eyeglasses or spectacles, instrument or device for aiding and correcting defective sight. Eyeglasses usually consist of a pair of lenses mounted in a frame to hold them in position before the eyes. The first device of this kind was probably invented by Roger Bacon in the 13th cent. although similar devices are believed to have existed in ancient times in China and in the Mediterranean civilizations. Early forms were crude and clumsy and were not improved until the 18th cent. when the grinding of lenses was first based upon the principles of light refraction. Lenses are made of clear or rock crystal glass or plastic ground to suit the defect of the eye. Concave glass is used for nearsightedness, so that the rays of light are diverged. Convex lenses are used for farsightedness, so that the light rays are converged. Astigmatism is remedied by cylindrical lenses. Bifocal lenses, with a lower part for viewing objects near at hand (as in reading), were first devised by Benjamin Franklin. Telescopic lenses are used by the near-blind. A contact lens is shaped to fit the eye and is worn under the eyelid. Incorrect eyeglasses may do harm, and lenses should be prescribed by an ophthalmologist or optometrist and fitted by a skilled optician. Eyeglasses to protect the eyes from glare or from foreign bodies are made of tinted or polarized glass and of wire mesh. The Eskimo make and use wooden eyeglasses that have only narrow slits for eyepieces to protect the eyes from glare reflected by ice and snow.

Definition

Eyeglasses and contact lenses are devices that correct refractive errors in vision. Eyeglass lenses are mounted in frames that are worn on the face, sitting mostly on the ears and nose, so that the lenses are positioned in front of the eyes. Contact lenses appear to be worn in direct contact with the cornea, but they actually float on a layer of tears that separates them from the cornea.

Purpose

The purpose of eyeglasses and contact lenses is to correct or improve the vision of people with nearsightedness (myopia), farsightedness (hyperopia), presbyopia, and astigmatism.

Description

Eyes are examined by optometrists (OD) or by ophthalmologists (MD or DO). Prescriptions, if necessary, are then given to patients for glasses. Eyeglasses are generally made by an optician. A separate contact lens-fitting exam is necessary if an individual wants contact lenses, because an eyeglass prescription can differ from a contact lens prescription.

Eyeglasses

More than 140 million people in the United States wear eyeglasses. People whose eyes have refractive errors do not see clearly without glasses, because the light emitted from the objects they are observing does not come into focus on their retinas. For people who are farsighted, images come into focus behind the retina; for people who are nearsighted, images come into focus in front of the retina. For both, the result is a blurring of vision.

LENSES Lenses work by changing the direction of light so that images come into focus on the retina. The greater the index of refraction of the lens material and the greater the difference in the curvature between the two surfaces of the lens, the greater the change in direction of light that passes through it and the greater the correction.

Lenses can be unifocal, with one correction for all distances, or they can correct for more than one distance (multifocal). One type of multifocal, the bifocal, has an area of the lens (usually at the bottom) that corrects for nearby objects (about 14 in [35 cm] from the eyes); the remainder of the lens corrects for distant objects (about 20 ft [6 m] from the eyes). Another type of multifocal, a trifocal, has an area in-between that corrects for intermediate distances (usually about 28 in [71 cm]) Conventional bifocals and trifocals have visible lines between the areas of different correction; however, lenses in which the correction gradually changes from one area to the other, without visible lines, have been available since the 1970s. Such lenses are sometimes called progressives or no-line bifocals.

To be suitable for eyeglass lenses, a material must be transparent, without bubbles, and have a high index of refraction. The greater the index of refraction, the thinner the lens can be. Lenses are made from either glass or plastic (hard resin). The advantage of plastic is that it is lightweight and more impact-resistant than glass. The advantage of glass is that it is scratch resistant and provides the clearest possible vision.

Glass was the first material to be used for eyeglass lenses and was used for several hundred years before plastic was introduced. The crown glass used for eyeglass lenses has an index of refraction of 1.52.

