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The Difference in 1.59 & 1.67 Eyeglass Lenses

Picking out eyeglasses should not be as complicated as a college physics class, but trying to understand the difference in lenses is challenging. The numbers 1.59 and 1.67 in lenses refers to the index of refraction, or IR, of a lens material. When choosing your eyeglass lenses, the IR number affects your lens thickness and lens weight.

  1. Index of Refraction Explained

    • The difference in the numbers 1.59 and 1.67 in spectacle lenses means that light bends differently in lenses made from different material or construction. The speed of light changes when it travels through a lens. A material with a higher IR number slows and bends light more than a lens with a lower IR number. The more the light bends, the less material needed to make the prescription. Less lens material is a benefit if you have a strong prescription because your lenses will be lighter and thinner with a higher IR material.

    Material

    • Lenses are made from glass, plastic or polycarbonate, a more shatter-resistant type of plastic. Glass is the hardest lens material but breaks easier and is twice as heavy as plastic. Polycarbonate has an IR of 1.59. Hi-index plastic, a compressed material, has an IR of 1.67. According to All About Vision (allaboutvision.com), any lens material with an IR greater than regular plastic or glass is referred to as "hi-index" or "high-index."

    Nearsighted or Farsighted

    • Also confusing to many eyeglass wearers is the prescription and how it relates to the thickness and weight of your lenses. If you are nearsighted, or myopic, your lenses are thinner in the center and thicker at the edges. Your prescription is written with a (-) sign. If you are farsighted, or hyperopic, your lenses are thicker at the center and thinner at the edges and your prescription begins with a plus (+) sign. Lenses with a higher refractive index reduce the weight and thickness of nearsighted and farsighted prescriptions.

    Cost

    • The higher the refractive index of a lens, the more cost involved in making the lens and usually the more expensive for you. IR 1.67 lenses are the most popular lenses and tend to be the best choice for cost savings when you select a thinner, lighter premium lens. High-index lenses are available as high as 1.74. Your eye care practitioner can help you select the best style IR lens based on your individual needs.

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