Differences in Eyeglass Thickness

When eyeglass lenses were invented, they were originally made from glass and continued to be made that way for hundreds of years. Lens innovations in the 20th century paved the way for different materials and now most lenses are made from plastic according to the Cleveland Clinic. Each lens material has a different thickness and index of refraction, the ratio of the speed of light as it moves from air through the lens. The higher the index of refraction number, the thinner the lens.

Plastic Lenses

• Your glasses help you see better by bending, or refracting, light as it passes through your lenses and into your eyes. Your eyeglass prescription determines the light-bending ability of your lenses. The power of the prescription remains the same in each material but the thickness changes. Plastic lenses have a refraction index of 1.498, which means that light travels 1.498 times faster in a vacuum than it does through plastic lenses. Plastic lenses are lighter but a little thicker than glass lenses.

Polycarbonate Lenses

• Polycarbonate lenses have a refraction index of 1.586, which means the material is thinner than plastic or glass lenses. Polycarbonate is a very shatter-resistant material and is recommended for children because of the safety feature of the lenses; however many people choose polycarbonate lenses for the lightweight properties and thinner lens.

Hi-Index Lenses

• Hi-index lenses, as they are referred to, are about 50 percent thinner than plastic lenses. Hi-index lens material bends light more efficiently and is available in different refraction indexes ranging from 1.54 to 1.74 in the United States. The most popular hi-index material has an index of refraction of 1.67 and is about half as thick as plastic lenses.

Glass Lenses

• Glass lenses are slightly thinner than plastic lenses but about twice as heavy. The index of refraction for crown glass, the type used to make eyeglass lenses, is 1.523.