A wide variety of materials is used for the manufacture of ophthalmic lenses. They are mainly classified into mineral and organic lenses. Mineral glass was used for spectacles for the first time around 1250. The first organic ophthalmic lenses were developed in Columbia (USA) in 1939. The lenses were made of a plastic polymer called CR-39, an abbreviation for “Columbia Resin 39”. Eyeglasses are nowadays available with mineral and organic lenses. Since the mid-1980s, materials with a higher index of refraction have become available, making it possible to manufacture thinner lenses with more appealing aesthetics. The lower curvature of high-index lenses means that the risk of blurry vision close to the edges of the lenses due to colour fringing is higher.
Glass lenses consist of many individual components. Their main ingredient is silica sand which is mixed with flux agents, stabilizers and chemical additives. Other components are added to optimize the performance properties and working conditions. One advantage of mineral glass is its scratch resistance due to its high density, which lengthens the life of the lenses. Since glass is also very heat-resistant, mineral lenses are not prone to deformation. Mineral lenses are available in refractive indices from 1.5 to 1.9. Due to the higher refractive index, higher power lenses can be manufactured much thinner and are thus aesthetically more appealing than plastic lenses. This is primarily beneficial for plus lenses to reduce the centre thickness. Minus lenses are usually thinnest at the centre, which is why this optimization offers little advantage. The lens must have a certain centre thickness to ensure stability/breaking strength.
The following raw materials are required for the production of fully synthetic ophthalmic lenses: mineral oil, natural gas, coal, cellulose, protein, natural rubber, natural resin, etc. Plastic lenses are considerably lighter than comparable mineral lenses and thus very comfortable to wear. They are ideal for sports activities and children due to their high breaking strength. Plastic lenses can be tinted through a dipping process, enabling the production of a wide range of colours. They must be hard coated to achieve the same surface hardness as mineral glass has. Plastic lenses are available in refractive indices from 1.5 to 1.74.
Thermosetting plastic allyl diglycol carbonate
The oldest and most popular allyl diglycol carbonate material is CR-39 with a refraction index of 1.5. Around 180 different cross-links of this monomer were tested and the 39th attempt offered the most promising material properties. Early CR-39 lenses were only available with a low refractive index over the course of time, this has changed.
Thermoplastics such as polycarbonate and poly(methyl) methacrylate (PMMA)
Polycarbonate is a type of plastic that is often used for sports eyewear and safety googles since it is 4x more impact-resistant than CR-39 plastic. Its index of refraction is 1.59. Poly(methyl) methacrylate (PMMA) is shatter resistant and therefore used for sports eyewear and safety goggles. Its index of refraction is somewhat lower at 1.49.