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109-481: A monocle is a type of corrective lens used to correct or enhance the visual perception in only one eye . It consists of a circular lens placed in front of the eye and held in place by the eye socket itself. Often, to avoid losing the monocle, a string or wire is connected to the wearer's clothing at one end and, at the other end, to either a hole in the lens or, more often, a wire ring around its circumference. The Prussian antiquarian Philipp von Stosch wore

218-455: A diffraction limited system, which has an increased depth of field, similar to using a small aperture in photography. This form of correction has many limitations that prevent it from gaining popularity in everyday use. Pinhole glasses can be made in a DIY fashion by making small holes in a piece of card which is then held in front of the eyes with a strap or cardboard arms. Glasses may also house other corrective or assistive devices. After

327-443: A refractive index of 1.9) and in certain occupations where the hard surface of glass offers more protection from sparks or shards of material. If the highest Abbe number is desired, the only choices for common lens optical material are optical crown glass and CR-39. Higher-quality optical-grade glass materials exist (e.g. Borosilicate crown glasses such as BK7 ( n d = 1.51680 , V d = 64.17 , D = 2.51 g/cm³ ), which

436-533: A top hat , the monocle completed the costume of the stereotypical 1890s capitalist . Monocles were also accessories of German military officers from this period, especially from World War I and World War II . German military officers known to have worn a monocle include Helmuth Weidling , Hans Krebs , Werner von Fritsch , Erich Ludendorff , Walter Model , Walter von Reichenau , Dietrich von Saucken , Wilhelm Keitel , Dietrich von Choltiz , Hans von Seeckt , and Hugo Sperrle . Monocles were most prevalent in

545-410: A circular lens called a lens blank. Lens blanks are cut to fit the shape of the frame that will hold them. Frame styles vary and fashion trends change over time, resulting in a multitude of lens shapes. For lower power lenses, there are few restrictions, allowing for many trendy and fashionable shapes. Higher power lenses can distort peripheral vision and may become thick and heavy if a large lens shape

654-415: A continuously variable parametric surface that begins using one spherical surface base curve and ends at another, with the radius of curvature continuously varying as the transition is made from one surface to the other. This shift in curvature results in different powers being delivered from different locations on the lens. Vertex distance is the space between the front of the eye and the back surface of

763-429: A fraction of this. This is also true with aspheric design lenses. The lens's minimum thickness can also be varied. The FDA ball drop test (5/8" 0.56-ounce steel ball dropped from 50 inches) effectively sets the minimum thickness of materials. Glass or CR-39 requires 2.0 mm, but some newer materials only require 1.5 mm or even 1.0 mm minimum thickness. Material density typically increases as lens thickness

872-533: A frame that holds them in front of a person's eyes , typically utilizing a bridge over the nose and hinged arms, known as temples or temple pieces, that rest over the ears. Glasses are typically used for vision correction , such as with reading glasses and glasses used for nearsightedness ; however, without the specialized lenses, they are sometimes used for cosmetic purposes. Safety glasses provide eye protection against flying debris for construction workers or lab technicians; these glasses may have protection on

981-529: A full-sized welding helmet is inconvenient or uncomfortable. These are often called "flash goggles" because they provide protection from welding flash. Nylon frames are usually used for protective eyewear for sports because of their lightweight and flexible properties. Unlike most regular glasses, safety glasses often include protection beside the eyes as well as in front of the eyes. Sunglasses provide more comfort and protection against bright light and often against ultraviolet (UV) light. To properly protect

1090-441: A given material. Ways to reduce the weight and thickness of corrective lenses, in approximate order of importance are these: It is not always possible to follow the above points, because of the rarity of such frames, and the need for a more pleasing appearance. However, these are the main factors to consider if ever it should become necessary and possible to do so. Eyeglasses for a high-diopter nearsighted or farsighted person cause

1199-404: A lens as the eye looks through various points on the area of the lens. Generally, it is least present at the optic center and gets progressively worse as one looks towards the edges of the lens. The actual amount of power error is highly dependent on the strength of the prescription as well as whether a best spherical form of lens or an optically optimal aspherical form was used in the manufacture of

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1308-603: A material with improved ABBE. The best way to combat lens induced power error is to limit the choice of corrective lens to one that is in the best spherical form. A lens designer determines the best-form spherical curve using the Oswalt curve on the Tscherning ellipse . This design gives the best achievable optical quality and least sensitivity to lens fitting. A flatter base-curve is sometimes selected for cosmetic reasons. Aspheric or atoric design can reduce errors induced by using

1417-419: A monocle could easily fall off with the wrong facial expression . A once-standard comedic device exploits this: an upper-class gentleman affects a shocked expression in response to some event, and his monocle falls into his drink or smashes to pieces on the floor. During the late 19th and early 20th centuries, the monocle was generally associated with wealthy upper-class men. Combined with a morning coat and

1526-463: A monocle in Rome in the 1720s, in order to closely examine engravings and antique engraved gems , but the monocle did not become an article of gentlemen's apparel until the 19th century. The dandy 's quizzing glass of the 1790s was an article of high fashion, which differs from the monocle in being held to one's eye with a handle in a fashion similar to a lorgnette , rather than being held in place by

