(Vd): 42–32 (higher indexes generally result in lower Abbe values) Density: 1.3–1.5 (g/cm3) UV cutoff: 380–400 nm High-index plastics allow for thinner lenses. The lenses may not be lighter, however, due to the increase in density vs. mid- and normal index materials. A disadvantage is that high-index plastic lenses have a much higher level of chromatic aberrations, which can be seen from their lower Abbe value. Aside from the thinness of the lens, another advantage of high-index plastics is their strength and shatter resistance, although not as shatter resistant as polycarbonate. This makes them particularly suitable for rimless eyeglasses. These high-refractive-index plastics are typically thiourethanes, with the sulfur atoms in the polymer being responsible for the high refractive index. The sulfur content can be up to 60 percent by weight for an n=1.74 material. Ophthalmic material property tables Indices of refraction for a range of materials can be found in the list of refractive indices. Lens coatings Anti-reflective Anti-reflective coatings help to make the eye behind the lens more visible. They also help lessen back reflections of the white of the eye as well as bright objects behind the eyeglasses wearer (e.g. windows, lamps). Such a reduction of back reflections increases the apparent contrast of surroundings. At night, anti-reflective coatings help to reduce headlight glare from oncoming cars, street lamps, and heavily lit or neon signs. One problem with anti-reflective coatings is that historically they have been very easy to scratch. Newer coatings try to address this problem by combining scratch resistance with the anti-reflective coating. They also offer a measure of dirt and smudge resistance, due to their hydrophobic qualities. Ultraviolet protection A UV coating is used to reduce the transmission of light in the ultraviolet spectrum. UV-B radiation increases the likelihood of cataracts, while long-term exposure to UV-A radiation can damage the retina. DNA damage from UV light is cumulative and irreversible. Some materials such as Trivex and Polycarbonate, naturally block most UV light; they have UV-cutoff wavelengths just outside the visible range and do not benefit from the application of a UV coating. Many modern anti-reflective coatings also block UV. Scratch resistance Resists damage to lens surfaces from minor scratches. Confusing corrective lens industry terminology Spheric vs. aspheric, atoric, etc. Lens manufacturers claim that aspheric lenses improve vision over traditional spheric lenses. This statement could be misleading to individuals who do not know that the lenses are