Eyeglass lenses are mainly corrective lenses though people tend to wear eyeglasses for sheer fashion. A corrective lens is worn in front of the eyes and is usually used for the treatment of myopia, hyperopia, astigmatism and problems like presbyopia. Glasses are worn at a distance on the face, contact lenses are worn directly of the surface of the eye and intraocular lenses are surgically fitted in the eyes. For the treatment of myopia one needs a divergent lens and a convergent lens is needed for the treatment of hyperopia. A corrective lens prescription is written by an optometrist and the prescription consists of the power specification of each eye. The lens can be for sphero cylindrical correction. The first component for this is sphere component and each power specification in this is the spherical correction that is in diopters. The cylindrical power component consists of correction of two different meridians horizontally and vertically. The third component is the axis component where the axis defines two powers sphere and cylindrical the sphere is always almost 90º from the cylinder.
The different types of lenses are:
- Single vision lens which has the same optical correction over the entire area.
- Bifocal lenses are lenses in which the upper potion is generally for distant vision and the lower portion is for near vision. But there is a segment line between the two.
- A trifocal lens is similar to a bifocal lens. The only exception is that there are two segment lines dividing the three different correcting segments.
- Progressive lenses provide a smooth transition from distance correction to near correction eliminating the lines in between enabling a clear intermediate vision.
The most common shape of corrective lenses is ophthalmic or convex-concave. These lenses have the same radius from the back and the front. The difference in the curvature between the front and the back of the lens leads to the corrective nature of these lenses. The bifocals and the trifocals are more complex lens shape, whereas the progressive lenses eliminate the line which is there in the bifocals and the trifocals. The base curve can be changed in order to give the best optic and the cosmetic characteristics across the entire lens.
The greatest cosmetic effect that has come up in the recent years is that of reduction in lens thickness. This results in the increase in material density. The lenses are made from plastics, polycarbonates and high index plastics called the polyurethanes. The lenses can have a scratch resistant coating, an anti glare coating and a coating for protection from ultraviolet rays of the sun.
Posted in 
Tags: