In the past, eyeglass lenses were made exclusively of glass. Today, most eyeglasses are made of high-tech plastics. These new lenses are lighter, do not break as easily as glass lenses, and can be treated with a filter to shield your eyes from damaging ultraviolet light. The following modern eyeglass lenses are lighter, thinner, and more scratch-resistant than glass lenses or the older, common plastic lenses. Polycarbonate lenses. These eyeglass lenses are impact-resistant and are a good choice for people who regularly participate in sporting activities, work in a job environment in which their eyeglasses may be easily scratched or broken, and for children who may easily drop and scratch their eyeglasses. Polycarbonate lenses also provide ultraviolet protection. Trivex lenses. Made from a newer plastic with similar characteristics of polycarbonate lenses. It is lightweight, thin, and impact-resistant and may result in better vision correction than the polycarbonate lenses in some people. High index plastic lenses. Designed for people who require strong prescriptions, these eyeglass lenses are lighter and thinner than the standard, thick "coke bottle" lenses that may otherwise be needed. Aspheric lenses. These eyeglass lenses are unlike typical lenses, which are spherical in shape. Aspheric lenses are made up of differing degrees of curvature over its surface, which allows the lens to be thinner and flatter than other lenses. This also creates an eyeglass lens with a much larger usable portion than the standard lens. Photochromic lenses. Made from either glass or plastic, these eyeglasses change from clear to tinted when exposed to sunlight. This eliminates the need for prescription sunglasses. These eyeglass lenses may not darken in a car because the windshield could block the ultraviolet rays from the sun. Polarized sunglasses. Light reflected from water or a flat surface can cause unwanted glare. Polarized lenses reduce glare and are useful for sports and driving. These lenses may cause the liquid crystal displays on the dashboard of cars to appear invisible. The type of vision problem that you have determines the shape of the eyeglass lens. For example, a lens that is concave, or curves inward, is used to correct nearsightedness, while a lens that is convex, or curves outward, is used to correct farsightedness. To correct astigmatism, which is caused by distortions in the shape of the cornea, a cylinder shaped lens is frequently used. Simply put, the eyeglass lens is a tool you use to focus light appropriately onto your retina.