In the early 2000s eyeglass wearers can choose polycarbonate or polyurethane materials for their lenses. Polycarbonate is the most impact-resistant material available for eyewear, and polyurethane has exceptional optical qualities and an index of refraction of up to 1.66, much higher than the conventional plastics used for lenses and even higher than glass. Parents whose children have high prescriptions should ask about high-index material options for their lenses. Aspheric lenses are also useful for high prescriptions. They are flatter and lighter than conventional lenses.

There are many lenses and lens-coating options for individual needs, including coatings that block the ultraviolet (UV) light and/or blue light which have been found to be harmful to the eyes. Such coatings are not needed on polycarbonate lenses, which already have UV protection. UV coatings are particularly important on sunglasses and ski goggles. Sunglasses, when nonprescription, should be labeled with an indication that they block out 99 to 100 percent of both UV-A and UV-B rays.

There are anti-scratch coatings that increase the surface hardness of lenses (an important feature when using plastic lenses) and anti-reflective (AR) coatings that eliminate almost all glare and allow other people to see the eyes of the wearer. AR coatings may be particularly helpful to people who use computers or who drive at night. Mirror coatings that prevent other people from seeing the wearer's eyes are also available. There is a whole spectrum of tints, from light to dark, used in sunglasses. Tint, however, does not block-out UV rays, so a UV coating is needed. Polaroid lenses that block out much of the reflected light also allow better vision in sunny weather and are helpful for people who enjoy boating. Photosensitive (photochromatic) lenses that darken in the presence of bright light are handy for people who do not want to carry an extra set of glasses. Photochromatic lenses are available in glass and plastic.

FRAMES Frames can be made from metal or plastic, and they can be rimless. There is an almost unlimited variety of shapes, colors, and sizes. The type and degree of refractive correction in the lens determine to some extent the type of frame most suitable. Some lenses are too thick to fit in metal rims, and some large-correction prescriptions are best suited to frames with small-area lenses.

Rimless frames are the least noticeable type, and they are lightweight because the nosepiece and temples are attached directly to the lenses, eliminating the weight of the rims. They tend not to be as sturdy as frames with rims, so they are not a good choice for children, or for people who frequently remove their glasses and put them on again. They are also not very suitable for lenses that correct a high degree of farsightedness, because such lenses are thin at the edges.

Metal frames are less noticeable than plastic, and they are lightweight. They are available in solid gold, gold-filled, anodized aluminum, nickel, silver, stainless steel, and titanium and titanium alloy. Until the late 1980s, when titanium-nickel alloy and titanium frames were introduced, metal frames were, in general, more fragile than plastic frames. Titanium frames, however, are very strong and lightweight. An alloy of titanium and nickel, called Flexon, is strong, lightweight, and returns to its original shape after being twisted or dented. It is not perfect for everyone, though, because some young people are sensitive to its nickel content. Flexon frames are also relatively expensive.

Plastic frames are durable, can accommodate just about any lens prescription, and are available in a wide range of prices. They are also offered in a variety of plastics, including acrylic, epoxy, cellulose acetate, cellulose propionate, polyamide, and nylon, and in different colors, shapes, and levels of resistance to breakage. Epoxy frames are resilient and return to their original shape after being deformed, so they do not need to be adjusted as frequently as other types. Nylon frames are almost unbreakable. They revert to their original shape after extreme trauma and distortion; because of this property, though, they cannot be readjusted after they are manufactured.

FIT An individual should have the distance between the eyes (PD) measured, so that the optical centers of the lenses will be in front of the person's pupils. Bifocal heights also have to be measured with the chosen frame in place and adjusted on the person. Again, this is so that the lenses will be positioned correctly. If not positioned correctly, the individual may experience eyestrain, headache, or other problems.

Children may sometimes need a few days to adjust to a new prescription. However, problems should be reported, because the glasses may need to be rechecked.

