Poly ( methyl methacrylate ) ( PMMA ) is the synthetic polymer derived from methyl methacrylate . It is used as an engineering plastic , and it is a transparent thermoplastic . PMMA is also known as acrylic , acrylic glass , as well as by the trade names and brands Crylux , Hesalite , Plexiglas , Acrylite , Lucite , and Perspex , among several others ( see below ). This plastic is often used in sheet form as a lightweight or shatter-resistant alternative to glass . It can also be used as a casting resin, in inks and coatings, and for many other purposes.
107-495: 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
214-450: A refraction to determine the proper power of the lens and an assessment of the health of the eye's anterior segment. Many eye diseases inhibit contact lens wear, such as active infections, allergies, and dry eye. Keratometry is especially important in the fitting of rigid lenses. Lens (optics) A lens is a transmissive optical device that focuses or disperses a light beam by means of refraction . A simple lens consists of
321-405: A biconcave or plano-concave lens in a lower-index medium, a collimated beam of light passing through the lens is diverged (spread); the lens is thus called a negative or diverging lens. The beam, after passing through the lens, appears to emanate from a particular point on the axis in front of the lens. For a thin lens in air, the distance from this point to the lens is the focal length, though it
428-845: A brittle manner when under load, especially under an impact force , and is more prone to scratching than conventional inorganic glass, but modified PMMA is sometimes able to achieve high scratch and impact resistance. PMMA is a strong, tough, and lightweight material. It has a density of 1.17–1.20 g/cm , which is approximately half that of glass, which is generally, depending on composition, 2.2–2.53 g/cm . It also has good impact strength, higher than both glass and polystyrene, but significantly lower than polycarbonate and some engineered polymers. PMMA ignites at 460 °C (860 °F) and burns , forming carbon dioxide , water , carbon monoxide , and low-molecular-weight compounds, including formaldehyde . PMMA transmits up to 92% of visible light (3 mm (0.12 in) thickness), and gives
535-426: 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
642-399: 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
749-642: A focus. This led to the invention of the compound achromatic lens by Chester Moore Hall in England in 1733, an invention also claimed by fellow Englishman John Dollond in a 1758 patent. Developments in transatlantic commerce were the impetus for the construction of modern lighthouses in the 18th century, which utilize a combination of elevated sightlines, lighting sources, and lenses to provide navigational aid overseas. With maximal distance of visibility needed in lighthouses, conventional convex lenses would need to be significantly sized which would negatively affect
856-519: 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
963-412: A great deal of experimentation with lens shapes in the 17th and early 18th centuries by those trying to correct chromatic errors seen in lenses. Opticians tried to construct lenses of varying forms of curvature, wrongly assuming errors arose from defects in the spherical figure of their surfaces. Optical theory on refraction and experimentation was showing no single-element lens could bring all colours to
1070-537: A lens in air, f is then given by 1 f ≈ ( n − 1 ) [ 1 R 1 − 1 R 2 ] . {\displaystyle \ {\frac {1}{\ f\ }}\approx \left(n-1\right)\left[\ {\frac {1}{\ R_{1}\ }}-{\frac {1}{\ R_{2}\ }}\ \right]~.} The spherical thin lens equation in paraxial approximation
1177-443: 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
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#17327762380961284-579: A magnifying glass, or a burning glass. Others have suggested that certain Egyptian hieroglyphs depict "simple glass meniscal lenses". The oldest certain reference to the use of lenses is from Aristophanes ' play The Clouds (424 BCE) mentioning a burning-glass. Pliny the Elder (1st century) confirms that burning-glasses were known in the Roman period. Pliny also has the earliest known reference to
1391-413: 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. 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,
1498-463: 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 [ ja ] 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
1605-524: 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. A cosmetic contact lens
1712-453: 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
1819-649: 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
