Corrective lens

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A corrective lens is a lens worn on or before the eye, used to treat myopia, hyperopia, astigmatism, and presbyopia. The most common types of corrective lenses are spectacle lenses and contact lenses. Intraocular lenses are also beginning to become common. Myopia (near sightedness) requires biconcave or diverging lenses, whereas hypermetropia (far sightedness) requires biconvex or converging lenses.

In some cases, mild farsightedness can be treated with simple magnifying lenses or "reading glasses". However, an optometrist may need to prescribe a dioptre for both eyes or each eye, allowing lenses to be custom ground to the patient's specific needs. Usually, the dioptre (or "prescription") is equal for both eyes; in some cases, however, the two eyes may need different prescriptions.

Patients with presbyopia or other disorders of accommodation often benefit from bifocals, or lenses with separate sections ground to different prescriptions for different circumstances. Typically a person with myopia would have one section of a prescription lens that has a certain diverging power while another section of the lens would have a lower diverging power for close-up work. Similarly a person with hypermetropia would have one section of the lens with a certain converging power and another section with a greater power for close-up work.

Contents

1 Types of lens material

1.1 Glass Lenses
1.2 Plastic Lenses (CR-39)
1.3 Polycarbonate
1.4 High Index Plastics

2 Lens material

2.1 Anti Reflective Coating
2.2 Scratch Resistant Coating

3 See also

Types of lens material

Glass Lenses

Glass lenses have become less common in recent years due to the danger of shattering and their relatively high weight compared to CR-39 plastic lenses. They still remain in use for specialised circumstances, for example in extremely high prescriptions (currently, glass lenses can be manufactured up to a refractive index of 1.9) and in certain occupations where the hard surface of glass offers more protection from sparks or shards of material.

Plastic Lenses (CR-39)

Plastic lenses are currently the most commonly prescribed lens, due to their relative safety, low cost and ease of production. The main drawbacks are the ease by which a lens can be scratched, and the limitations and costs of producing higher index lenses. Standard CR-39 plastic lenses have an index of 1.5, however thinner lenses are now common, available up to a 1.74 index.

Polycarbonate

Index of Refraction: 1.61? Lighter weight than normal plastic. Less tendency to iritate your nose or leave red marks on your nose where the glasses touch your nose. Polycarb blocks UV rays, is shatter resistant and is used in sports glasses and glasses for children and teenages. Polycarb is soft and will scratch easily. You must get a scratch coating on a polycarb lens.

High Index Plastics

Index of Refraction: 1.67? Lighter weight due to thinner lenses. This allows people with high prescriptions (above 6.0 diopters) to get lenses that don't look the thickness of a wine bottle.

Lens material

Anti Reflective Coating

Modern AR is usually ion implanted. You must wash glasses with only water and no soap or the Anti Relective Coating will peel off and ruin the look of your lens.

Scratch Resistant Coating

Highly recommended to make your lenses last longer.