CR-39 Standard Plastic Lenses
For a long time glass was the only material for eyeglass lenses. It has excellent optical properties, protects from ultraviolet radiation, the lens surface of the glass is scratch resistant. The disadvantage is that such lenses are heavier and thicker compared to plastic, in the case of damage to their splinters can injure eye glass lens is not always possible to install some modern rim.
The first alternative to glass lenses, plastic lenses steel CR-39 Standard Plastic Lenses (refractive index 1,49-1,50). The material for the manufacture of spectacle lenses is the most common in the world, but its gradual displacement of lighter materials with high refractive index. CR-39 Standard Plastic Lenses cheapest organic material, the lenses there of two times lighter than glass, and more resistant to impact. Easy coloring with organic dyes is an advantage of this material.
The disadvantage of these lenses include low resistance to scratches, little protection from ultraviolet radiation. However, these problems are easily solved by applying special coatings. Such lenses may be recommended at low and moderate refractive error, and optionally coloring.
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