1926-790: A reflection of about 4% from each of its surfaces due to its refractive index (1.4905 at 589.3 nm). It filters ultraviolet (UV) light at wavelengths below about 300 nm (similar to ordinary window glass). Some manufacturers add coatings or additives to PMMA to improve absorption in the 300–400 nm range. PMMA passes infrared light of up to 2,800 nm and blocks IR of longer wavelengths up to 25,000 nm. Colored PMMA varieties allow specific IR wavelengths to pass while blocking visible light (for remote control or heat sensor applications, for example). PMMA swells and dissolves in many organic solvents ; it also has poor resistance to many other chemicals due to its easily hydrolyzed ester groups. Nevertheless, its environmental stability
2033-407: 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
2140-774: 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. 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,
2247-833: A single piece of transparent material , while a compound lens consists of several simple lenses ( elements ), usually arranged along a common axis . Lenses are made from materials such as glass or plastic and are ground , polished , or molded to the required shape. A lens can focus light to form an image , unlike a prism , which refracts light without focusing. Devices that similarly focus or disperse waves and radiation other than visible light are also called "lenses", such as microwave lenses, electron lenses , acoustic lenses , or explosive lenses . Lenses are used in various imaging devices such as telescopes , binoculars , and cameras . They are also used as visual aids in glasses to correct defects of vision such as myopia and hypermetropia . The word lens comes from lēns ,
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#17327762380962354-831: A spherical lens in air or vacuum for paraxial rays can be calculated from the lensmaker's equation : 1 f = ( n − 1 ) [ 1 R 1 − 1 R 2 + ( n − 1 ) d n R 1 R 2 ] , {\displaystyle {\frac {1}{\ f\ }}=\left(n-1\right)\left[\ {\frac {1}{\ R_{1}\ }}-{\frac {1}{\ R_{2}\ }}+{\frac {\ \left(n-1\right)\ d~}{\ n\ R_{1}\ R_{2}\ }}\ \right]\ ,} where The focal length f {\textstyle \ f\ }
2461-457: 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
2568-403: Is biconvex (or double convex , or just convex ) if both surfaces are convex . If both surfaces have the same radius of curvature, the lens is equiconvex . A lens with two concave surfaces is biconcave (or just concave ). If one of the surfaces is flat, the lens is plano-convex or plano-concave depending on the curvature of the other surface. A lens with one convex and one concave side
2675-407: Is convex-concave or meniscus . Convex-concave lenses are most commonly used in corrective lenses , since the shape minimizes some aberrations. For a biconvex or plano-convex lens in a lower-index medium, a collimated beam of light passing through the lens converges to a spot (a focus ) behind the lens. In this case, the lens is called a positive or converging lens. For a thin lens in air,
2782-445: Is h ), and v {\textstyle v} is the on-axis image distance from the line. Due to paraxial approximation where the line of h is close to the vertex of the spherical surface meeting the optical axis on the left, u {\textstyle u} and v {\textstyle v} are also considered distances with respect to the vertex. Moving v {\textstyle v} toward
2889-643: Is 105 °C (221 °F). The T g values of commercial grades of PMMA range from 85 to 165 °C (185 to 329 °F); the range is so wide because of the vast number of commercial compositions that are copolymers with co-monomers other than methyl methacrylate. PMMA is thus an organic glass at room temperature; i.e., it is below its T g . The forming temperature starts at the glass transition temperature and goes up from there. All common molding processes may be used, including injection molding , compression molding , and extrusion . The highest quality PMMA sheets are produced by cell casting , but in this case,
2996-400: Is a commonly used material in modern dentistry, particularly in the fabrication of dental prosthetics, artificial teeth, and orthodontic appliances. Methyl methacrylate " synthetic resin " for casting (simply the bulk liquid chemical) may be used in conjunction with a polymerization catalyst such as methyl ethyl ketone peroxide (MEKP), to produce hardened transparent PMMA in any shape, from
3103-515: Is an economical alternative to polycarbonate (PC) when tensile strength , flexural strength , transparency , polishability, and UV tolerance are more important than impact strength , chemical resistance, and heat resistance. Additionally, PMMA does not contain the potentially harmful bisphenol-A subunits found in polycarbonate and is a far better choice for laser cutting. It is often preferred because of its moderate properties, easy handling and processing, and low cost. Non-modified PMMA behaves in
