This article was updated on the 11th September 2019.
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A corrective lens is a lens worn on or before the eye - contact lenses or eyeglass lenses respectively. They are used to treat short-sightedness, long-sightedness, presbyopia, and where vision is blurred by an irregularly shaped cornea. Biconcave or diverging lenses are required for near-sightedness and biconvex or converging lenses are required for far-sightedness.
An optometrist usually prescribes corrective lenses. The prescription will involve the necessary corrections for refraction errors in each eye - individually for distance vision and near vision, for a total of four possible correction specifications.
Prescription Glasses - Lens Design
Single powered lenses to correct short-sightedness and long-sightedness are required by many people for both full-time and part-time use, such as for reading. Single powered lenses can be made for very strong prescriptions, but they are only designed to deal with one vision deficiency. People who have more than one sight deficiency, requiring multiple prescriptions will require multifocal lenses.
Four types of lens designs are available:
- Single vision – lenses that have the same optical correction over the entire area of the lens
- Bifocals - lenses that have a segment line separating the two halves of the lens. The upper half of the lens is generally used for distance vision, whereas the lower half is for near vision.
- Trifocals - lenses that are divided into three regions by two segment lines. Similar to bifocals, the top half and bottom half are for distant vision and near vision respectively. However, the middle area is used for intermediate distance vision as it has no corrections
- Progressive or varifocal lens - eliminates the use of segment lines and provides a smooth transition in bifocal and trifocal lens.
Convex-concave and plano-concave are the most common shapes used in single vision lenses for the treatment of near-sightedness. The corrective power of the lens is governed by the difference in curvature between the front and rear surface of the lens.
Prescription Glasses - Lens Materials
The role of a lens material is used in the production of the corrective function of the lens, but also acts as a support for coatings such as anti-reflective coatings. Glass and plastic are the two major materials used for the production of lenses.
Due to the danger of shattering and their relatively high weight compared to plastic lenses, glass lenses have become less common in recent years. However, they still remain in use for specialized circumstances. Examples of this are in extremely strong prescriptions, and they are also used in occupations where the mechanical properties of glass will give more protection for the wearer from sparks or shards of materials, i.e. in welding applications. Plastic lenses, such as CR39 or polycarbonate are currently the most commonly prescribed lens, due to their relative safety, low cost, ease of production, and outstanding optical quality, as well as their shatter resistance. The major disadvantage, when compared to glass, is its limited resistance to abrasion.
Prescription Glasses - Optical Coatings
Optical coating is a thin layer of material placed on an optical component that alters the way in which the optic reflects and transmits light. The most common types of coatings include anti-reflective, which reduce the reflections of the white of the eye as well as bright objects for the spectacle wearer; UV coatings which reduce the light transmission in the UV spectrum; and scratch resistant coatings which increase the abrasive properties of plastic lenses.