High index lenses are made of materials that are "compressed," or denser, so the same amount of visual correction is taking place using less lens material than traditional plastic or glass requires. "High index" means that the lenses are constructed of a plastic or glass material that has a higher index of refraction. The "index of refraction" refers to the speed that light travels as it passes through the lens material.

Why High Index Lenses Are Thinner

Eyeglass lenses work to correct vision by bending light as it passes through the lens. The amount that the light must be bent in order to give you good vision is determined by the prescription that your eye doctor provides. The higher the number in the prescription, the stronger that prescription is, and the more the light must be bent in order to correct your vision. (For example, if your prescription is - 5.00, you need a stronger lens than someone with a -2.00 prescription.) So people with stronger prescriptions traditionally have had thick lenses -- that is, their lenses provided more visual correction than the thin lenses worn by others.

But lens materials with a higher index of refraction (or IOR) allow light to pass more quickly through the lens, so less of the actual lens material needs to be used. That's why lenses with a higher index of refraction, or high index lenses, can be thinner than traditional lenses.

High index lenses are available in either glass or plastic. Different manufacturers make different high index lenses, and what sets each lens apart from the others is its index of refraction. The higher the IOR, the denser the material. All things being equal, a 1.66 IOR material will result in a thinner and lighter-weight lens than a 1.57 IOR material will.

Generally, the higher the IOR, the higher the cost of the lenses will be. The thinnest, lightest-weight lenses are typically the most expensive. With high index lenses, it seems that the less you get, the more you pay.

Regardless of the index of refraction, plastic high index lenses tend to be lighter in weight than glass high index lenses, simply because plastic weighs less than glass to begin with.

Polycarbonate lenses are not only thinner and lighter in weight than traditional plastic lenses, they also offer ultraviolet protection and scratch-resistance. In addition, they are very impact-resistant. This extra toughness makes them the lenses of choice for children's, sports and safety eyewear.

The flexibility or softness of the lens material dictates that a scratch-resistant coating be applied to the front and back surfaces of polycarbonate lenses. Manufacturers automatically coat them, and fabrication laboratories add a scratch-resistant coating to any uncoated lens surface before sending the lens to a dispenser. So any polycarbonate lens you buy already has built-in scratch-resistance.

Impact-Resistance

Polycarbonate lenses are safer than traditional lens materials because they aren't brittle, so they won't usually break if they come into contact with fast-moving objects such as balls, racquets or BBs. Regular glass and plastic eyeglass lens materials shatter upon violent contact in the same way that car safety glass does. Pieces of the lens are sprayed around, which potentially can cause injury to the eye. That's why polycarbonate lenses, which hold up better under impact, are required in sports goggles and are recommended for all children's glasses and some safety eyewear.

Multifocals let you focus through different prescriptions at different distances through the same lens -- hence the name. Bifocals (meaning a lens with two points of focus -- usually one for distance and one for near) are the most commonly prescribed multifocal lenses.

Many people need some visual correction in order to read or see things close up. Often, bifocals are necessary because people's arms "become shorter" (actually a condition called "presbyopia") as they enter middle age and things closer to them become more difficult to see. But other conditions can cause people of any age to need more help seeing properly in the near range. Overconvergence, when the eyes work too hard to see close up, is one.

Regardless of the reason you need a prescription for near-vision correction, bifocals all work in the same way. A small portion of the eyeglass lens is reserved for the near-vision correction. The rest of the lens is usually a distance correction, but sometimes has no correction at all in it.

The segment that is devoted to near-vision correction can be in one of several shapes:

• A half-moon, also called a flat-top, straight-top or D segment;
• A round segment;
• A narrow rectangular area, known as a ribbon segment;
• Or a full bottom half of a lens, called the Franklin, Executive or E style.

Similar to bifocals are trifocals, or lenses with three points of focus -- usually for distance, intermediate and near. Trifocals have an added segment above the bifocal for viewing things in the intermediate zone, which is farther than the near zone -- about arm's length away. Computers are an excellent example of something that is in a person's intermediate zone. Motorists who need to see in the distance to drive, to see the gauges on the dashboard, and to read a map also would benefit from a trifocal. Flat-top and Executive lens styles are the most common trifocals.

Coatings can enhance the performance and appearance of eyeglass lenses.

Scratch-Resistant Coatings

No lens material -- not even glass -- is scratch-proof. But a lens that is treated front and back with a clear, hard coating does become more resistant to scratching. Nowadays, most types of plastic lenses, including high index, polycarbonate and traditional plastic materials, have built-in scratch-resistant coatings.

Ultraviolet Treatment

Another lens treatment that is beneficial but invisible to the naked eye is ultraviolet protection. Just as we use sunscreen to keep the sun's UV rays from harming our skin, UV protection in eyewear blocks those same rays from damaging our eyes. Overexposure to ultraviolet light is thought to be a cause of cataracts, retinal damage and other eye problems.

An ultraviolet treatment is simple and quick to apply to most plastic eyeglass lenses, and it does not change the appearance of the lenses at all. The exception is polycarbonate lenses, which don't need anti- UV treatment because it is an inherent property of the material.

Antireflective Coatings

To improve both the vision through the lenses and the appearance of the glasses, an antireflective (AR) coating is applied. AR coatings are similar to the coatings found on microscopes and camera lenses. They consist of several layers of metal oxides applied to the front and back lens surfaces. Because of the layering effect, AR coatings sometimes have a hint of a green or purple color, depending on the individual manufacturer's formula.

Also, the coating reduces both internal and external reflections on the lenses themselves, creating a nicer cosmetic appearance. Internal reflections appear as rings that make lenses look thick. External reflections mask your eyes from a clear, complete view when someone is looking at you. So with an antireflective coating, eyeglass lenses appear thin or non-existent, and your eyes look more natural.

Mirror Coatings

In contrast to antireflective coatings, which are very clear, mirror or "flash" coatings are bold statements of color. Just as the name implies, a mirror coating is highly reflective. The mirrored shades associated with state troopers are one example of a flash coating. The technology has advanced, however, so that today's choices include colors of the rainbow as well as silver, gold and copper metallic mirror coatings. Hot pink, blue -- almost any color is available.

Mirror coatings are purely cosmetic: the wearer perceives no difference in vision regardless of what color the coating is. Only those looking at the person wearing the glasses can see the color of the mirror coating. Mirror coatings are generally applied over sunglass-dark lenses.

Photochromic lenses change from light to dark depending on the amount of ultraviolet light they are exposed to. Early photochromics were strictly glass lenses, but today, regular plastic as well as high index glass, plastic, and polycarbonate lenses are also available.

The active ingredient that causes the lenses to transform is called silver halide and is mixed evenly throughout the lens. This means the whole lens will change when exposed to light. It also means that if a particularly strong prescription is made, the strongest, thickest part of the lens will be darker than the thinner parts. Also, if there is a large difference in prescription between the two eyes, the lens with the stronger prescription will be darker than the weaker one.

Tints

As opposed to the changeable photochromics, another option in colored lenses is a tint, which remains constant at all times. Tints are available on plastic as well as glass lenses and can be had in almost any color of the rainbow. Lighter, fashion tints are used primarily for cosmetic purposes to enhance a wearer’s looks. Darker tints allow the wearer to use the lenses as sunglasses.

Typically, fashion tints are applied in light pink, brown or gray, while sunglasses are usually gray or brown. A tint can be solid, when the entire lens is the same color, or gradient, which is a gradual fade from dark to light, usually fading from the top down.

Other colors can be applied to lenses for different purposes:

• Yellow, sometimes referred to as a "blue-blocker" because the color keeps blue light from entering the lens, is often the color of choice for target shooters because it decreases haze and makes objects appear sharper, with more contrast.
• Green, or its cousin G-15 (the Ray-Ban lens color), is sometimes used as a sunglass, though brown and gray are the most popular sun shades.
• Red is a very uncomfortable color to look through, though it does have applications for certain ocular pathologies. However, some people enjoy seeing the world through "rose-colored glasses."