1635-703: A pair of glasses is 12-14mm. A contact lens is placed directly on the eye and thus has a vertex distance of zero. In the UK and the US , the refractive index is generally specified concerning the yellow He -d Fraunhofer line , commonly abbreviated as n d . Lens materials are classified by their refractive index, as follows: This is a general classification. Often for marketing purposes, materials with n d values that are at or above 1.60 can be referred to as "high-index". Likewise, Trivex and other borderline normal / mid-index materials may be referred to as mid-index. Of all of

1744-440: A property of mid and higher index lenses that cannot be avoided, regardless of the material used. The Abbe number for a material at a particular refractive index formulation is usually specified as its Abbe value. In practice, an Abbe number change from 30 to 32 will not have a practically noticeable benefit, but a change from 30 to 47 could be beneficial for users with strong prescriptions that move their eyes and look "off-axis" of

1853-480: A recent ophthalmic prescription are required. People who need glasses to see often have corrective lens restrictions on their driver's licenses that require them to wear their glasses every time they drive or risk fines or jail time. Some militaries issue prescription glasses to servicemen and women. These are typically GI glasses . Many state prisons in the United States issue glasses to inmates, often in

1962-725: A rifle at a shooting range during his initial training. Corrective lens Corrective lenses are typically prescribed by an ophthalmologist or an optometrist . The prescription consists of all the specifications necessary to make the lens. Prescriptions typically include the power specifications of each lens (for each eye). Strengths are generally prescribed in quarter- diopter steps (0.25 D), because most people cannot generally distinguish between smaller increments (e.g., eighth-diopter steps / 0.125 D). The use of improper corrective lenses may not be helpful and can even exacerbate binocular vision disorders . Eyecare professionals (optometrists and ophthalmologists) are trained to determine

2071-432: A separate set of glasses for focusing on close by objects. Reading glasses are available without prescription from drugstores , and offer a cheap, practical solution, though these have a pair of simple lenses of equal power, and so will not correct refraction problems like astigmatism or refractive or prismatic variations between the left and right eye. For the total correction of the individual's sight, glasses complying to

2180-448: A slight bias toward plano is introduced it may be negligible cosmetically and optically. These optical degradations due to base-curve grouping also apply to aspherics since their shapes are intentionally flattened and then asphericized to minimize error for the average base curve in the grouping. The greatest cosmetic improvement on lens thickness (and weight) benefits from choosing a frame that holds physically small lenses. The smallest of

2289-442: A slightly lower density. Its lower refractive index of n d = 1.532 vs. polycarbonate's 1.586 may result in slightly thicker lenses depending upon the prescription. Along with polycarbonate and the various high-index plastics, Trivex is a lab favorite for use in rimless frames, owing to the ease with which it can be drilled and its resistance to cracking around the drill holes. One other advantage that Trivex has over polycarbonate

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2398-552: A smooth transition from distance correction to near correction, eliminating segment lines and allowing clear vision at all distances, including intermediate (roughly arms' length). The lack of any abrupt change in power and the uniform appearance of the lens gives rise to the name "no-line bifocal". 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

2507-411: A spherically designed center area for improved vision and fit. In practice, labs tend to produce pre-finished and finished lenses in groups of narrow power ranges to reduce inventory. Lens powers that fall into the range of the prescriptions of each group share a constant base curve. For example, corrections from -4.00D to -4.50D may be grouped and forced to share the same base curve characteristics, but

2616-526: A suboptimal flatter base-curve. They cannot surpass the optical quality of a spherical best-form lens but can reduce the error induced by using a flatter-than-optimal base curve. The improvement due to flattening is most evident for strong farsighted lenses. High myopes (-6D) may see a slight cosmetic benefit with larger lenses. Mild prescriptions will have no perceptible benefit (-2D). Even at high prescriptions, some high myope prescriptions with small lenses may not see any difference, since some aspheric lenses have

2725-409: A user. Chromatic aberration is independent of the lens being of spherical, aspheric, or atoric design. The eye's Abbe number is independent of the importance of the corrective lens's Abbe, since the human eye: In contrast, the eye moves to look through various parts of a corrective lens as it shifts its gaze, some of which can be as much as several centimeters off of the optical center. Thus, despite

2834-526: A visible distortion of their face as seen by other people, in the apparent size of the eyes and facial features visible through the eyeglasses. Either situation can result in social stigma due to some facial distortions. This can result in the low self-esteem of the eyeglass wearer and lead to difficulty in making friends and developing relationships. People with very high-power corrective lenses can benefit socially from contact lenses because these distortions are minimized and their facial appearance to others

2943-650: A wide range of fashions are available, using plastic, metal, wire, and other materials for frames. Glasses can be marked or found by their primary function, but also appear in combinations such as prescription sunglasses or safety glasses with enhanced magnification. Corrective lenses are used to correct refractive errors by bending the light entering the eye in order to alleviate the effects of conditions such as nearsightedness (myopia) , farsightedness (hypermetropia) or astigmatism . The ability of one's eyes to accommodate their focus to near and distant focus alters over time. A common condition in people over forty years old

