Optical crown glass

Glass lenses have become less common in recent years due to the danger of shattering and their relatively high weight compared to CR-39 plastic lenses. They still remain in use for specialised 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 kg/m3), which is commonly used in telescopes and binoculars, and fluorite crown glasses such as Schott N-FK51A (nd=1.48656 / Vd=84.47 / D=3.675 kg/m3), which is 16.2 times the price of a comparable amount of BK7, and are commonly used in high-end camera lenses). However, one would be very hard pressed to find a laboratory that would be willing to acquire or shape custom eyeglass lenses, considering that the order would most likely consist of just two different lenses, out of these materials. Generally, Vd values above 60 are of dubious value, except in combinations of extreme prescriptions, large lens sizes, a high wearer sensitivity to dispersion, and occupations that involve work with high contrast elements (e.g., reading dark print on very bright white paper, construction involving contrast of building elements against a cloudy white sky, a workplace with recessed can or other concentrated small area lighting, etc...).

Plastic (CR-39™)

Plastic lenses are currently the most commonly prescribed lens, due to their relative safety, low cost, ease of production, and outstanding optical quality. The main drawbacks are the ease by which a lens can be scratched, and the limitations and costs of producing higher index lenses.

Trivex™

Trivex™ is a relative newcomer that posseses 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, however, may result in slightly thicker lenses. Along with polycarbonate and the various high-index plastics, Trivex is a lab favorite for use in rimless frames, due to the ease with which it can be drilled as well as its resistance to cracking around the drill holes. One other advantage that Trivex has over polycarbonate is that it can be easily tinted, if desired.

Polycarbonate

Lighter weight than normal plastic. Less tendency to irritate your nose or leave red marks on your nose where the glasses touch your nose. Polycarb blocks UV rays, is shatter resistant and is used in sports glasses and glasses for children and teenagers. Polycarb is soft and will scratch easily. You must get a scratch coating on a polycarb 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.

High-index plastics (polyurethanes)

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. Despite being popular with customers, due their thinner appearance, high-index lenses also suffer from a much higher level of chromatic aberrations due to their lower Abbe value. For people with strong prescriptions, the significant reduction in thickness may warrant the reduction in optical quality. Aside from thinness of the lens, another advantage of high-index plastics is their strength and shatter resistance, which is often comparable to or even better than polycarbonate. This makes them another excellent choice for rimless eyeglasses.

documentation taken from www.wikipedia.com