3210-522: Is called the focal plane . For paraxial rays , if the distances from an object to a spherical thin lens (a lens of negligible thickness) and from the lens to the image are S 1 and S 2 respectively, the distances are related by the (Gaussian) thin lens formula : Polymethyl methacrylate It is often technically classified as a type of glass , in that it is a non-crystalline vitreous substance—hence its occasional historic designation as acrylic glass . The first acrylic acid
3317-416: Is completely round. When used in novelty photography it is often called a "lensball". A ball-shaped lens has the advantage of being omnidirectional, but for most optical glass types, its focal point lies close to the ball's surface. Because of the ball's curvature extremes compared to the lens size, optical aberration is much worse than thin lenses, with the notable exception of chromatic aberration . For
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3424-794: Is derived here with respect to the right figure. The 1st spherical lens surface (which meets the optical axis at V 1 {\textstyle \ V_{1}\ } as its vertex) images an on-axis object point O to the virtual image I , which can be described by the following equation, n 1 u + n 2 v ′ = n 2 − n 1 R 1 . {\displaystyle \ {\frac {\ n_{1}\ }{\ u\ }}+{\frac {\ n_{2}\ }{\ v'\ }}={\frac {\ n_{2}-n_{1}\ }{\ R_{1}\ }}~.} For
3531-705: 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
3638-406: 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. He did not suggest his idea be used for correcting vision; he
3745-460: Is further along in the direction of the ray travel (right, in the accompanying diagrams), while negative R means that rays reaching the surface have already passed the center of curvature. Consequently, for external lens surfaces as diagrammed above, R 1 > 0 and R 2 < 0 indicate convex surfaces (used to converge light in a positive lens), while R 1 < 0 and R 2 > 0 indicate concave surfaces. The reciprocal of
3852-689: Is giant papillary conjunctivitis (GPC), sometimes associated with a poorly fitting contact lens. 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
3959-413: Is important compared to 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
4066-418: Is naturally wet; its water content is 46%. Contact lenses are classified in diverse ways, namely, 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). Corrective contact lenses are designed to improve vision, most commonly by correcting refractive error . This is done by directly focusing light so it enters
4173-407: Is negative with respect to the focal length of a converging lens. The behavior reverses when a lens is placed in a medium with higher refractive index than the material of the lens. In this case a biconvex or plano-convex lens diverges light, and a biconcave or plano-concave one converges it. Convex-concave (meniscus) lenses can be either positive or negative, depending on the relative curvatures of
4280-519: Is poly(methyl 2-methylprop en oate). (It is a common mistake to use "an" instead of "en".) Although PMMA is often called simply "acrylic", acrylic can also refer to other polymers or copolymers containing polyacrylonitrile . Notable trade names and brands include Acrylite, Altuglas, Astariglas, Cho Chen, Crystallite, Cyrolite, Hesalite (when used in Omega watches ), Lucite, Optix, Oroglas, PerClax, Perspex, Plexiglas, R-Cast, and Sumipex. PMMA
4387-433: Is positive for converging lenses, and negative for diverging lenses. The reciprocal of the focal length, 1 f , {\textstyle \ {\tfrac {1}{\ f\ }}\ ,} is the optical power of the lens. If the focal length is in metres, this gives the optical power in dioptres (reciprocal metres). Lenses have the same focal length when light travels from
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4494-574: Is rarely sold as an end product, since it is not optimized for most applications. Rather, modified formulations with varying amounts of other comonomers , additives, and fillers are created for uses where specific properties are required. For example: PMMA is routinely produced by emulsion polymerization , solution polymerization , and bulk polymerization . Generally, radical initiation is used (including living polymerization methods), but anionic polymerization of PMMA can also be performed. The glass transition temperature ( T g ) of atactic PMMA
4601-402: Is somewhat misleading as soft contact lenses are also gas permeable in that they allow oxygen to get through to the ocular surface. 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
4708-500: Is superior to most other plastics such as polystyrene and polyethylene, and therefore it is often the material of choice for outdoor applications. PMMA has a maximum water absorption ratio of 0.3–0.4% by weight. Tensile strength decreases with increased water absorption. Its coefficient of thermal expansion is relatively high at (5–10)×10 °C . The Futuro house was made of fibreglass-reinforced polyester plastic, polyester-polyurethane, and poly(methylmethacrylate); one of them