3052-404: Is presbyopia , which is caused by the eye's crystalline lens losing elasticity, progressively reducing the ability of the lens to accommodate (i.e. to focus on objects close to the eye). Few people have a pair of eyes that show exactly equal refractive characteristics; one eye may need a "stronger" (i.e. more refracting) lens than the other. Corrective lenses bring the image back into focus on

3161-455: Is "single vision", which has a uniform refractive index . For people with presbyopia and hyperopia , bifocal and trifocal glasses provide two or three different refractive indices, respectively, and progressive lenses have a continuous gradient. Lenses can also be manufactured with high refractive indices, which allow them to be more lightweight and thinner than their counterparts with "low" refractive indices. Reading glasses provide

3270-502: Is achieved over the entire field of view , in any direction. Switching between distance and near vision is accomplished by re-adjusting the lens, instead of by tilting and/or rotating the head. The need for constant adjustment when the person's attention switches to an object at a different distance is a design challenge of such a lens. Manual adjustment is more cumbersome than bifocals or similar lenses. Automated systems require electronic systems, power supplies, and sensors that increase

3379-413: Is also known to have written on the magnifying properties of lenses. The development of the first eyeglasses took place in northern Italy in the second half of the 13th century. Independently of the development of optical lenses, some cultures developed " sunglasses " for eye protection, without any corrective properties. For example, flat panes of smoky quartz were used in 12th-century China , and

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3488-493: Is commonly used in telescopes and binoculars, and fluorite crown glasses such as the best optical quality low dispersion glass currently in production, N-FK58 made by the German company Schott with the following characteristics ( n d = 1.456 , V d = 90.90 , D = 3.65 g/cm³ ) and are commonly used in high-end camera lenses). One must bear in mind that the human eye itself has an Abbe number V d ≈ 50.2 so

3597-517: Is filtered so that each eye receives a slightly different image. The filters only work for the type of signal they were designed for. Anaglyph 3D glasses have a different colored filter for each eye, typically red and blue or red and green. A polarized 3D system on the other hand uses polarized filters. Polarized 3D glasses allow for color 3D, while the red-blue lenses produce an image with distorted coloration. An active shutter 3D system uses electronic shutters . Head-mounted displays can filter

3706-439: Is no measurable UV light from computer monitors. The problem of computer vision syndrome (CVS) can result from focusing the eyes on a screen for long, continuous periods. Many times the glasses do not appear to have much of a tint, or, if any, a slight yellow tint, but they may be more heavily tinted. Long hours of computer use (not blue light) may cause eye strain. Many eye symptoms caused by computer use will lessen after

3815-423: Is normal. Aspheric/atoric eyeglass design can also reduce minification and magnification of the eye for observers at some angles. Glass lenses have become less common owing to the danger of shattering and their relatively high weight compared to CR-39 plastic lenses. They still remain in use for specialised circumstances, for example in extremely high prescriptions (currently, glass lenses can be manufactured up to

3924-404: Is not yet twenty years since there was found the art of making eyeglasses, which make for good vision ... And it is so short a time that this new art, never before extant, was discovered. ... I saw the one who first discovered and practiced it, and I talked to him." Giordano's colleague Friar Alessandro della Spina of Pisa (d. 1313) was soon making eyeglasses. The Ancient Chronicle of

4033-417: Is of concern. Along with Trivex and the high-index plastics, polycarbonate is an excellent choice for rimless eyeglasses. Similar to the high-index plastics, polycarbonate has a very low Abbe number, which may be bothersome to individuals sensitive to chromatic aberrations. Glasses Glasses , also known as eyeglasses and spectacles , are vision eyewear with clear or tinted lenses mounted in

4142-509: Is often hard-pressed to find an optical laboratory that has the machinery needed shape custom glass lenses. A further complication for seeking lenses made of better, even-lower dispersion glass, is that the specialty glass is often expensive. Also, many exotic glass types, with Abbe number V d ≳ 65 , contain oxides of heavy metals such as arsenic or lanthanum , some of which are toxic. The need for special venting to protect technicians from exposure to powdered toxic glass further limits

4251-427: Is reduced by increasing index. There is also a minimum lens thickness required to support the lens shape. These factors result in a thinner lens which is not lighter than the original. There are lens materials with lower density at higher index which can result in a truly lighter lens. These materials can be found in a material property table. Reducing frame lens size will give the most noticeable improvement in weight for

4360-459: Is that it can be tinted. Polycarbonate is lighter weight than normal plastic. It blocks UV rays, is shatter resistant, and is used in sports glasses and glasses for children and teenagers. Because polycarbonate is soft and will scratch easily, a scratch-resistant coating is typically applied after shaping and polishing the lens. Standard polycarbonate with an Abbe number of 30 is one of the worst materials optically if chromatic aberration intolerance

4469-401: Is that they may alleviate symptoms, causing a person to forgo the other benefits of routine vision exams, such as early diagnosis of chronic disease. Although lenses are normally prescribed by optometrists or ophthalmologists, there is evidence from developing countries that allowing people to select lenses for themselves produces good results in the majority of cases and is less than a tenth of

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4578-404: Is used. However, if the lens is too small, it can drastically reduce the field of view. Bifocal , trifocal , and progressive lenses generally require a taller lens shape to leave room for the different segments while preserving an adequate field of view through each segment. Frames with rounded edges are the most efficient for correcting myopic prescriptions, with perfectly round frames being

