High-Index Lenses - Better Looking, More Comfortable Eyeglasses
How do eyeglasses work?
Eyeglasses correct your vision by bending incoming light rays, so that they focus properly on your retinas. A stronger prescription means that more light-bending is required, and therefore the lenses must be thicker.
What are high-index lenses?
Eyeglass lenses can be made of different materials, some of which bend light more efficiently than others. In other words, it can take less of one material to bend light the same amount as another material. The measurement of this property is called the "refractive index" of the material. The higher this refractive index, the more efficient the material, and the thinner the resulting lenses will be.
The refractive index of glass is 1.52, for conventional plastic it is 1.50. A lens material is considered high-index if it has a refractive index higher than glass. There is a range of such materials used to make eyeglass lenses, each with a different refractive index. When comparing lenses of the same prescription, the higher the index of the material used, the thinner the lenses will be, and typically, the higher the price will be.
Most high-index lenses are also aspheric lenses. This gives them a flatter, more appealing shape, allows the glasses to fit closer to your face, and reduces unwanted cosmetic affects sometimes seen with stronger prescriptions.
High-index materials can achieve the same prescription using less material, resulting in thinner, lighter lenses for the wearer. This change is especially noticeable for myopic prescriptions, because myopic lenses are thicker on the edges than in the center, and many modern frames are too thin to hide these thick edges. In the past, strong myopic prescriptions meant thick "coke-bottle" lenses with unattractive edges, but not so today!
Buying high-index lenses
If you are interested in purchasing eyewear with high-index lenses, you should speak with your eye care professional. He or she can help you choose lenses that are right for you. There are multiple high-index materials available, and not all of them are appropriate for all prescriptions. With a little help, you should be able to find lenses that give you great vision and offer a good balance between thinness and cost.
If you are purchasing high-index lenses, another option to ask about is anti-reflective (AR) coating. This coating is helpful for regular lenses, but downright essential for high-index lenses, as the design of these lenses causes them to reflect a lot more light rays. Without this coating, you will find your high-index lenses do not look as nice and, more importantly, you will experience distracting reflections that can interfere with critical tasks, such as night driving.
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