4815-400: Is the radius of the spherical surface, n 2 is the refractive index of the material of the surface, n 1 is the refractive index of medium (the medium other than the spherical surface material), u {\textstyle u} is the on-axis (on the optical axis) object distance from the line perpendicular to the axis toward the refraction point on the surface (which height
4922-671: Is very important to follow the instructions of the eye care professional prescribing the lenses. When comparing traditional hydrogel soft lens contacts with silicone hydrogel versions, there is no clear evidence to recommend a superior lens. 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: A small number of hybrid lenses exist. Typically, these contact lenses consist of
5029-1135: Is with respect to the principal planes of the lens, and the locations of the principal planes h 1 {\textstyle \ h_{1}\ } and h 2 {\textstyle \ h_{2}\ } with respect to the respective lens vertices are given by the following formulas, where it is a positive value if it is right to the respective vertex. h 1 = − ( n − 1 ) f d n R 2 {\displaystyle \ h_{1}=-\ {\frac {\ \left(n-1\right)f\ d~}{\ n\ R_{2}\ }}\ } h 2 = − ( n − 1 ) f d n R 1 {\displaystyle \ h_{2}=-\ {\frac {\ \left(n-1\right)f\ d~}{\ n\ R_{1}\ }}\ } The focal length f {\displaystyle \ f\ }
5136-593: The trademark Plexiglas. Polymethyl methacrylate was discovered in the early 1930s by British chemists Rowland Hill and John Crawford at Imperial Chemical Industries (ICI) in the United Kingdom. ICI registered the product under the trademark Perspex. About the same time, chemist and industrialist Otto Röhm of Röhm and Haas AG in Germany attempted to produce safety glass by polymerizing methyl methacrylate between two layers of glass. The polymer separated from
5243-480: The 11th and 13th century " reading stones " were invented. These were primitive plano-convex lenses initially made by cutting a glass sphere in half. The medieval (11th or 12th century) rock crystal Visby lenses may or may not have been intended for use as burning glasses. Spectacles were invented as an improvement of the "reading stones" of the high medieval period in Northern Italy in the second half of
5350-512: The 13th century. This was the start of the optical industry of grinding and polishing lenses for spectacles, first in Venice and Florence in the late 13th century, and later in the spectacle-making centres in both the Netherlands and Germany . Spectacle makers created improved types of lenses for the correction of vision based more on empirical knowledge gained from observing the effects of
5457-611: 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 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
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#17327762380965564-979: The Gaussian thin lens equation is 1 u + 1 v = 1 f . {\displaystyle \ {\frac {1}{\ u\ }}+{\frac {1}{\ v\ }}={\frac {1}{\ f\ }}~.} For the thin lens in air or vacuum where n 1 = 1 {\textstyle \ n_{1}=1\ } can be assumed, f {\textstyle \ f\ } becomes 1 f = ( n − 1 ) ( 1 R 1 − 1 R 2 ) {\displaystyle \ {\frac {1}{\ f\ }}=\left(n-1\right)\left({\frac {1}{\ R_{1}\ }}-{\frac {1}{\ R_{2}\ }}\right)\ } where
5671-484: The Latin name of the lentil (a seed of a lentil plant), because a double-convex lens is lentil-shaped. The lentil also gives its name to a geometric figure . Some scholars argue that the archeological evidence indicates that there was widespread use of lenses in antiquity, spanning several millennia. The so-called Nimrud lens is a rock crystal artifact dated to the 7th century BCE which may or may not have been used as
5778-471: The Latin translation of an incomplete and very poor Arabic translation. The book was, however, received by medieval scholars in the Islamic world, and commented upon by Ibn Sahl (10th century), who was in turn improved upon by Alhazen ( Book of Optics , 11th century). The Arabic translation of Ptolemy's Optics became available in Latin translation in the 12th century ( Eugenius of Palermo 1154). Between
5885-626: The M1911A1 pistol or clear handle grips for the M1 bayonet or theater knifes so that soldiers could put small photos of loved ones or pin-up girls' pictures inside. They were called "Sweetheart Grips" or "Pin-up Grips". Others were used to make handles for theater knives made from scrap materials and people who made them got artistic or creative. Civilian applications followed after the war. Common orthographic stylings include polymethyl methacrylate and polymethylmethacrylate . The full IUPAC chemical name
5992-438: 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
6099-466: 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