Contact lenses

Over 32 million people in the United States wear these small lenses that fit on top of the cornea. They provide a field of view unobstructed by eyeglass frames. They do not fog up or get splattered, so it is possible to see well while walking in the rain. They are less noticeable than any style of eyeglass. On the other hand, they take time to get accustomed to; require more measurements for fitting; require many follow-up visits to an eye doctor; can lead to complications such as infections and corneal damage; and may not correct astigmatism as well as eyeglasses, especially if the astigmatism is severe.

Originally, hard contact lenses were made of a material called PMMA. The more common types of contact lenses are listed below:

• Rigid gas-permeable (RGP) daily-wear lenses are made of plastic that does not absorb water but allows oxygen to get from the atmosphere to the cornea. (This is important because the cornea has no blood supply and needs to get its oxygen from the atmosphere through the film of tears that moves beneath the lens.) These lenses must be removed and cleaned each night.
• Rigid gas-permeable (RGP) extended-wear lenses are made from plastic that also does not absorb water but is more permeable to oxygen than the plastic used for daily-wear lenses. They can be worn up to a week.
• Daily wear soft lenses are made of plastic that is permeable to oxygen and absorbs water; therefore, they are soft and flexible. These lenses must be removed and cleaned each night, and they do not correct all vision problems. Soft lenses are easier to get used to than rigid lenses but are more prone to tears and do not last as long.
• Extended-wear soft lenses are highly permeable to oxygen, are flexible by virtue of their ability to absorb water, and can usually be worn for up to one week. They do not correct all vision problems. There is more risk of infection with extended-wear lenses than with daily-wear lenses.
• Extended-wear disposable lenses are soft lenses worn continually for up to six days and then discarded, with no need for cleaning.
• Planned-replacement soft lenses are daily-wear lenses that are replaced on a regular schedule, which is usually every two weeks, monthly, or quarterly. They must also be cleaned.

Soft contact lenses come in a variety of materials. There are also different kinds of RGP and soft multifocal contact lenses available. Monovision, where one contact lens corrects for distance vision while the other corrects for near vision, may be an option for persons with presbyopia. Monovision, however, may affect depth perception and may not be appropriate for everyone. Contact lenses also come in a variety of tints. Soft contacts are available that can change the color of dark-colored eyes. Even though such lenses have no prescription, they must be fitted and checked to make sure that an eye infection does not occur. People should never wear someone else's contact lenses. Doing so can lead to infection or damage to the eye.

Risks

Young people allergic to certain plastics should not wear contact lenses or eyeglass frames or lenses manufactured from that type of plastic. People allergic to nickel should not wear Flexon frames. Children and teens at risk of being in accidents that might shatter glass lenses should wear plastic lenses, preferably polycarbonate. (Lenses made from polycarbonate, the same type of plastic used for the space shuttle windshield, are about 50 times stronger than other lens materials.) Also, young people whose work places them at risk of receiving electric shock should avoid metal frames.

People employed in certain occupations may be prohibited from wearing contact lenses or may be required to wear safety eyewear over the contact lenses. Some occupations, such as construction or auto repair, may require safety lenses and safety frames. Physicians and employers should be consulted for recommendations.

Contact lens wearers must be examined periodically by their eye doctors to make sure that the lenses fit properly and that there is no infection. Both infection and lenses that do not fit properly can damage the cornea. People can be allergic to certain solutions that are used to clean or lubricate lenses. For that reason, individuals should not randomly switch products unless they speak with their doctor. Contact lens wearers should seek immediate attention if they experience eye pain, a burning sensation, red eyes, intolerable sensitivity to light, cloudy vision, or an inability to keep the eyes open.

To avoid infection, it is important for contact lens wearers to exactly follow their instructions for lens insertion and removal, as well as cleaning. Soft contact lens wearers should never use tap water to rinse their lenses or to make-up solutions. All contact lens wearers should also always have a pair of glasses and a carrying case for their contacts with them, in case the contacts have to be removed due to eye irritation. Wearing contact lenses increases the risk of corneal damage and eye infections.

Normal results

Improved vision is the primary result of corrective lenses. The normal expectation is that people will achieve 20/20 vision while wearing corrective lenses. Contact lenses may contribute to improved cosmetic appearance for some users.

Parental concerns

Parents of young children requiring corrective lenses should be prepared for broken or lost glasses. Parents of adolescents who wear contact lenses should help their children to maintain a regular cleaning schedule. Glasses normally last one to two years. Growing children may require changes of prescription more frequently than people in other age groups.

Resources

BOOKS

Douglas, Lloyd G. My Eyes. Bridgeport, CT: Children's Press, 2004.

Milder, Benjamin, and Melvin L. Rubin. The Fine Art of Prescribing Glasses Without Making a Spectacle of Yourself, 3rd ed. Gainesville, FL: Triad Publishing, 2004.

Olitsky, Scott, and Leonard B. Nelson. "Disorders of Vision." In Nelson Textbook of Pediatrics, 17th ed. Richard E. Behrman et al. Philadelphia: Saunders, 2003, pp. 2087–2089.

Willson, Sarah. Hocus Focus. New York: Kane Press, 2004.

PERIODICALS

Glavas, I. P., et al. "Sunglasses- and photochromic lens-wearing patterns in spectacle and/or contact lens-wearing individuals." Eye and Contact Lens 30, no. 2 (2004): 81–84.

Kemper, A. R., D. Bruckman, and G. L. Freed. "Prevalence and distribution of corrective lenses among school-age children." Optometry and Vision Science 81, no. 1 (2004): 7–10.

Kemper, A. R., L. M. Cohn, and K. J. Dombkowski. "Patterns of vision care among Medicaid-enrolled children." Journal of Pediatrics 113, no. 3 pt 1 (2004): e190–e196.

ORGANIZATIONS

American Academy of Ophthalmology. PO Box 7424, San Francisco, CA 94120. Web site: <www.eyenet.org>.

American Optometric Association. 243 North Lindbergh Blvd., St. Louis, MO 63141. Web site: <www.aoanet.org/>.

Contact Lens Society of America. 441 Carlisle Drive, Herndon, VA 20170. Web site: <www.clsa.info/index.shtml>.

Opticians Association of America. 44 Carlisle Drive, Herndon, VA 20170. Web site: <www.oaa.org>.

WEB SITES

"Contact Lenses." Contact Lens Council. Available online at <www.coontactlenscouncil.org/> (accessed September 28, 2004).

"Eyeglasses." Available online at <www.allaboutvision.com/eyeglasses/> (accessed September 28, 2004).

"Eyeglass Recycling." Available online at <www.lionsclubs.org/EN/content/vision_eyeglass_recycling.shtml> (accessed September 28, 2004).

"How to Read Your Eyeglasses Prescription." Available online at <www.medem.com/MedLB/article_detaillb.cfm?article_ID=ZZZG178LH4C&sub_cat=2017> (accessed September 28, 2004).

L. Fleming Fallon, Jr., MD, DrPH

"Glasses" can also be the plural of "glass".

Glasses, also called eyeglasses or spectacles, are frames bearing lenses worn in front of the eyes, normally for vision correction, eye protection, or for protection from UV rays.

Modern glasses are typically supported by pads on the bridge of the nose and by temples placed over the ears. Historical types include the pince-nez, monocle, and lorgnette.

Eyeglass frames are commonly made from metal or plastic. Lenses were originally made from glass, but many are now made from various types of plastic, including CR-39 or polycarbonate. These materials reduce the danger of breakage and weigh less than glass lenses. Some plastics also have more advantageous optical properties than glass, such as better transmission of visible light and greater absorption of ultraviolet light. Some plastics have a greater index of refraction than most types of glass; this is useful in the making of corrective lenses shaped to correct various vision abnormalities such as myopia, allowing thinner lenses for a given prescription.

Scratch-resistant coatings can be applied to most plastic lenses giving them similar scratch resistance to glass. Hydrophobic coatings designed to ease cleaning are also available, as are anti-reflective coatings intended to improve night vision and make the wearer's eyes more visible.

