Glass
lenses. In the early days of vision correction, all eyeglass lenses were made
of glass.
Although
glass lenses offer exceptional optics, they are heavy and can break easily,
potentially causing serious harm to the eye or even loss of an eye. For these
reasons, glass lenses are no longer widely used for eyeglasses.
Because
of its light weight (about half the weight of glass), low cost and excellent
optical qualities, CR-39 plastic remains a popular material for eyeglass lenses
even today.
Polycarbonate
lenses. In the early 1970s, Gentex Corporation introduced the first
polycarbonate lenses for safety glasses. Later that decade and in the 1980s,
polycarbonate lenses became increasing popular and remain so today.
Originally
developed for helmet visors for the Air Force, for "bulletproof
glass" for banks and other safety applications, polycarbonate is lighter
and significantly more impact-resistant than CR-39 plastic, making it a
preferred material for children's eyewear, safety glasses and sports eyewear.
A
newer lightweight eyeglass lens material with similar impact-resistant
properties as polycarbonate is called Trivex (PPG Industries), which was
introduced for eyewear in 2001. A potential visual advantage of Trivex is its
higher Abbe value (see below).
High-index
plastic lenses. In the past 20 years, in response to the demand for thinner,
lighter eyeglasses, a number of lens manufacturers have introduced high-index plastic
lenses. These lenses are thinner and lighter than CR-39 plastic lenses because
they have a higher index of refraction (see below) and may also have a lower
specific gravity.
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