6206-399: The above contact lens types—sclerals, PMMAs and RGPs—could be correctly referred to as "rigid" or "hard", the latter term is now used for 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
6313-499: The appearance and, to some extent the function, of a damaged or missing iris . 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
6420-405: The back to the front as when light goes from the front to the back. Other properties of the lens, such as the aberrations are not the same in both directions. The signs of the lens' radii of curvature indicate whether the corresponding surfaces are convex or concave. The sign convention used to represent this varies, but in this article a positive R indicates a surface's center of curvature
6527-400: 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,
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#17327762380966634-494: 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
6741-421: The development of lighthouses in terms of cost, design, and implementation. Fresnel lens were developed that considered these constraints by featuring less material through their concentric annular sectioning. They were first fully implemented into a lighthouse in 1823. Most lenses are spherical lenses : their two surfaces are parts of the surfaces of spheres. Each surface can be convex (bulging outwards from
6848-403: The distance from the lens to the spot is the focal length of the lens, which is commonly represented by f in diagrams and equations. An extended hemispherical lens is a special type of plano-convex lens, in which the lens's curved surface is a full hemisphere and the lens is much thicker than the radius of curvature. Another extreme case of a thick convex lens is a ball lens , whose shape
6955-426: The effect of the lens' thickness. For a single refraction for a circular boundary, the relation between object and its image in the paraxial approximation is given by n 1 u + n 2 v = n 2 − n 1 R {\displaystyle {\frac {n_{1}}{u}}+{\frac {n_{2}}{v}}={\frac {n_{2}-n_{1}}{R}}} where R
7062-586: The entire surface of the cut parts, at great distances from the stressed edge. Annealing the PMMA sheet/parts is therefore an obligatory post-processing step when intending to chemically bond lasercut parts together. In the majority of applications, PMMA will not shatter. Rather, it breaks into large dull pieces. Since PMMA is softer and more easily scratched than glass, scratch-resistant coatings are often added to PMMA sheets to protect it (as well as possible other functions). Pure poly(methyl methacrylate) homopolymer
7169-426: The eye have also been developed. 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
7276-410: 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
7383-552: 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. Soft lenses are often used in
7490-448: 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
7597-564: 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. One of
7704-746: The form of sheets affords to shatter resistant panels for building windows, skylights, bulletproof security barriers, signs and displays, sanitary ware (bathtubs), LCD screens, furniture and many other applications. It is also used for coating polymers based on MMA provides outstanding stability against environmental conditions with reduced emission of VOC. Methacrylate polymers are used extensively in medical and dental applications where purity and stability are critical to performance. In particular, acrylic-type lenses are useful for cataract surgery in patients that have recurrent ocular inflammation (uveitis), as acrylic material induces less inflammation. Due to its aforementioned biocompatibility, poly(methyl methacrylate)
7811-637: The glass as a clear plastic sheet, which Röhm gave the trademarked name Plexiglas in 1933. Both Perspex and Plexiglas were commercialized in the late 1930s. In the United States, E.I. du Pont de Nemours & Company (now DuPont Company) subsequently introduced its own product under the trademark Lucite. In 1936 ICI Acrylics (now Lucite International) began the first commercially viable production of acrylic safety glass. During World War II both Allied and Axis forces used acrylic glass for submarine periscopes and aircraft windscreen, canopies, and gun turrets. Scraps of acrylic were also used to make clear pistol grips for
7918-1062: 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. 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 eye wear 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. Leonardo da Vinci
8025-461: 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
8132-1499: The imaging by second lens surface, by taking the above sign convention, u ′ = − v ′ + d {\textstyle \ u'=-v'+d\ } and n 2 − v ′ + d + n 1 v = n 1 − n 2 R 2 . {\displaystyle \ {\frac {n_{2}}{\ -v'+d\ }}+{\frac {\ n_{1}\ }{\ v\ }}={\frac {\ n_{1}-n_{2}\ }{\ R_{2}\ }}~.} Adding these two equations yields n 1 u + n 1 v = ( n 2 − n 1 ) ( 1 R 1 − 1 R 2 ) + n 2 d ( v ′ − d ) v ′ . {\displaystyle \ {\frac {\ n_{1}\ }{u}}+{\frac {\ n_{1}\ }{v}}=\left(n_{2}-n_{1}\right)\left({\frac {1}{\ R_{1}\ }}-{\frac {1}{\ R_{2}\ }}\right)+{\frac {\ n_{2}\ d\ }{\ \left(\ v'-d\ \right)\ v'\ }}~.} For
