<unk> es are<unk> assified 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). Functions Correction of refractive error Corrective<unk> es 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<unk> ear vision. A spherical<unk>  bends light evenly in every direction (horizontally, vertically, etc.). They are typically used to correct myopia and hypermetropia. There are two ways that<unk> es 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<unk> es. Two main strategies exist: multifocal lenses and monovision. Multifocal<unk> es (e.g. bifocals or progressives) are comparable to spectacles with bifocals or progressive lenses because they have multiple focal points. Multifocal<unk> es 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<unk> early 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<unk>  wearers. Alternatively, a person may simply wear reading glasses over their distance<unk> es. Other types of vision correction For those with certain color deficiencies, a red-tinted "X-Chrom"<unk>  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<unk> es can also be an option for extreme light sensitivity in some visual deficiencies such as achromatopsia. ChromaGen<unk> es 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<unk>  applications. Embedding of telescopic lenses and electronic components suggests that future uses of<unk> es may become extremely diverse. Cosmetic<unk> es Woman wearing a cosmetic type of<unk> ; 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<unk>  is designed to change the appearance of the eye. These lenses may also correct refractive error. Although many brands of<unk> es 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<unk>  fits in 2004. In the United States, the FDA labels non-corrective cosmetic<unk> es as decorative<unk> es. Like any<unk> , cosmetic lenses carry risks of mild to serious complications, including ocular redness, irritation and infection. Due to their medical nature, colored<unk> es, similar to regular ones, are illegal to purchase in the United States without a valid prescription. Those with perfect vision can buy color<unk> s 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<unk> es completely cover the iris, thus dramatically changing eye colour. Other colored<unk> es merely tint the iris, highlighting its natural colour. A new trend in Japan, South Korea and China is the circle<unk> , 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<unk> es 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<unk>  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 suffering from 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<unk>  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.<unk> es that deliver drugs to the eye have also been developed. Materials<unk> es, other than the cosmetic variety, become almost invisible once inserted in the eye. Most corrective<unk> es come with a light "handling tint" that renders the lens slightly more visible on the eye. Soft<unk> es 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,<unk> inical events. Starting in the late 1970s, improved rigid materials which were oxygen-permeable were developed.<unk> es 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<unk>  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 than 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
Contact lens
Bionic contact lens
Ocular prosthesis
Browline glasses
Tinnitus masker
Corrective lens
Electronic glasses
Trifocal lenses