Trifocal lenses Health Dictionary

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Trifocal Lenses: From 1 Different Sources

lenses in which there are three segments. The upper provides a clear image of distant objects; the lower is used for reading and close work; and the middle one for the intermediate distance. Musicians sometimes find the middle segment useful for reading the score during performance.
Health Source: Oxford | Concise Colour Medical Dictionary
Author: Jonathan Law, Elizabeth Martin

Contact Lenses

Contact lenses are lenses worn in contact with the EYE, behind the eyelids and in front of the cornea. They may be worn for cosmetic, optical or therapeutic reasons. The commonest reason for wear is cosmetic, many short-sighted people preferring to wear contact lenses instead of glasses. Optical reasons for contact-lens wear include cataract surgery (usually unilateral extraction) and the considerable improvement in overall standard of vision experienced by very short-sighted people when wearing contact lenses instead of glasses. Therapeutic lenses are those used in the treatment of eye disease: ‘bandage lenses’ are used in certain corneal diseases; contact lenses can be soaked in a particular drug and then put on the eye so that the drug slowly leaks out on to the eye. Contact lenses may be hard, soft or gas permeable. Hard lenses are more optically accurate (because they are rigid), cheaper and more durable than soft. The main advantage of soft lenses is that they are more comfortable to wear. Gas-permeable lenses are so-called because they are more permeable to oxygen than other lenses, thus allowing more oxygen to reach the cornea.

Disposable lenses are soft lenses designed to be thrown away after a short period of continuous use; their popularity rests on the fact that they need not be cleaned. The instructions on use should be followed carefully because the risk of complications, such as corneal infection, are higher than with other types of contact lenses.

Contraindications to the use of contact lenses include a history of ATOPY, ‘dry eyes’, previous GLAUCOMA surgery and a person’s inability to cope with the management of lenses. The best way to determine whether contact lenses are suitable, however, may be to try them out. Good hygiene is essential for wearers so as to minimise the risk of infection, which may lead to a corneal abscess – a serious complication. Corneal abrasions are fairly common and, if a contact-lens wearer develops a red eye, the lens should be removed and the eye tested with ?uorescein dye to identify any abrasions. Appropriate treatment should be given and the lens not worn again until the abrasion or infection has cleared up.... contact lenses

Achromatic Lenses

lenses that are specially designed to give clear images, eliminating the coloured fringes that are produced with ordinary lenses (caused by splitting of the light into different wavelengths and hence its component colours). Such lenses are useful in scientific instruments, such as the eyepieces of microscopes.... achromatic lenses

Multifocal Lenses

lenses containing more than one segment made to different prescriptions for visual correction. See bifocal lens; trifocal lenses.... multifocal lenses

Progressive Lenses

see varifocal lenses.... progressive lenses

Varifocal Lenses

(progressive lenses) lenses in which the power (see dioptre) gradually changes from one prescription to the other and there is no dividing line on the lens between the different segments (compare multifocal lenses). The wearer can see clearly at any distance by raising or lowering the eyes.... varifocal lenses

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