other types of vision correction for those with certain color deficiencies, a red-tinted "x-chrom" contact lens may be used. although such a lens does not restore normal color vision, it allows some color-blind people to distinguish colors better. red-filtering contact lenses can also be an option for extreme light sensitivity in some visual deficiencies such as achromatopsia. chromagen contact lenses have been used and shown to have some limitations with vision at night although otherwise producing significant improvements in color vision. an earlier study showed very significant improvements in color vision and patient satisfaction. later work that used these chromagen lenses with people with dyslexia in a randomised, double-blind, placebo controlled trial showed highly significant improvements in reading ability over reading without the lenses. this system has been granted fda approval for use in the united states. magnification is another area being researched for future contact lens applications. embedding of telescopic lenses and electronic components suggests that future uses of contact lenses may become extremely diverse. cosmetic contact lenses woman wearing a cosmetic type of contact lens; enlarged detail shows the grain produced during the manufacturing process. curving of the lines of printed dots suggests these lenses were manufactured by printing onto a flat sheet then shaping it. a cosmetic contact lens is designed to change the appearance of the eye. these lenses may also correct refractive error. although many brands of contact lenses are lightly tinted to make them easier to handle, cosmetic lenses worn to change eye color are far less common, accounting for only 3% of contact lens fits in 2004. in the united states, the fda labels non-corrective cosmetic contact lenses as decorative contact lenses. like any contact lens, cosmetic lenses carry risks of mild to serious complications, including ocular redness, irritation and infection. due to their medical nature, colored contact lenses, similar to regular ones, are illegal to purchase in the united states without a valid prescription. those with perfect vision can buy color contacts for cosmetic reasons, but they still need their eyes to be measured for a "plano" prescription, meaning one with zero vision correction. this is for safety reasons so the lenses will fit the eye without causing irritation or redness. some colored contact lenses completely cover the iris, thus dramatically changing eye color. other colored contact lenses merely tint the iris, highlighting its natural color. a new trend in japan, south korea and china is the circle contact lens, which extend the appearance of the iris onto the sclera by having a dark tinted area all around. the result is an appearance of a bigger, wider iris, a look reminiscent of dolls' eyes. cosmetic lenses can have more direct medical applications. for example, some contact lenses can restore the appearance and, to some extent the function, of a damaged or missing iris. therapeutic scleral lenses scleral lens, with visible outer edge resting on the sclera of a patient with severe dry eye syndrome a scleral lens is a large, firm, transparent, oxygen-permeable contact lens that rests on the sclera and creates a tear-filled vault over the cornea. the cause of this unique positioning is usually relevant to a specific patient whose cornea is too sensitive to support the lens directly. scleral lenses may be used to improve vision and reduce pain and light sensitivity for people with disorders or injuries to the eye, such as severe dry eye syndrome (keratoconjunctivitis sicca), microphthalmia, keratoconus, corneal ectasia, stevens–johnson syndrome, sjögren's syndrome, aniridia, neurotrophic keratitis (anesthetic corneas), complications post-lasik, high order aberrations of the eye, complications post-corneal transplant and pellucid degeneration. injuries to the eye such as surgical complications, distorted corneal implants, as well as chemical and burn injuries also may be treated with scleral lenses. therapeutic soft lenses soft lenses are often used in the treatment and management of non-refractive disorders of the eye. a bandage contact lens allows the patient to see while protecting an injured or diseased cornea from the constant rubbing of blinking eyelids, thereby allowing it to heal. they are used in the treatment of conditions including bullous keratopathy, dry eyes, corneal abrasions and erosion, keratitis, corneal edema, descemetocele, corneal ectasia, mooren's ulcer, anterior corneal dystrophy, and neurotrophic keratoconjunctivitis. contact lenses that deliver drugs to the eye have also been developed. materials contact lenses, other than the cosmetic variety, become almost invisible once inserted in the eye. most corrective contact lenses come with a light "handling tint" that renders the lens slightly more visible on the eye. soft contact lenses extend beyond the cornea, their rim sometimes visible against the sclera. rigid lenses glass lenses were never comfortable enough to gain widespread popularity. the first lenses to do so were those made from polymethyl methacrylate (pmma or perspex/plexiglas), now commonly referred to as "hard" lenses. their main disadvantage is they do not allow oxygen to pass through to the cornea, which can cause a number of adverse, and often serious, clinical events. starting in the late 1970s, improved rigid materials which were oxygen-permeable were developed. contact lenses made from these materials are called rigid gas permeable lenses or 'rgps'. a rigid lens is able to cover the natural shape of the cornea with a new refracting surface. this means that a spherical rigid contact lens can correct corneal astigmatism. rigid lenses can also be made as a front-toric, back-toric, or bitoric. rigid lenses can also correct corneas with irregular geometries, such as those with keratoconus or post surgical ectasias. in most cases, patients with keratoconus see better through rigid lenses than through glasses. rigid lenses are more chemically inert, allowing them to be worn in more challenging environments where chemical inertia is important compared to soft lenses. soft lenses soft lenses are more flexible than rigid lenses, and can be gently rolled or folded without damaging the lens. while rigid lenses require a period of adaptation before comfort is achieved, new soft lens wearers typically report lens awareness rather than pain or discomfort. hydrogel lenses rely on their water content to transmit oxygen through the lens to the