8239-563: 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
8346-437: The individual patient. The parameters specified in a contact lens prescription may include: Prescriptions for contact lenses and glasses may be similar but are not interchangeable. Prescribing of contact lenses is usually restricted to various combinations of ophthalmologists , optometrists and opticians . An eye examination is needed to determine an individual's suitability for contact lens wear. This typically includes
8453-458: 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
8560-769: The late 1970s, improved rigid materials which were oxygen-permeable were developed. Contact lenses made from these materials are called rigid gas permeable lenses (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
8667-417: The lens), concave (depressed into the lens), or planar (flat). The line joining the centres of the spheres making up the lens surfaces is called the axis of the lens. Typically the lens axis passes through the physical centre of the lens, because of the way they are manufactured. Lenses may be cut or ground after manufacturing to give them a different shape or size. The lens axis may then not pass through
8774-399: The lens. These two cases are examples of image formation in lenses. In the former case, an object at an infinite distance (as represented by a collimated beam of waves) is focused to an image at the focal point of the lens. In the latter, an object at the focal length distance from the lens is imaged at infinity. The plane perpendicular to the lens axis situated at a distance f from the lens
8881-409: 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 (a need for
8988-402: The lenses (probably without the knowledge of the rudimentary optical theory of the day). The practical development and experimentation with lenses led to the invention of the compound optical microscope around 1595, and the refracting telescope in 1608, both of which appeared in the spectacle-making centres in the Netherlands . With the invention of the telescope and microscope there was
9095-431: 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. 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
9202-560: The major disadvantages 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
9309-615: 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. Typically, soft contact lenses are mass-produced, while rigids are custom-made to exact specifications for
9416-482: 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
9523-467: 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. For those with certain color deficiencies ,
9630-406: The physical centre of the lens. Toric or sphero-cylindrical lenses have surfaces with two different radii of curvature in two orthogonal planes. They have a different focal power in different meridians. This forms an astigmatic lens. An example is eyeglass lenses that are used to correct astigmatism in someone's eye. Lenses are classified by the curvature of the two optical surfaces. A lens
9737-517: The polymerization and molding steps occur concurrently. The strength of the material is higher than molding grades owing to its extremely high molecular mass . Rubber toughening has been used to increase the toughness of PMMA to overcome its brittle behavior in response to applied loads. Being transparent and durable, PMMA is a versatile material and has been used in a wide range of fields and applications such as rear-lights and instrument clusters for vehicles, appliances, and lenses for glasses. PMMA in
9844-458: 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
9951-414: The radius of curvature is called the curvature . A flat surface has zero curvature, and its radius of curvature is infinite . This convention seems to be mainly used for this article, although there is another convention such as Cartesian sign convention requiring different lens equation forms. If d is small compared to R 1 and R 2 then the thin lens approximation can be made. For
10058-775: The right infinity leads to the first or object focal length f 0 {\textstyle f_{0}} for the spherical surface. Similarly, u {\textstyle u} toward the left infinity leads to the second or image focal length f i {\displaystyle f_{i}} . f 0 = n 1 n 2 − n 1 R , f i = n 2 n 2 − n 1 R {\displaystyle {\begin{aligned}f_{0}&={\frac {n_{1}}{n_{2}-n_{1}}}R,\\f_{i}&={\frac {n_{2}}{n_{2}-n_{1}}}R\end{aligned}}} Applying this equation on