4687-454: The College of Optometrists (UK) is that "the best scientific evidence currently available does not support the use of blue-blocking spectacle lenses in the general population to improve visual performance, alleviate the symptoms of eye fatigue or visual discomfort, improve sleep quality or conserve macula health." The ophthalmic frame is the part of a pair of glasses that is designed to hold

4796-635: The Inuit have used snow goggles for eye protection. The earliest recorded comment on the use of lenses for optical purposes was made in 1268 by Roger Bacon . The first eyeglasses were estimated to have been made in Central Italy , most likely in Pisa or Florence , by about 1290: In a sermon delivered on 23 February 1306, the Dominican friar Giordano da Pisa ( c.  1255 –1311) wrote "It

4905-492: The gallery . The gallery was designed to help secure the monocle in place by raising it out of the eye's orbit slightly so that the eyelashes would not jar it. Monocles with galleries were often the most expensive. The wealthy would have the frames custom-made to fit their eye sockets. A sub-category of the galleried monocle was the "sprung gallery", where the gallery was replaced by an incomplete circle of flattened, ridged wire supported by three posts. The ends were pulled together,

5014-457: The "night mode" of different operating systems, which can usually be activated outside of nighttime hours. The American Academy of Ophthalmology (AAO) does not recommend special eyewear for computer use, although it recommends using prescription glasses measured specifically for computer screen distance (depending on individuals, but possibly 20–26 inches from the face), which are not the same as "blue-light blocking" glasses. The position of

5123-585: The 12th century, coinciding with the development of " reading stones ". There are claims that single lens magnifying glasses were being used in China during the Northern Song dynasty (960–1127). Robert Grosseteste 's treatise De iride ( On the Rainbow ), written between 1220 and 1235, mentions using optics to "read the smallest letters at incredible distances". A few years later in 1262, Roger Bacon

5232-526: The 1930s to assist people bedbound by chronic illness or spinal injury, recumbent glasses have more recently been marketed not simply as an assistive device but also as 'lazy glasses'. They do not assist with vision, although they can be worn over regular corrective glasses. Yellow-tinted glasses are a type of glasses with a minor yellow tint. They perform a slight color correction, on top of reducing eyestrain from lack of blinking. They may also be considered minor corrective non-prescription glasses. Depending on

5341-593: The Dominican Monastery of St. Catherine in Pisa records: "Eyeglasses, having first been made by someone else, who was unwilling to share them, he [Spina] made them and shared them with everyone with a cheerful and willing heart." Venice quickly became an important center of manufacture, especially due to using the high-quality glass made at Murano . By 1301, there were guild regulations in Venice governing

5450-408: The basic fixed frame with another pair of lenses (optional), that are connected by four-bar linkage . For example, sun lenses could be easily lifted up and down while mixed with myopia lenses that always stay on. Presbyopia lenses could be also combined and easily removed from the field of view if needed without taking off glasses. These glasses are often used for drivers going through tunnels, with

5559-491: The beginning of the 20th century as the lens could be cut to fit any shape eye orbit inexpensively, without the cost of a customized frame. If customized, monocles can be worn securely with little effort. However, periodic adjustment is common for monocle wearers to keep the monocle from popping. Often only the rich could afford to have a monocle custom-fabricated, while the poor had to settle for ill-fitting monocles that were less comfortable and less secure. In popular perception,

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5668-404: The best optic and cosmetic characteristics across the entire surface of the lens. Optometrists may choose to specify a particular base curve when prescribing a corrective lens for either of these reasons. A multitude of mathematical formulas and professional clinical experience has allowed optometrists and lens designers to determine standard base curves that are ideal for most people. As a result,

5777-517: The cardinal Hugh de Saint-Cher reading in a scriptorium . Another early example would be a depiction of eyeglasses found north of the Alps in an altarpiece of the church of Bad Wildungen , Germany, in 1403. These early glasses had convex lenses that could correct both hyperopia (farsightedness), and the presbyopia that commonly develops as a symptom of aging . Although concave lenses for myopia (near-sightedness) had made their first appearance in

5886-405: 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). The power or focal length of adjustable or variable focus can be changed to suit the needs of the wearer. A typical application of such a lens is to refocus the correction allowing clear vision at any distance. Unlike with bifocals, near-vision correction

5995-433: The clarity of vision in the periphery. As corrective power increases, even optimally designed lenses will have distortion that can be noticed by a user. This particularly affects individuals that use the off-axis areas of their lenses for visually demanding tasks. For individuals sensitive to lens errors, the best way to eliminate lens induced aberrations is to use contact lenses. Contacts eliminate all these aberrations since

6104-764: The company, these computer or gaming glasses can also filter out high energy blue and ultra-violet light from LCD screens , fluorescent lighting , and other sources of light. This allows for reduced eye-strain. These glasses can be ordered as standard or prescription lenses that fit into standard optical frames. By the end of the 2010s, eyeglasses that filter out blue light from computers , smartphones and tablets are becoming increasingly popular in response to concerns about problems caused by blue light overexposure. The problems claimed range from dry eyes to eye strain , sleep cycle disruption, up to macular degeneration which can cause partial blindness. They may also block out ultraviolet (UV) radiation. However, there