Polycarbonate lenses are the lightest and most shatter-resistant, making them the best for impact protection, yet offer poor optics due to high dispersion, and having a low Abbe number of 31. CR-39 lenses are the most common plastic lenses due to their low weight, high scratch resistance, and low transparency for ultra violet and infrared radiation.

Not all glasses are designed solely for vision correction, but rather for protection, viewing visual information (such as stereoscopy) or simply just for aesthetic or fashion values. Safety glasses are a kind of eye protection against flying debris or against visible and near visible light or radiation. Sunglasses allow better vision in bright daylight, and may protect against damage from high levels of ultraviolet light.

History

Precursors

The first suspected recorded use of a corrective lens may have been by the emperor Nero in the 1st century, who was known to watch the gladiatorial games using an emerald.

Corrective lenses were said to be used by Abbas Ibn Firnas in the 9th century. He had devised a way to finish sand into glass; which until this time, was secret to the Egyptians. These glasses could be shaped and polished into round rocks used for viewing - known as reading stones. Sunglasses, in the form of flat panes of smoky quartz, protected the eyes from glare and were used in China in the 12th century or possibly earlier. However, they did not offer any corrective powers.

Invention of eyeglasses

Around 1284 in Italy, Salvino D'Armate is credited with inventing the first wearable eye glasses. The earliest pictorial evidence for the use of eyeglasses, however, is Tomaso da Modena's 1352 portrait of the cardinal Hugh de Provence reading in a scriptorium. Another early example would be a depiction of eyeglasses found north of the Alpes in an altarpiece of the church of Bad Wildungen, Germany, in 1403.

Many theories abound for to whom the credit for the invention of traditional eyeglasses belong. In 1676, Francesco Redi, a professor of medicine at the University of Pisa, wrote that he possessed a 1289 manuscript whose author complains that he would be unable to read or write were it not for the recent invention of glasses. He also produced a record of a sermon given in 1305, in which the speaker, a Dominican monk named Fra Giordano da Rivalto, remarked that glasses had been invented less than twenty years previously, and that he had met the inventor. Based on this evidence, Redi credited another Dominican monk, Fra Alessandro da Spina of Pisa, with the re-invention of glasses after their original inventor kept them a secret, a claim contained in da Spina's obituary record.

Other stories, possibly legendary, credit Roger Bacon with the invention. Bacon is known to have made the first recorded reference to the magnifying properties of lenses in 1262. His treatise De iride ("On the Rainbow"), which was written while he was a student of Robert Grosseteste, no later than 1235, mentions using optics to "read the smallest letters at incredible distances". While the exact date and inventor may be forever disputed, it is almost certainly clear that spectacles were invented between 1280 and 1300 in Italy.

These early spectacles had convex lenses that could correct the presbyopia (farsightedness) that commonly develops as a symptom of aging. Nicholas of Cusa is believed to have discovered the benefits of concave lens in the treatment of myopia (nearsightedness). However, it was not until 1604 that Johannes Kepler published in his treatise on optics and astronomy, the first correct explanation as to why convex and concave lenses could correct presbyopia and myopia.

Later developments

The American scientist Benjamin Franklin, who suffered from both myopia and presbyopia, invented bifocals in 1784 to avoid having to regularly switch between two pairs of glasses. The first lenses for correcting astigmatism were constructed by the British astronomer George Airy in 1825.

Over time, the construction of spectacle frames also evolved. Early eyepieces were designed to be either held in place by hand or by exerting pressure on the nose (pince-nez). Girolamo Savonarola suggested that eyepieces could be held in place by a ribbon passed over the wearer's head, this in turn secured by the weight of a hat. The modern style of glasses, held by temples passing over the ears, was developed in 1727 by the British optician Edward Scarlett. These designs were not immediately successful, however, and various styles with attached handles such as "scissors-glasses" and lorgnettes remained fashionable throughout the 18th and into the early 19th century.

In the early 20th century, Moritz von Rohr at Zeiss (with the assistance of H. Boegehold and A. Sonnefeld), developed the Zeiss Punktal spherical point-focus lenses that dominated the eyeglass lens field for many years.

