Contact lens
Contact lenses, or simply contacts, are thin lenses placed directly on the surface of the eyes. Contact lenses are ocular prosthetic devices used by over 150 million people worldwide,[1] and they can be worn to correct vision or for cosmetic or therapeutic reasons.[2] In 2010, the worldwide market for contact lenses was estimated at $6.1 billion, while the US soft lens market was estimated at $2.1 billion.[3] Multiple analysts estimated that the global market for contact lenses would reach $11.7 billion by 2015.[3] As of 2010[update], the average age of contact lens wearers globally was 31 years old, and two-thirds of wearers were female.[4]
People choose to wear contact lenses for many reasons.[5] Aesthetics and cosmetics are the main motivating factors for people who want to avoid wearing glasses or to change the appearance or color of their eyes.[6] Others wear contact lenses for functional or optical reasons.[7] When compared with spectacles, contact lenses typically provide better peripheral vision, and do not collect moisture (from rain, snow, condensation, etc.) or perspiration. This can make them preferable for sports and other outdoor activities. Contact lens wearers can also wear sunglasses, goggles, or other eyewear of their choice without having to fit them with prescription lenses or worry about compatibility with glasses. Additionally, there are conditions such as keratoconus and aniseikonia that are typically corrected better with contact lenses than with glasses.[8]
Leonardo da Vinci is frequently credited with introducing the idea of contact lenses in his 1508 Codex of the eye, Manual D,[9] wherein he described a method of directly altering corneal power by either submerging the head in a bowl of water or wearing a water-filled glass hemisphere over the eye. Neither idea was practically implementable in da Vinci's time.[10]: 9 He did not suggest his idea be used for correcting vision; he was more interested in exploring mechanisms of accommodation.[9]
Descartes proposed a device for correcting vision consisting of a liquid-filled glass tube capped with a lens. However, the idea was impracticable, since the device was to be placed in direct contact with the cornea and thus would have made blinking impossible.[11]
In 1801, Thomas Young fashioned a pair of basic contact lenses based on Descartes' model. He used wax to affix water-filled lenses to his eyes, neutralizing their refractive power, which he corrected with another pair of lenses.[10][11]
Sir John Herschel, in a footnote to the 1845 edition of the Encyclopedia Metropolitana, posed two ideas for the visual correction: the first "a spherical capsule of glass filled with animal jelly",[12] the second "a mould of the cornea" that could be impressed on "some sort of transparent medium".[13] Though Herschel reportedly never tested these ideas, they were later advanced by independent inventors, including Hungarian physician Joseph Dallos, who perfected a method of making molds from living eyes.[14] This enabled the manufacture of lenses that, for the first time, conformed to the actual shape of the eye.[15]