6213-434: The corrective glass and improve aesthetic appearance (mini telescopic spectacles). They may take the form of self-contained glasses that resemble goggles or binoculars , or may be attached to existing glasses. Recumbent or prism glasses are glasses that use a prism with a 90° refraction to allow the wearer to read or view a screen while lying on their back. Developed by Liverpudlian ophthalmologist Andrew McKie Reid in

6322-458: The cost of prescription lenses. Single vision lenses correct for only one distance. If they correct for far distance, the person must accommodate to see up close. If the person cannot accommodate, they may need a separate correction for near distances, or else use a multifocal lens (see below). Reading glasses are single vision lenses designed for near work and include over the counter glasses. They come in two main styles: full frames, in which

6431-442: The cost, size, and weight of the correction. A corrective lens with a power of zero is called a plano lens. These lenses are used when one or both eyes do not require correction of a refractive error . Some people with good natural eyesight like to wear eyeglasses as a style accessory, or want to change the appearance of their eyes using novelty contact lenses. Although corrective lenses can be produced in many different profiles,

6540-466: The development of the transistor in the 1940s, combined eyeglass-hearing aids became popular. With thick-rimmed glasses the fashion at the time, a hearing aid could be concealed in the temple part of the frame. These fell out of fashion after the 1970s, but there are still occasions when combined eyeglass-hearing aids may be useful. Safety glasses are worn to protect the eyes in various situations. They are made with break-proof plastic lenses to protect

6649-414: The divergent back surface. For myopia the opposite is true: the divergent back surface is greater in magnitude than the convergent front surface. To correct for presbyopia , the lens, or section of the lens, must be more convergent or less divergent than the person's distance lens. The base curve (usually determined from the profile of the front surface of an ophthalmic lens) can be changed to result in

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6758-468: The ease by which a lens can be scratched, and the limitations and costs of producing higher-index lenses. CR-39 lenses are inherently scratch resistant. Trivex was invented by Edwin C. Slagel and patented in September 1998. Trivex was developed in 2001 by PPG Industries for the military as transparent armor. With Hoya Corporation and Younger Optics, PPG announced the availability of Trivex for

6867-415: The entire lens is made in the reading prescription, and half-eyes, style glasses that sit lower down on the nose . Full frame readers must be removed to see distance clearly, while the distance can be clearly viewed over the top of half-eye readers. A bifocal is a lens with two sections, separated by a line (see image to the right). Generally, the upper part of the lens is used for distance vision, while

6976-566: The expensive, high-end optical glass types mentioned above have little value for central vision; however, the wearer's view through the side of a glass lens is not comparable to central vision through the eye: Low dispersion glass definitely makes optically superior corrective lenses since it greatly reduces color-fringing of edge-wise viewed contrasty objects, compared to all available plastics. But glass lenses are much heavier, and making them requires specialized glass grinding equipment no longer common in ordinary prescription lens labs: At present, one

7085-615: The eye from flying debris or other matter. Construction workers, factory workers, machinists and lab technicians are often required to wear safety glasses to shield the eyes from flying debris or hazardous splatters such as blood or chemicals. As of 2017, dentists and surgeons in Canada and other countries are required to wear safety glasses to protect against infection from patients' blood or other body fluids. There are also safety glasses for welding , which are styled like wraparound sunglasses, but with much darker lenses, for use in welding where

7194-461: The eye socket itself. There are three additional styles of the monocle. The first style consists of a simple loop of metal with a lens that was slotted into the eye orbit . These were the first monocles worn in England and could be found from the 1830s onwards. The second style, which was developed in the 1890s, was the most elaborate, consisting of a frame with a raised edge-like extension known as

7303-409: The eye's dispersive properties, the corrective lens's dispersion cannot be dismissed. People who are sensitive to the effects of chromatic aberrations, or who have stronger prescriptions, or who often look off the lens's optical center, or who prefer larger corrective lens sizes may be impacted by chromatic aberration. To minimize chromatic aberration: Power error is the change in the optical power of

7412-455: The eyes from the dangers of UV light, sunglasses should have UV-400 blocker to provide good coverage against the entire light spectrum that poses a danger. Light polarization is an added feature that can be applied to sunglass lenses. Polarization filters are positioned to remove horizontally polarized rays of light, which eliminates glare from horizontal surfaces (allowing wearers to see into water when reflected light would otherwise overwhelm

7521-568: The floorboards at Kloster Wienhausen , a convent near Celle in Germany; they have been dated to circa 1400. The world's first specialist shop for spectacles—what we might regard today as an optician —opened in Strasbourg (then Holy Roman Empire , now France) in 1466. The 17th-century claim by Francesco Redi that Salvino degli Armati of Florence invented eyeglasses in the 13th century has been exposed as erroneous. Marco Polo

7630-414: The form of clear plastic aviators. Adjustable-focus eyeglasses might be used to replace bifocals or trifocals, or might be used to produce cheaper single-vision glasses (since they do not have to be custom-manufactured for every person). Pinhole glasses are a type of corrective glasses that do not use a lens. Pinhole glasses do not actually refract the light or change focal length. Instead, they create

