The most commonly used natural material is crystal lens, which is finely ground from quartz and mainly composed of silica, pure crystal and so on. There are two major categories, one is colorless lens, and it is of high purity silicon and the other one is brown lens, which iscommonly known as citrine lenses. Because of the presence of other elements and different colors, greatly reducing the visible lights' transmittance, people who are sensitive to lights will wear this eyewear comfortably. However, this kind of lenses can not dully absorb short wave UV and reduce the infrared rays' transmittance. Moreover, its density is uneven, so if it contains impurities, there will be streaks and bubbles, then the birefringence will be raised and the vision will be affected. This material is scarce and hard to find, and the price is relatively expensive.
The glass that can be used in the production of lenses is optical glass. They are very strict, such as refractive index, chromatic dispersion, transmission and so on. It is mainly composed of silica, followed by boron oxide and phosphorus pentoxide. Moreover, they are hard and they are better than crystal as well as can reduce the absorption of UV light stimulation and infrared and other harmful rays. However, they are heavy and proportion is double as resin material. Besides, its impact resistance is poor, mechanical damage likely to cause eye damage, so the security is poor.
It is mainly composed of organic matter, carbon, hydrogen, nitrogen and oxygen-based. The resin material used to make lenses should be homogeneous, transparent and hardly deformed. Resins currently used are summarized as follows: Acrylic, CR39, and Polycarbonate (PC). Trend of lenses development is ultra-thin, ultra-light, ultra-wearable, ultra-smooth, ultra transparent, ultra-rugged, ultra-UV and electromagnetic waves. At present, the lenses on the market have gradually transited from the glass to resin, from no coating to the coating. Different types of glasses, or different customers, all require different lenses. Moreover, in our daily life the most common optical glasses are now mostly made of resin lenses, such as CR39 or PC.