contact lens 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, and they can be worn to correct vision or for cosmetic or therapeutic reasons. 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. multiple analysts estimated that the global market for contact lenses would reach $11.7 billion by 2015. as of 2010, the average age of contact lens wearers globally was 31 years old, and two-thirds of wearers were female. a pair of contact lenses, positioned with the concave side facing upward one-day disposable contact lenses with blue handling tint in blister-pack packaging people choose to wear contact lenses for many reasons. aesthetics and cosmetics are main motivating factors for people who want to avoid wearing glasses or to change the appearance or color of their eyes. others wear contact lenses for functional or optical reasons. 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. history origins and first functional prototypes artist's impression of leonardo's method for neutralizing the refractive power of the cornea leonardo da vinci is frequently credited with introducing the idea of contact lenses in his 1508 codex of the eye, manual d, 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.: 9 he did not suggest his idea be used for correcting vision; he was more interested in exploring mechanisms of accommodation. 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. 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. 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", the second "a mould of the cornea" that could be impressed on "some sort of transparent medium". 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. this enabled the manufacture of lenses that, for the first time, conformed to the actual shape of the eye. in 1888, adolf gaston eugen fick was the first to successfully fit contact lenses, which were made from blown glass although louis j. girard invented a scleral contact lens in 1887, it was german ophthalmologist adolf gaston eugen fick who in 1888 fabricated the first successful afocal scleral contact lens. approximately 18–21 mm (0.71–0.83 in) in diameter, the heavy blown-glass shells rested on the less sensitive rim of tissue surrounding the cornea and floated on a dextrose solution. he experimented with fitting the lenses initially on rabbits, then on himself, and lastly on a small group of volunteers, publishing his work, "contactbrille", in the march 1888 edition of archiv für augenheilkunde. large and unwieldy, fick's lens could be worn only for a couple of hours at a time. august müller of kiel, germany, corrected his own severe myopia with a more convenient blown-glass scleral contact lens of his own manufacture in 1888. the development of polymethyl methacrylate (pmma) in the 1930s paved the way for the manufacture of plastic scleral lenses. in 1936, optometrist william feinbloom introduced a hybrid lens composed of glass and plastic, and in 1937 it was reported that some 3,000 americans were already wearing contact lenses. in 1939, hungarian ophthalmologist dr.istván györffy produced the first fully plastic contact lens. the following year, german optometrist heinrich wöhlk produced his own version of plastic lenses based on experiments performed during the 1930s. corneal and rigid lenses (1949–1960s) in 1949, the first "corneal" lenses were developed. these were much smaller than the original scleral lenses, as they sat only on the cornea rather than across all of the visible ocular surface, and could be worn up to 16 hours a day. pmma corneal lenses became the first contact lenses to have mass appeal through the 1960s, as lens designs became more sophisticated with improving manufacturing technology. on october 18, 1964, in a television studio in washington, d.c., lyndon baines johnson became the first president in the history of the united states to appear in public wearing contact lenses, under the supervision of dr. alan isen, who developed the first commercially viable soft-contact lenses in the united states. early corneal lenses of the 1950s and 1960s were relatively expensive and fragile, resulting in the development of a market for contact lens insurance. replacement lens insurance, inc. (now known as rli corp.) phased out its original flagship product in 1994 after contact lenses became more affordable and easier to replace. gas permeable and soft lenses (1959-current) one major disadvantage of pmma lenses is that they allow no oxygen to get through to the conjunctiva and cornea, causing a number of adverse and potentially serious clinical effects. by the end of the 1970s and through the 1980s and 1990s, a range of oxygen-permeable but rigid materials were developed to overcome this problem. chemist norman gaylord played a prominent role in the development of these new oxygen-permeable contact lenses. collectively, these polymers are referred to as rigid gas permeable or rgp materials or lenses. though all the above contact lens types—sclerals, pmmas and rgps—could be correctly referred to as "rigid" or "hard", the latter term is now used to the original pmmas, which are still occasionally fitted and worn, whereas "rigid" is a generic term for all these lens types; thus hard lenses (pmmas) are a subset of rigid contact lenses. occasionally, the term "gas permeable" is used to describe rgps, which is somewhat misleading as soft contact lenses are also gas permeable in that they allow oxygen to get through to the ocular surface. otto wichterle (pictured) and drahoslav lím introduced modern soft hydrogel lenses in 1959. the principal breakthrough in soft lenses was made by czech chemists otto wichterle and drahoslav lím, who published their work "hydrophilic gels for biological