7739-403: The front surface curve is more standardized and the characteristics that generate a person's unique prescription are typically derived from the geometry of the back surface of the lens. Bifocals and trifocals result in a more complex lens profile, compounding multiple surfaces. The main lens is composed of a typical ophthalmic lens. Thus the base curve defines the front surface of the main part of

7848-419: The geometry into a single piece of lens material. There are many locations, profiles, and sizes of add segments typically referred to as segment type. Some "seg type" examples include Flat top, Kryptok, Orthogon, Tillyer Executive, and Ultex A. Trifocals contain two add segments to achieve a lens that corrects the person's vision for three distinct distances. The optical center of the add segment may be placed on

7957-487: The glasses, which would be required with single vision correction. Trifocal lenses are similar to bifocals, except that the two focal areas are separated by a third area (with intermediate focus correction) in the middle. This segment corrects the wearer's vision for intermediate distances roughly at arms' length, e.g. computer distance. This lens type has two segment lines, dividing the three different correcting segments. Progressive addition or varifocal lenses provide

8066-445: The image of the viewed object bigger, their main advantage comes from focusing the image, not magnification. These glasses are not tailored to a person's individual needs. A difference in refractive error between the eyes or the presence of astigmatism will not be accounted for. People with little to no need for correction in the distance may find off-the-shelf glasses work quite well for seeing better during near vision tasks. But if

8175-519: The late 19th century, but are rarely worn today. This is due in large part to advances in optometry which allow for better measurement of refractive error , so that glasses and contact lenses can be prescribed with different strengths in each eye. The monocle did, however, gain a following in the stylish lesbian circles of the early 20th century, when lesbians would wear a monocle for effect. Such women included Una Lady Troubridge , Radclyffe Hall , and Weimar German reporter Sylvia von Harden ;

8284-429: The lens surface or may hang off into an empty space near the lens surface. Although the surface profile of a bifocal segment is spherical, it is often trimmed to have straight edges so that it is contained within a small region of the overall lens surface. The progressive addition lens (PAL, also commonly called a no-line or varifocal lens) eliminates the line in bi/tri-focals and is very complex in its profile. PALs are

8393-412: The lens then moves with the eye. Barring contacts, a good lens designer does not have many parameters that can be traded off to improve vision. The index has little effect on error. Note that, although chromatic aberration is often perceived as "blurry vision" in the lens periphery and gives the impression of power error, this is actually due to color shifting. Chromatic aberration can be improved by using

8502-440: The lens while the back surface geometry is changed to achieve the desired distance power. The "bifocal" is a third spherical segment, called an add segment , found on the front surface of the lens. Steeper and more convergent than the base curve, the add segment combines with the back surface to deliver the person's near correction. Early manufacturing techniques fused a separate lens to the front surface, but modern processes cut all

8611-411: The lens. Generally, the best spherical form lenses attempt to keep the ocular curve between four and seven diopters. As the eye shifts its gaze from looking through the optical center of the corrective lens, the lens-induced astigmatism value increases. In a spherical lens, especially one with a strong correction whose base curve is not in the best spherical form, such increases can significantly impact

8720-530: The lens. In glasses with powers beyond ±4.00D, the vertex distance can affect the effective power of the glasses. A shorter vertex distance can expand the field of view, but if the vertex distance is too small, the eyelashes will come into contact with the back of the lens, smudging the lens and causing annoyance for the wearer. A skilled frame stylist will help the wearer select a good balance of fashionable frame size with good vertex distance to achieve ideal aesthetics and field of view. The average vertex distance in

8829-463: The lenses in the proper position. Ophthalmic frames come in a variety of styles, sizes, materials, shapes, and colors. Various metals and alloys may be used to make glasses, such as gold, silver, aluminum, beryllium , stainless steel , titanium , Monel , and nickel titanium . Natural materials such as wood, bone, ivory, leather and semi-precious or precious stones may also be used. Corrective lenses can be produced in many different shapes from

8938-495: The lenses; however, special prescription sunglasses can be made. People with conditions that have photophobia as a primary symptom (like certain migraine disorders) often wear sunglasses or precision tinted glasses, even indoors and at night. Specialized glasses may be used for viewing specific visual information, for example, 3D glasses for 3D films ( stereoscopy ). Sometimes glasses are worn purely for fashion or aesthetic purposes. Even with glasses used for vision correction,

9047-473: The light more tolerable. Sunglasses may also have corrective lenses, which requires a prescription. Clip-on sunglasses or sunglass clips can be attached to another pair of glasses. Some wrap-around sunglasses are large enough to be worn over another pair of glasses. Otherwise, many people opt to wear contact lenses to correct their vision so that standard sunglasses can be used. The double frame uplifting glasses have one moving frame with one pair of lenses and

9156-517: The loss and breaking of the glasses. Sunglasses allow for better vision in bright daylight and are used to protect one's eyes against damage from excessive levels of ultraviolet light . Typical sunglasses lenses are tinted for protection against bright light or polarized to remove glare; photochromic glasses are clear or lightly tinted in dark or indoor conditions, but turn into sunglasses when they come into contact with ultraviolet light. Most over-the-counter sunglasses do not have corrective power in

