High-Index Lenses Are Thinner and Lighter

Thinner and lighter high-index lens materials have impacted eyeglasses in a very significant way. Thin eyeglasses are attractive; thick ones aren't. Light eyeglasses are comfortable; heavy ones aren't. So it's no surprise that most of us want the thinnest, lightest eyeglasses possible.

Most eyeglass wearers are nearsighted, and require the basic physical property of lenses with edges that are thicker than their centers. The stronger the prescription, the thicker the lens edge will be.

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

How High-Index Lenses Differ From Regular Lenses

Eyeglass lenses are able to correct vision because they bend light as it passes through the lens. The amount of light-bending that's needed to provide good vision is determined by the eyeglass prescription provided by your 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 diopters, for example, you are very nearsighted and need a stronger lens than someone with a -2.00 prescription.

To bend light more, stronger minus lenses require thicker edges than weaker minus lenses. It's not unusual for a nearsighted prescription to worsen over time, which means the edges of your lenses will grow increasingly thicker with each prescription change.

Fortunately, chemists have created a variety of new plastic lens materials that bend light more efficiently than the conventional plastic lenses used for eyeglasses. This means less material can be used in high-index lenses to correct the same amount of nearsightedness.

Advantages of High-Index Lenses

Thinner. Because of the ability to bend light more efficiently, nearsighted lenses made of high-index materials have thinner edges than the same prescription made from conventional plastic materials of the same prescription power.

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. Most high-index lenses also have an aspheric design, which makes them flatter and reduces the magnified "bug-eye" look that conventional lenses cause in strong farsighted prescriptions.

Many High-Index Lens Choices

Different varieties of thinner, lighter high-index lenses are classified by how well they bend light. The ability to bend light is controlled by the material's "index of refraction," a ratio that compares the speed of light when it travels through air with the speed of light when it passes through a clear material. If a material bends light more, speed is slowed as well. So the higher the refractive index of a lens material, the thinner the lens will be.

For conventional plastic, the ratio (or "index") is 1.50. For glass, it's 1.52. Any lens material with a refractive index that's higher than that of glass or plastic is considered to be high-index. High-index plastic lenses are now available in a wide variety of refractive indices, typically ranging from 1.53 to 1.74.
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. Lenses with an index of refraction of 1.70 or higher are typically at least 50 percent thinner than conventional plastic lenses. Your eyeglass prescription also determines what kind of high-index material you want for your lens. The highest index materials are used primarily for the strongest prescriptions.

AR Technology: A Perfect Companion for High-Index Lenses

All lens materials block some light from passing through the lens. This light reflects back from the lens surface, causing distractions and reducing the clarity of night vision.

Conventional plastic lenses reflect about 8 percent of the light that otherwise would reach the eye. High-index lenses reflect up to 50 percent more light than conventional plastic lenses. However, when an anti-reflective lens technology is applied, high-index lenses transmit 99.5 percent of the light. And by allowing more light to enter the eye, AR technology provides sharper night vision with less annoying glare which is a real advantage for night drivers.

Because AR technologies also eliminate lens reflections, they make high-index lenses appear even thinner. This is a big plus if you want to improve your appearance in eyeglasses.