Ophthalmic Lens Materials

Optical crown glass: (B270 - Soda/lime glass) Refractive index (nd): 1.52288 Abbe value (Vd): 58.5 Density: 2.56 g/cm3 (the heaviest corrective lens material in common use, today) UV cutoff: 320 nm Glass lenses have become less common owing to the danger of shattering and their relatively high weight compared to CR-39 plastic lenses. They still remain in use for specialized circumstances, for example in extremely high prescriptions (currently, glass lenses can be manufactured up to a refractive index of 1.9) and in certain occupations where the hard surface of glass offers more protection from sparks or shards of material. If the highest Abbe value is desired, the only choices for common lens optical material are optical crown glass and CR-39.

Higher-quality optical-grade glass materials exist (e.g. Borosilicate crown glasses such as BK7 (nd=1.51680 / Vd=64.17 / D=2.51 g/cm3), which is commonly used in telescopes and binoculars, and fluorite crown glasses such as the best optical quality low dispersion glass currently in production, N-FK58 made by the German company Schott with the following characteristics (nd=1.456 / Vd=90.90 / D=3.65 g/cm3) and are commonly used in high-end camera lenses).

One must bear in mind that the human eye itself has an Abbe value Vd≈50.2 so the extremely expensive high-end optical glass types mentioned above would be of dubious value when used to make corrective lenses. Also, one would be very hard-pressed to find a laboratory that would be willing to acquire or shape custom eyeglass lenses from these materials, considering that such an order would most likely consist of just two different lenses that are specific to the wearer. In general, Vd values above that of Crown Glass and CR-39 are of dubious value, except in combinations of extreme prescriptions, very large lens sizes that cover a good portion of the face, high wearer sensitivity to dispersion, and occupations that involve work with very high contrast elements (e.g. reading dark print on very bright white paper, construction involving contrast of (dark) building elements against a cloudy white sky, a workplace with recessed can or other concentrated small area lighting shining on very bright white surfaces, etc.).

CR-39 Plastic: Refractive index (nd): 1.498 (standard) Abbe value (Vd): 59.3 Density: 1.31 g/cm3 UV cutoff: 355 nm Plastic lenses are currently the most commonly prescribed lens, owing to their relative safety, low cost, ease of production, and high optical quality. The main drawbacks of many types of plastic lenses are the ease by which a lens can be scratched, and the limitations and costs of producing higher-index lenses. CR-39 lenses are an exception in that they are inherently scratch resistant.

Trivex: Refractive index (nd): 1.532 Abbe value (Vd): 43–45 (depending on licensing manufacturer) Density: 1.1 g/cm3 (the lightest corrective lens material in common use) UV cutoff: 394 nm Trivex was developed in 2001 by PPG Industries for the military as transparent armor. With Hoya Corporation and Younger Optics PPG announced the availability of Trivex for the optical industry in 2001. Trivex is a urethane-based pre-polymer. PPG named the material Trivex because of its three main performance properties, superior optics, ultra lightweight, and extreme strength. Trivex is a relative newcomer that possesses the UV-blocking properties and shatter resistance of polycarbonate while at the same time offering far superior optical quality (i.e., higher Abbe value) and a slightly lower density. Its lower refractive index of 1.532 vs. polycarbonate's 1.586 may result in slightly thicker lenses depending upon the prescription. Along with polycarbonate and the various high-index plastics, Trivex is a lab favorite for use in rimless frames, owing to the ease with which it can be drilled and its resistance to cracking around the drill holes. One other advantage that Trivex has over polycarbonate is that it can be tinted. The material was invented by Edwin C. Slagel and patented in September 1998.

Polycarbonate: Refractive index (nd): 1.586 Abbe value (Vd): 30 Density: 1.2 g/cm3 UV cutoff: 385 nm Polycarbonate is lighter weight than normal plastic. It blocks UV rays, is shatter resistant, and is used in sports glasses and glasses for children and teenagers. Because polycarbonate is soft and will scratch easily, a scratch-resistant coating is typically applied after shaping and polishing the lens. Standard polycarbonate with an Abbe value of 30 is one of the worst materials optically if chromatic aberration intolerance is of concern. Along with Trivex and the high-index plastics, polycarbonate is an excellent choice for rimless eyeglasses. Similar to the high-index plastics, polycarbonate has a very low Abbe value, which may be bothersome to individuals sensitive to chromatic aberrations.

High-index plastics: (thiourethanes) Refractive index (nd): 1.600–1.740 Abbe value (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 suffer from 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.