9265-433: The lower segment is used for near vision. The area of the lens that caters to near vision is called the add segment. There are many different shapes, sizes, and positions for the add segment, such as rounded bifocals and D segment bifocals, that are selected for functional differences as well as the visual demands of the patient. Bifocals allow people with presbyopia to see clearly at distance and near without having to remove

9374-406: The mid-15th century, it was not until 1604 that Johannes Kepler published the first correct explanation as to why convex and concave lenses could correct presbyopia and myopia. Early frames for glasses consisted of two magnifying glasses riveted together by the handles so that they could grip the nose. These are referred to as "rivet spectacles". The earliest surviving examples were found under

9483-401: The monocle was placed in the eye orbit, and the ends were released, causing the gallery to spring out and keep the monocle in place. The third style of monocle was frameless. This consisted of a cut piece of glass, with a serrated edge to provide a grip and sometimes a hole drilled into one side for a cord. Often the frameless monocle had no cord and would be worn freely. This style was popular at

9592-417: The most common is ophthalmic or convex-concave. In an ophthalmic lens, both the front and back surfaces have a positive radius, resulting in a positive/convergent front surface and a negative/divergent back surface. The difference in curvature between the front and rear surface leads to the corrective power of the lens. In hyperopia a convergent lens is needed; therefore, the convergent front surface overpowers

9701-534: The most efficient. Before the advent of eyeglasses as a fashion item, when frames were constructed with only functionality in mind, virtually all eyeglasses were either round , oval , panto, rectangular , octagonal , or square . It was not until glasses began to be seen as an accessory that different shapes were introduced to be more aesthetically pleasing than functional. Scattered evidence exists for use of visual aid devices in Greek and Roman times, most prominently

9810-426: The number of optical labs that can safely grind super-low dispersion / large Abbe number glass. Abbe numbers ( V d ) in excess of Crown Glass and CR-39 are mainly warranted only for unusual special uses, such as Plastic lenses are currently the most commonly prescribed lens, owing to their relative safety, low cost, ease of production, and high optical quality. The main drawbacks of many types of plastic lenses are

9919-451: The optical center of the lens. Some users do not sense color fringing directly but will just describe "off-axis blurriness". Abbe values even as high as that of ( V d ≤ 45 ) produce chromatic aberrations which can be perceptible to a user in lenses larger than 40 mm in diameter and especially in strengths that are in excess of ±4 D. At ±8 D even glass ( V d ≤ 58 ) produces chromatic aberration that can be noticed by

10028-478: The optical industry in 2001. Trivex is a urethane-based pre-polymer. PPG named the material Trivex because of its three main performance properties: Superior optics, ultra lightweight, and extreme strength. Trivex is a relative newcomer that possesses the UV-blocking properties and shatter resistance of polycarbonate while at the same time offering far superior optical quality (i.e., higher Abbe number) and

10137-1138: The painting Portrait of the Journalist Sylvia Von Harden by German expressionist painter Otto Dix depicts its subject wearing a monocle. Monocle wearers have included British politicians Joseph Chamberlain , his son Austen , Henry Chaplin , and Angus Maude . Percy Toplis ( The Monocled Mutineer ), Astronomer and The Sky at Night Presenter Sir Patrick Moore , founder of Pakistan Mohammad Ali Jinnah , Portuguese President António de Spínola , filmmakers Fritz Lang and Erich von Stroheim , 19th-century Portuguese writer Eça de Queiroz , Soviet writer Mikhail Bulgakov , actor Conrad Veidt , Dadaists Tristan Tzara and Raoul Hausmann , esoteric-fascist Julius Evola , French collaborationist politician Louis Darquier de Pellepoix , Poet laureate Alfred Lord Tennyson , singer Richard Tauber , diplomat Christopher Ewart-Biggs (a smoked-glass monocle, to disguise his glass eye), Major Johnnie Cradock , actors Ralph Lynn , George Arliss and Martyn Green , and Karl Marx . In another vein, G. E. M. Anscombe

10246-445: The person has a significant need for distance correction, it is less likely that the over-the-counter glasses will be perfectly effective. Although such glasses are generally considered safe, an individual prescription, as determined by an ophthalmologist or optometrist and made by a qualified optician , usually results in better visual correction and fewer headaches and visual discomfort. Another criticism of over-the-counter glasses

10355-403: The popular adult lens sizes available in retail outlets is about 50 mm (2.0 in) across. There are a few adult sizes of 40 mm (1.6 in), and although they are quite rare, can reduce lens weight to about half of the 50 mm versions. The curves on the front and back of a lens are ideally formed with the specific radius of a sphere. This radius is set by the lens designer based on

10464-403: The prescription and cosmetic consideration. Selecting a smaller lens will mean less of this sphere surface is represented by the lens surface, meaning the lens will have a thinner edge (myopia) or center (hyperopia). A thinner edge reduces light entering into the edge, reducing an additional source of internal reflections. Extremely thick lenses for myopia can be beveled to reduce flaring out of

