Its not an uncommon sight to see a person wearing glasses or plastic contact lenses as a lot of people wear these nowadays. Though nature has provided us with a pair of eyes, man would never have thought that he would have to invent more power for his natural eyesight due to weakening of the eyes. These vision-aids were invented way back in Rome by a Popozo family in 1289. The first visual evidence is a picture painted in 1352 by Tommaso da Modena in which eyeglasses are depicted.
Different experiments were carried out in different centuries and evolved various kinds of glasses like magnifying glasses in 1000 A. D., hand held glasses and distant vision glasses in 15th century, eye glasses with resting on ear by London optics in 17th century and so on. But it was Benjamin Franklin who with the invention of bifocals in 1784 really revolutionized the glass industry. One of the most interesting and funniest parts of this whole journey is that till 19th century there was no instrument or standardized testing procedure to recommend which type of glasses should be worn and hence the person suffering from vision weakness would simply try all the available glasses and the one suited the most was kept for further use.
In the 19th century John Bausch and Henry Lomb developed a highly refined quality of glasses in their factory located in Rochester, N. Y. The first plastic lenses were developed by Pittsburgh Plate Glass Company during 1940. The material then developed was called as CR-39.
Plastic high index lenses are also called as high index, or hi-index, lenses. These are comparatively thinner than the regular plastic or glass lenses and are specially made for people requiring high power vision. The word index refers to the index of refraction which is affected by the material thickness and type of material through which the light is passing. This causes the light to bend in a different way. The material used in plastic high index lenses is different from the regular plastic or glass lenses and hence bends the light with more efficiency. The plastic high index lenses are made from polycarbonate material which is unbreakable unlike the regular glass lenses. These are thin, light and offer high resistance to ultra violet rays. These are compressed so as to give them a high power index of refraction (IOR). Since these lenses bend to a great extent, the wearer does not have to opt for lot of corrections. Many American Manufacturers offer different kind of plastic high index lenses depending upon the preferences. Although expensive, these lenses are more prone to scratch and breakage. But they look more appealing cosmetically.
High index material is also used in Aspheric lenses. These are light and thin and have special non spherical curves moving from the center to the sides of the lenses. These high index lenses are available in bifocals, progressive-addition designs and single vision. Plastic high index lenses tend to cause glare problems which is minimized by coating it with an anti reflective coatings.