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 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, 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 (or refraction) 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. 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.
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 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.
Left: High-index lenses can be much thinner and lighter, even in a strong prescription. Right: The three basic types of eyeglass lenses: no correction (such as non-prescription sunglasses); farsighted lenses; and nearsighted lenses. (Photo: Vision Consultants, Inc.)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. Also, generally speaking, the higher the index, the higher the cost of the 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.
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. Your eye care practitioner can explain which index makes the most sense for your exact prescription. Most popular lens designs and features (single vision, bifocal, progressive, photochromic, etc.) are available in high-index material. But there are exceptions, and your eye doctor or optician will know which high-index lens options are available in your prescription.
AR Coating: 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 glass or 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 glass or plastic lenses. However, when an anti-reflective lens coating (AR coating) is applied, high-index lenses transmit 99.5 percent of the light. And by allowing more light to enter the eye, AR coatings provide sharper night vision with less glare — a real advantage for night drivers.
Because AR coatings 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.
