are graded in numbers, such as 1.50 or 1.67. The higher the number, the thinner the lens. Because they are thinner, hi-index lenses are also lighter, making them more comfortable to wear. This is important to patients with high prescriptions, as their glasses can be made more cosmetically attractive and appealing. Hi-index lens materials tend to cost more than standard plastic lenses.
Polycarbonate lenses are made of a type of plastic that is more impact-resistant than standard plastic lenses. Polycarbonate lenses are also much thinner and lighter in weight. They are considered a high index plastic. Polycarbonate lenses also have built-in ultraviolet protection. Because of these properties, it is the lens material of choice for children’s lenses, sport lenses and safety lenses. However, some people complain that polycarbonate lenses fail to give them the crispest, clearest vision.
Trivex is a relatively new material that is similar to polycarbonate lenses but with higher quality optics, and thus provides clearer vision. Trivex is lighter in weight than standard plastic but not quite as thin as polycarbonate. Trivex is a more rigid material, making it a better selection for rimless or drill mount frames. Trivex is just as impact resistant as polycarbonate and may also be prescribed for children’s lenses, sport lenses and safety lenses.
An aspheric lens design gives several advantages to eyeglass lenses. An aspheric lens has flatter peripheral curvatures than a regular spherical lens. This reduces the amount a lens may bulge out of an eyeglass frame, reducing magnification of the eyes, which improves cosmetic appearance. Flatter peripheral curves also reduce distortions that occur when looking away from the center of the lens, making vision much crisper. Aspheric lenses are also much lighter in weight than standard spherical lenses.
Anti-reflective coating is applied to eyeglass lenses to reduce the amount of internal and external reflections on a lens. This increases the amount of light transmitted through the lens, which improves quality of vision. Anti-reflective coating also decreases unwanted glare and halos at nighttime. It also makes the lenses appear somewhat invisible and very thin. While everyone could benefit from an anti-reflective coating, it is especially beneficial for people with high prescriptions, people who have a decrease in vision at night, and professions in which cosmetic appearance is important.
Scratch-resistant coatings are applied to the front and back of lenses in the manufacturing process. Although it is important to realize that no lens is scratch-proof, this special coating does make lenses harder to scratch when dropped or rubbed against a hard surface. While most lenses are made with scratch-resistant coating, sometimes the term scratch-resistant coating indicates a type of “scratch warranty.” These warranties guarantee against scratches, ensuring the lenses will be remade if a scratch does occur. Be sure to clarify any warranty of this nature.
Ultraviolet treatment is applied to lenses to protect against harmful UV sunrays that can accelerate the development of cataracts and macular degeneration. It is extremely important to protect eyes from the damaging effects of the sun. UV treatment is easy to apply to lenses and is often included with the purchase of eyeglasses.
Polarized lenses are usually used to make sunglasses. They are available most commonly in grey or brown tint but many other colors are available. Vertically polarized lenses decrease bright glare and reflections by blocking horizontal polarized reflected light. Polarized lenses have been used by fishermen for years to better deal with bright light being reflected off water and to see deeper into the water.
Photochromatic lenses have a special chemical coating that makes them change to a dark tint in the sunlight and turn clear indoors. Photochromatic lenses are great for people who do not wish to carry a separate pair of prescription sunglasses. It is important to recognize that these lenses do not darken as well while driving a car. The windshield prevents most of the UV light from reaching the lens.