Want Thinner, Lighter Lenses?
Order Them in a High-Index Material

As consumers, our interest in thin and light extends to more than just potato chips or beer — it has impacted eyeglasses in a very significant way. Most eyeglass wearers are nearsighted, and a basic physical property of nearsighted lenses is that their edges are thicker than their centers. The stronger the prescription, the thicker the edges (see lens drawings below).

Most of today's fashionable frames are made of plastic or metal with rims thinner than the lens itself. Also very popular are rimless mountings, where the lens edges are completely exposed. In either case, the lens edges are highly visible, and thicker edges can detract from the appearance of the eyewear.

Eyeglass lenses are able to correct vision because they bend light as it passes through the lens. The amount that light is bent in order to provide good vision is determined by the prescription provided by the eye doctor.

For weaker eyes, the number in the prescription is higher, and the lenses must bend the light more to provide clear vision. Prescriptions for nearsighted people begin with a minus symbol (-). If your prescription is -5.00, for example, you are very nearsighted and need a stronger lens than someone with a -2.00 prescription. (Read more about what the numbers in an eyeglass prescription mean.)

To bend light more, stronger minus lenses require thicker edges than weaker minus lenses. It's not unusual for a nearsighted prescription to increase over time, meaning lens edges will grow increasingly thicker.

This is where the new high-index lens materials come in. Chemists have created a variety of new plastic materials that bend light more than the conventional plastic used for eyeglasses. This means less lens material can be used to correct the same amount of nearsightedness.

Thinner. As a result of the ability to bend light more, nearsighted lenses made of high-index materials have thinner edges than the same prescription made from conventional plastic material.

Lighter. Thinner edges require less lens material, which reduces the overall weight of the lenses. Lenses made of high-index plastic are lighter than the same lenses made in conventional plastic, so they're more comfortable to wear. High-index glass lenses also have thinner edges, but high-index glass is heavier than conventional glass, so there is not as much weight savings with glass as there is with plastic lenses.

The overwhelming popularity of the first high-index lenses has led to the development of a variety of high-index materials, which are classified by how they bend light. The ability to bend light is controlled by the material's "index of refraction." This is a ratio that compares the speed of light traveling through a clear material with the speed of light traveling through air. For conventional plastic, the ratio (or "index") is 1.50. For glass, it is 1.52. Any lens material with an index higher than that of glass or plastic is considered to be high index. High-index plastic lenses are now available in a wide variety ranging from 1.53 to 1.74. High-index glass lenses are available as high as 1.90 index.
 

All other things being equal, a lens made from a 1.66 index material has thinner edges than a lens made from a 1.56 material. Also, generally speaking, the higher the index, the higher the cost.

The other controlling factor is the prescription. The highest index materials are used primarily for the strongest prescriptions.

If you want high-index lenses, be sure to ask for them, but rely on your eye doctor's or optician's advice regarding which index to use. They can explain which index makes the most sense for your exact prescription. Most popular lens designs (single vision, bifocal, progressive, photochromic, etc.) come in high-index materials, and your doctor or optician will know which ones are available in your prescription.

All lens materials block some light from passing through the lens. Conventional glass or plastic lenses keep eight percent of the light from reaching the eye. A minor side effect of higher index lens materials is that they block slightly more light (10 to 12 percent). However, when an anti-reflective coating is applied, high-index lenses transmit 99.5 percent of the light. For this and other reasons, anti-reflective lens coatings are a valuable addition when you order high-index lenses.