Materials of lenses
There are two great families of ophthalmic lens materials: the mineral (glass) and the organic (plastic). The properties of each differ. Before, however, it is best to explain the most important characteristics of lenses:
1- The refraction index is the capacity of an optical material to bend light. The higher, the thinner and more aesthetic are the lenses. The usual values of the refractive index are: normal (1.50), reduced (1.60) and very reduced (18.104.22.168.1.74, 1.80 and 1.90)
2- Abbe number: the light is made up of all colors. An optical material has a different refractive index for each color, producing chromatic dispersion (the image of each color is formed at slightly different points). Lenses with chromatic dispersion, generally at the ends, show less definition of color separation. A value above 40 has good optical quality. The usual values of the Abbe number are above 50 in the Crown mineral glass, below 40 in the high refractive indexes.
3- Density is related to the weight of the lens and, therefore, comfort.
4- Ultra-violet rays (UV): it corresponds to the protection provided by the lens.
1- Organic 1.5 or CR-39 (Columbian Resin 39): it was the first organic material available for ophthalmic lenses. It was invented in the 40s. It is very light and difficult to break. As limitations, it is not so resistant to scratches as those that are in glass.
2- Trivex: it is a material that was developed for military industry in 1994 and adapted to the optics as of 2001. Chemically, it is a urethane monomer. The name of Trivex comes from its three great properties: a very sharp vision, the lighter material of all the lenses used in optics and a very high impact resistance. It has good protection against UV rays and goes well for drilled lenses.
3- Polycarbonate: it was developed in the 70's for the aerospace industry and incorporated in the optics in the 80's. It is a lightweight and highly impact-resistant material, with good UV protection. It's soft, which is why it always comes with anti-scratch treatment. Optical quality is not as perfect as other lenses due to its low Abbe number and its manufacturing process. It is used mainly in protection lenses against impact.
4- High-index organics (from 1.60 to 1.76): chemically, they come from the ti urethane. The main advantage is that they are thinner. They have a good cut in the UV, but they are softer (they need an anti-scratch treatment). The thinner they are, the more dispersal of Abbe. Suitable for outdoor frames.
Within the mineral glass, there are the crowns (Abbe number greater than 55) and the flint (Abbe number less than 50). The properties are common to all materials: very good optical quality, very resistant to scratches and more weight than organic materials.
1- Crown: they are the index 1.5. They were the first material available to make lenses. It has very little chromatic dispersion.
2- Crown of bari: they are index 1.6
3- Flint: they are index 1.7, 1.8 and 1.9. These indexes are achieved with the addition of titanium or lanthanum.