There are many different types of lens materials on the market today. Lenses correct vision by bending or changing the direction of light. The amount of bending or change of direction depends on the shape of the lens and the density of the material from which the lens is made. Virtually every material on the market is available to the patient at Kirkland Vision Center. Below are brief descriptions of these products.
CR-39 , usually referred to as plastic, is the most commonly used material. It's optics approach that of crown glass but it is 50% lighter weight, making it more comfortable to wear.
CROWN GLASS was the first material used for making eyeglass lenses. Because of its weight and potential for shattering on impact, it is rarely used today.
HI-INDEX lenses are lenses with an index of refraction higher than CR-39 or crown glass. The greater the density of the lens material, the more efficiently it bends light. The density rating of a material is referred to as its index of refraction. The denser the material the higher its index of refraction. Air, which has a negligible effect on light, has an index of refraction of 1. Water, being denser than air, has an index of refraction of 1.33. So the direction of light is changed as it hits the surface of water. This is why a stick in water appears bent. CR-39 has an index of refraction of 1.49. Crown glass, with an index of refraction of 1.523, bends light slightly more efficiently than CR-39. Beyond these basic materials, there is a variety of high index plastics and glass materials with an index of refraction as high as 1.9. So the greater the index the more efficiently the lenses bend light. Therefore, lenses can be thinner.
There is a downside to high index materials. As the index of refraction increases, the potential for optical aberrations also increase. These aberrations cause images to appear distorted or blurred as one looks through the periphery of a lens. There is more inherent aberration in polycarbonate than any other lens material. In addition, polycarbonate is softer than other materials, making it more scratchable. On the other hand, polycarbonate is virtually shatterproof making it an excellent industrial safety material. It is the responsibility of the optician to carefully match the patients needs with the most appropriate lens material.
There's more to lenses than meets the eye. 