use" in the journal nature in 1959. in 1965, national patent development corporation (npdc) bought the american rights to produce the lenses and then sublicensed the rights to bausch & lomb, which started to manufacture them in the united states. the czech scientists' work led to the launch of the first soft (hydrogel) contact lenses in some countries in the 1960s and the first approval of the soflens material by the us food and drug administration (fda) in 1971. these soft lenses were soon prescribed more often than rigid ones, due to the immediate and much greater comfort (rigid lenses require a period of adaptation before full comfort is achieved). polymers from which soft lenses are manufactured improved over the next 25 years, primarily in terms of increasing oxygen permeability, by varying the ingredients. in 1972, british optometrist rishi agarwal was the first to suggest disposable soft contact lenses. in 1998, the first silicone hydrogel contact lenses were released by ciba vision in mexico. these new materials encapsulated the benefits of silicone which has extremely high oxygen permeability—with the comfort and clinical performance of the conventional hydrogels that had been used for the previous 30 years. these contact lenses were initially advocated primarily for extended (overnight) wear, although more recently, daily (no overnight) wear silicone hydrogels have been launched. in a slightly modified molecule, a polar group is added without changing the structure of the silicone hydrogel. this is referred to as the tanaka monomer because it was invented and patented by kyoichi tanaka of menicon co. of japan in 1979. second-generation silicone hydrogels, such as galyfilcon a (acuvue advance, vistakon) and senofilcon a (acuvue oasys, vistakon), use the tanaka monomer. vistakon improved the tanaka monomer even further and added other molecules, which serve as an internal wetting agent. comfilcon a (biofinity, coopervision) was the first third-generation polymer. its patent claims that the material uses two siloxy macromers of diverse sizes that, when used in combination, produce very high oxygen permeability (for a given water content). enfilcon a (avaira, coopervision) is another third-generation material that is naturally wettable; its water content is 46%. types contact lenses are classified in diverse ways: by their primary function, material, wear schedule (how long a lens can be worn), and replacement schedule (how long before a lens needs to be discarded). correction of refractive error corrective contact lenses are designed to improve vision, most commonly by correcting refractive error. this is done by directly focusing light so it enters the eye with the proper power for clear vision. a spherical contact lens bends light evenly in every direction (horizontally, vertically, etc.). they are typically used to correct myopia and hypermetropia. there are two ways that contact lenses can correct astigmatism. one way is with toric soft lenses that work essentially the same way as eyeglasses with cylindrical correction; a toric lens has a different focusing power horizontally than vertically, and as a result can correct for astigmatism. another way is by using a rigid gas permeable lens; since most astigmatism is caused by the shape of the cornea, rigid lenses can improve vision because the front surface of the optical system is the perfectly spherical lens. both approaches have advantages and drawbacks. toric lenses must have the proper orientation to correct for astigmatism, so such lenses must have additional design characteristics to prevent them from rotating out of alignment. this can be done by weighting the bottom of the lens or by using other physical characteristics to rotate the lens back into position, but these mechanisms rarely work perfectly, so some misalignment is common and results in somewhat imperfect correction, and blurring of sight after blinking rotates the lens. toric soft lenses have all the advantages of soft lenses in general, which are low initial cost, ease of fitting, and minimal adjustment period. rigid gas permeable lenses usually provide superior optical correction, but have become less popular relative to soft lenses due to higher initial costs, longer initial adjustment period, and more involved fitting. correction of presbyopia correction of presbyopia (a need for a reading prescription different from the prescription needed for distance) presents an additional challenge in the fitting of contact lenses. two main strategies exist: multifocal lenses and monovision. multifocal contact lenses (e.g. bifocals or progressives) are comparable to spectacles with bifocals or progressive lenses because they have multiple focal points. multifocal contact lenses are typically designed for constant viewing through the center of the lens, but some designs do incorporate a shift in lens position to view through the reading power (similar to bifocal glasses). monovision is the use of single-vision lenses (one focal point per lens) to focus an eye (typically the dominant one) for distance vision and the other for near work. the brain then learns to use this setup to see clearly at all distances. a technique called modified monovision uses multifocal lenses and also specializes one eye for distance and the other for near, thus gaining the benefits of both systems. care is advised for persons with a previous history of strabismus and those with significant phorias, who are at risk of eye misalignment under monovision. studies have shown no adverse effect to driving performance in adapted monovision contact lens wearers. alternatively, a person may simply wear reading glasses over their distance contact lenses.