cornea. as a result, higher water content lenses allowed more oxygen to the cornea. in 1998, silicone hydrogel, or si-hy lenses became available. these materials have both the extremely high oxygen permeability of silicone and the comfort and clinical performance of the conventional hydrogels. because silicone allows more oxygen permeability than water, oxygen permeability of silicone hydrogels is not tied to the lenses' water content. lenses have now been developed with so much oxygen permeability that they are approved for overnight wear (extended wear). lenses approved for daily wear are also available in silicone hydrogel materials. disadvantages of silicone hydrogels are that they are slightly stiffer and the lens surface can be hydrophobic, thus less "wettable" – factors that can influence comfort of lens use. new manufacturing techniques and changes to multipurpose solutions have minimized these effects. those new techniques are often broken down into 3 generations: 1st generation (plasma coating): a surface modification process called plasma coating alters the lens surface's hydrophobic nature; 2nd generation (wetting agents): another technique incorporates internal rewetting agents to make the lens surface hydrophilic; 3rd generation (inherently wettable): a third process uses longer backbone polymer chains that results in less cross linking and increased wetting without surface alterations or additive agents. current brands of soft lenses are either traditional hydrogel or silicone hydrogel. because of drastic differences in oxygen permeability, replacement schedule, and other design characteristics, it is very important to follow the instructions of the eye care professional prescribing the lenses. hybrid a small number of hybrid lenses exist. typically these contact lenses consist of a rigid center and a soft "skirt". a similar technique is the "piggybacking" of a smaller, rigid lens on the surface of a larger, soft lens. these techniques are often chosen to give the vision correction benefits of a rigid lens and the comfort of a soft lens. wear schedule a "daily wear" (dw) contact lens is designed to be worn for one day and removed before sleeping. an "extended wear" (ew) contact lens is designed for continuous overnight wear, typically for up to 6 consecutive nights. newer materials, such as silicone hydrogels, allow for even longer wear periods of up to 30 consecutive nights; these longer-wear lenses are often referred to as "continuous wear" (cw). ew and cw contact lenses can be worn overnight because of their high oxygen permeability. while awake, the eyes are mostly open, allowing oxygen from the air to dissolve into the tears and pass through the lens to the cornea. while asleep, oxygen is supplied from the blood vessels in the back of the eyelid. a lens hindering passage of oxygen to the cornea causes corneal hypoxia which can result in serious complications, such as corneal ulcer that, if left untreated, can permanently decrease vision. ew and cw contact lenses typically allow for a transfer of 5–6 times more oxygen than conventional softs, allowing the cornea to remain healthy, even with closed eyelids. wearing lenses designed for daily wear overnight has an increased risk for corneal infections, corneal ulcers and corneal neovascularization—this latter condition, once it sets in, cannot be reversed and will eventually spoil vision acuity through diminishing corneal transparency. the most common complication of extended wear is giant papillary conjunctivitis (gpc), sometimes associated with a poorly fitting contact lens. replacement schedule contact lenses are often categorized by their replacement schedule. single use lenses (called 1-day or daily disposables) are discarded after one use. because they do not have to stand up to the wear and tear of repeated uses, these lenses can be made thinner and lighter, greatly improving their comfort. lenses replaced frequently gather fewer deposits of allergens and germs, making these lenses preferable for patients with ocular allergies or for those who are prone to infection. single-use lenses are also useful for people who wear contact lenses infrequently, or when losing a lens is likely or not easily replaced (such as when on vacation). they are also considered useful for children because cleaning or disinfecting is not needed, leading to improved compliance. other disposable contact lenses are designed for replacement every two or four weeks. quarterly or annual lenses, which used to be very common, are now much less so. rigid gas permeable lenses are very durable and may last for several years without the need for replacement. pmma hards were very durable and were commonly worn for 5 to 10 years, but had several drawbacks. lenses with different replacement schedules can be made of the same material. although the materials are alike, differences in the manufacturing processes determine if the resulting lens will be a "daily disposable" or one recommended for two or four week replacement. however, sometimes manufacturers use absolutely identical lenses and just repackage them with different labels. manufacturing molecular structure of silicone hydrogel used in flexible, oxygen-permeable contact lenses. typically, soft contact lenses are mass-produced, while rigids are custom-made to exact specifications for the individual patient. spin-cast lenses – a soft lens manufactured by whirling liquid silicone in a revolving mold at high speed. diamond turning – this type is cut and polished on a cnc lathe. the lens starts out as a cylindrical disk held in the jaws of the lathe that is equipped with an industrial-grade diamond as the cutting tool. the cnc lathe may turn at nearly 6000 rpm as the cutter removes the desired amount of material from the inside of the lens. the concave (inner) surface of the lens is then polished with some fine abrasive paste, oil, and a small polyester cotton ball turned at high speeds. to hold the delicate lens in reverse manner, wax is used as an adhesive. the lens' convex (outer) surface is thus cut and polished by the same process. this method can be used to shape rigid as well as soft lenses. in the case of softs, the lens is cut from a dehydrated polymer that is rigid until water is reintroduced. molded – molding is used to manufacture some brands of soft contact lenses. rotating molds are used and the molten material is added and shaped by centripetal forces. injection molding and computer control are also used to create nearly perfect lenses. the lens is kept moist throughout the entire