10165-410: The sign) would have zero optical power (as its focal length becomes infinity as shown in the lensmaker's equation ), meaning that it would neither converge nor diverge light. All real lenses have a nonzero thickness, however, which makes a real lens with identical curved surfaces slightly positive. To obtain exactly zero optical power, a meniscus lens must have slightly unequal curvatures to account for
10272-408: The subscript of 2 in n 2 {\textstyle \ n_{2}\ } is dropped. As mentioned above, a positive or converging lens in air focuses a collimated beam travelling along the lens axis to a spot (known as the focal point ) at a distance f from the lens. Conversely, a point source of light placed at the focal point is converted into a collimated beam by
10379-564: The surface of the material. Laser cutting may be used to form intricate designs from PMMA sheets. PMMA vaporizes to gaseous compounds (including its monomers) upon laser cutting, so a very clean cut is made, and cutting is performed very easily. However, the pulsed lasercutting introduces high internal stresses, which on exposure to solvents produce undesirable "stress- crazing " at the cut edge and several millimetres deep. Even ammonium-based glass-cleaner and almost everything short of soap-and-water produces similar undesirable crazing, sometimes over
10486-862: The thin lens approximation where d → 0 , {\displaystyle \ d\rightarrow 0\ ,} the 2nd term of the RHS (Right Hand Side) is gone, so n 1 u + n 1 v = ( n 2 − n 1 ) ( 1 R 1 − 1 R 2 ) . {\displaystyle \ {\frac {\ n_{1}\ }{u}}+{\frac {\ n_{1}\ }{v}}=\left(n_{2}-n_{1}\right)\left({\frac {1}{\ R_{1}\ }}-{\frac {1}{\ R_{2}\ }}\right)~.} The focal length f {\displaystyle \ f\ } of
10593-1110: The thin lens is found by limiting u → − ∞ , {\displaystyle \ u\rightarrow -\infty \ ,} n 1 f = ( n 2 − n 1 ) ( 1 R 1 − 1 R 2 ) → 1 f = ( n 2 n 1 − 1 ) ( 1 R 1 − 1 R 2 ) . {\displaystyle \ {\frac {\ n_{1}\ }{\ f\ }}=\left(n_{2}-n_{1}\right)\left({\frac {1}{\ R_{1}\ }}-{\frac {1}{\ R_{2}\ }}\right)\rightarrow {\frac {1}{\ f\ }}=\left({\frac {\ n_{2}\ }{\ n_{1}\ }}-1\right)\left({\frac {1}{\ R_{1}\ }}-{\frac {1}{\ R_{2}\ }}\right)~.} So,
10700-553: 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
10807-1258: The two spherical surfaces of a lens and approximating the lens thickness to zero (so a thin lens) leads to the lensmaker's formula . Applying Snell's law on the spherical surface, n 1 sin i = n 2 sin r . {\displaystyle n_{1}\sin i=n_{2}\sin r\,.} Also in the diagram, tan ( i − θ ) = h u tan ( θ − r ) = h v sin θ = h R {\displaystyle {\begin{aligned}\tan(i-\theta )&={\frac {h}{u}}\\\tan(\theta -r)&={\frac {h}{v}}\\\sin \theta &={\frac {h}{R}}\end{aligned}}} , and using small angle approximation (paraxial approximation) and eliminating i , r , and θ , n 2 v + n 1 u = n 2 − n 1 R . {\displaystyle {\frac {n_{2}}{v}}+{\frac {n_{1}}{u}}={\frac {n_{2}-n_{1}}{R}}\,.} The (effective) focal length f {\displaystyle f} of
10914-418: The two surfaces. A negative meniscus lens has a steeper concave surface (with a shorter radius than the convex surface) and is thinner at the centre than at the periphery. Conversely, a positive meniscus lens has a steeper convex surface (with a shorter radius than the concave surface) and is thicker at the centre than at the periphery. An ideal thin lens with two surfaces of equal curvature (also equal in
11021-467: The use of a corrective lens when he mentions that Nero was said to watch the gladiatorial games using an emerald (presumably concave to correct for nearsightedness , though the reference is vague). Both Pliny and Seneca the Younger (3 BC–65 AD) described the magnifying effect of a glass globe filled with water. Ptolemy (2nd century) wrote a book on Optics , which however survives only in
11128-515: Was created in 1843. Methacrylic acid , derived from acrylic acid , was formulated in 1865. The reaction between methacrylic acid and methanol results in the ester methyl methacrylate . It was developed in 1928 in several different laboratories by many chemists, such as William R. Conn, Otto Röhm , and Walter Bauer, and first brought to market in 1933 by German Röhm & Haas AG (as of January 2019, part of Evonik Industries ) and its partner and former U.S. affiliate Rohm and Haas Company under
11235-420: Was found to be degrading by cyanobacteria and Archaea . PMMA can be joined using cyanoacrylate cement (commonly known as superglue ), with heat (welding), or by using chlorinated solvents such as dichloromethane or trichloromethane (chloroform) to dissolve the plastic at the joint, which then fuses and sets, forming an almost invisible weld . Scratches may easily be removed by polishing or by heating
11342-590: 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. John Herschel , in
11449-406: 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. In 1949, the first "corneal" lenses were developed. These were much smaller than
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