10573-532: The properties of particular lens material, the one that most closely relates to its optical performance is its dispersion , which is specified by the Abbe number . Higher Abbe numbers mean a better lens material, and lower Abbe numbers result in the presence of chromatic aberration (i.e., color fringes above/below or to the left/right of a high contrast object), especially in larger lens sizes and stronger prescriptions (beyond ±4.00 D ). Generally, lower Abbe numbers are

10682-460: The retina. They are made to conform to the prescription of an ophthalmologist or optometrist . A lensmeter can be used to verify the specifications of an existing pair of glasses. Corrective eyeglasses can significantly improve the life quality of the wearer. Not only do they enhance the wearer's visual experience, but can also reduce problems that result from eye strain, such as headaches or squinting. The most common type of corrective lens

10791-423: The sale of eyeglasses and a separate guild of Venetian spectacle makers was formed in 1320. In the fourteenth century, they were very common objects: Francesco Petrarca says in one of his letters that, until he was 60, he did not need glasses, and Franco Sacchetti mentions them often in his Trecentonovelle . The earliest pictorial evidence for the use of eyeglasses is Tommaso da Modena 's 1352 portrait of

10900-639: The scene). Polarized sunglasses may present some difficulties for pilots since reflections from water and other structures often used to gauge altitude may be removed. Liquid-crystal displays emit polarized light, making them sometimes difficult to view with polarized sunglasses. Sunglasses may be worn for aesthetic purposes, or simply to hide the eyes. Examples of sunglasses that were popular for these reasons include tea shades and mirrorshades . Many blind people wear nearly opaque glasses to hide their eyes for cosmetic reasons. Many people with light sensitivity conditions wear sunglasses or other tinted glasses to make

11009-430: The sides of the eyes as well as in the lenses. Some types of safety glasses are used to protect against visible and near-visible light or radiation . Glasses are worn for eye protection in some sports, such as squash . Glasses wearers may use a strap to prevent the glasses from falling off. Wearers of glasses that are used only part of the time may have the glasses attached to a cord that goes around their neck to prevent

11118-681: The signal electronically and then transmit light directly into the viewer's eyes. Anaglyph and polarized glasses are distributed to audiences at 3D movies . Polarized and active shutter glasses are used with many home theaters. Head-mounted displays are used by a single person, but the input signal can be shared between multiple units. Glasses can also provide magnification that is useful for people with vision impairments or specific occupational demands. An example would be bioptics or bioptic telescopes which have small telescopes mounted on, in, or behind their regular lenses. Newer designs use smaller lightweight telescopes, which can be embedded into

11227-708: The specific corrective lenses that will provide the clearest, most comfortable, and most efficient vision, avoiding double vision and maximizing binocularity. Ready-made single-vision reading glasses go by many names, including over-the-counter glasses, ready readers, cheaters, magnifiers, non-prescription readers, or generic readers. They are designed to lessen the focusing burden of near work, such as reading. They are typically sold in retail locations such as pharmacies and grocery stores but are also available in book stores and clothing retailers. They are available in common reading prescriptions with strengths ranging from +0.75 to +3.50 diopters . While these "magnifiers" do indeed make

11336-420: The spherical form is only best for a -4.25D prescription. In this case, the error will be imperceptible to the human eye. However, some manufacturers may further cost-reduce inventory and group over a larger range which will result in perceptible error for some users in the range who also use the off-axis area of their lens. Additionally, some manufacturers may verge toward a slightly flatter curve. Although if only

11445-423: The upper frame serving as sunglasses and the second frame as transparent lenses. The illusion of three dimensions on a two-dimensional surface can be created by providing each eye with different visual information. 3D glasses create the illusion of three dimensions by filtering a signal containing information for both eyes. The signal, often light reflected off a movie screen or emitted from an electronic display,

11554-430: The usage of the computer is stopped. Decreasing evening screen time and setting devices to night mode will improve sleep. Several studies have shown that blue light from computers does not lead to eye diseases, including macular degeneration. The total amount of light entering the eyes can be adjusted without glasses using the screen brightness settings. Similarly, the blue light can often specifically be adjusted using

11663-657: The use of an emerald by Emperor Nero as mentioned by Pliny the Elder . The use of a convex lens to form an enlarged/magnified image was most likely described in Ptolemy 's Optics (which survives only in a poor Arabic translation). Ptolemy's description of lenses was commented upon and improved by Ibn Sahl (10th century) and most notably by Alhazen ( Book of Optics , c.  1021 ). Latin translations of Ptolemy's Optics and of Alhazen became available in Europe in

11772-439: The very thick edge. Thick myopic lenses are not usually mounted in wire frames, because the thin wire contrasts against the thick lens, to make its thickness much more obvious to others. The index can improve the lens thinness, but at a point, no more improvement will be realized. For example, if an index and lens size is selected with center to edge thickness difference of 1 mm then changing index can only improve thickness by

11881-481: Was one of only a few noted women who occasionally wore a monocle. Abstract expressionist painter Barnett Newman wore a monocle mainly for getting a closer look at artworks. Richard Tauber wore a monocle to mask a squint in one eye. The Irish poet William Butler Yeats wore them at times too. The English author, Evelyn Waugh , used a monocle when he was in the Army during World War II and needed to focus his vision using

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