Despite the increasing popularity of contact lenses and laser corrective eye surgery, glasses remain very common and their technology has not stood still. For instance, it is now possible to purchase frames made of special memory metal alloys that return to their correct shape after being bent. Other frames have spring-loaded hinges. Either of these designs offers dramatically better ability to withstand the stresses of daily wear and the occasional accident. Modern frames are also often made from strong, light-weight materials such as titanium alloys, which were not available in earlier times.

On May 1 1992 the United States Federal Trade Commission declared (section 456.2) that optometrists be required to provide the patient with a complete prescription immediately following an eye exam, effectively giving the patient the choice of where to purchase their glasses. The result was greater competition between the glasses manufacturers and thus lower prices for consumers. This trend has been accelerated by the proliferation of Internet technology, giving consumers the chance to bypass traditional distribution channels and buy glasses directly from the manufacturers.

Types

Corrective

Corrective lenses modify the focal length of the eye to alleviate the effects of nearsightedness (myopia), farsightedness (hyperopia) or astigmatism. As people age, the eye's crystalline lens loses elasticity, resulting in presbyopia, which limits their ability to change focus.

The power of a lens is generally measured in diopters. Over-the-counter reading glasses are typically rated at +1.00 to +4.00 diopters. Glasses correcting for myopia will have negative diopter strengths. Lenses made to conform to the prescription of an ophthalmologist or optometrist are called prescription lenses and are used to make prescription glasses.

Safety

Safety glasses are usually made with shatter-resistant plastic lenses to protect the eye from flying debris. Although safety lenses may be constructed from a variety of materials of various impact resistance, certain standards suggest that they maintain a minimum 1 millimeter thickness at the thinnest point, regardless of material. Safety glasses can vary in the level of protection they provide. For example, those used in medicine may be expected to protect against blood splatter while safety glasses in a factory might have stronger lenses and a stronger frame with additional shields at the temples. The lenses of safety glasses can also be shaped for correction.

The American National Standards Institute has established standard ANSI Z87.1 for safety glasses in the United States, and similar standards have been established elsewhere.

Some safety glasses are designed to fit over corrective glasses or sunglasses. They may provide less eye protection than goggles or other forms of eye protection, but their light weight increases the likelihood that they will actually be used. Recent safety glasses have tended to be given a more stylish design, in order to encourage their use. The pictured wraparound safety glasses are evidence of this style change with the close fitting nature of the wraparound dispensing with the need for side shields. Corrective glasses with plastic lenses can be used in the place of safety glasses in many environments; this is one advantage that they have over contact lenses.

There are also safety glasses for welding, which are styled like wraparound sunglasses, but with much darker lenses, for use in welding where 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 protection eyewear for sports because of their lightweight and flexible properties. They are able to bend slightly and return to their original shape instead of breaking when pressure is applied to them. Nylon frames can become very brittle with age and they can be difficult to adjust.

Sunglasses

Sunglasses may be made with either prescription or non-prescription lenses that are darkened to provide protection against bright visible and possibly ultraviolet light.

Glasses with photosensitive lenses, called photochromic lenses, become darker in the presence of UV light. Unfortunately, many car windshields block the passage of UV light, making photochromic lenses less effective whilst driving on bright days. Still, they offer the convenience of not having to carry both clear glasses and sunglasses to those who frequently go indoors and outdoors during the course of a day.

Light polarization is an added feature that can be applied to sunglass lenses. Polarization filters remove horizontal rays of light, which can cause glare. Popular among fishermen and hunters, polarized sunglasses allow wearers to see into water when normally glare or reflected light would be seen. Polarized sunglasses may present some difficulties for pilots since reflections from water and other structures often used to gauge altitude may be removed, or instrument readings on liquid crystal displays may be blocked.

Yellow lenses are commonly used by golfers and shooters for their contrast enhancement and depth perception properties. Brown lenses are also common among golfers, but cause color distortion. Blue, purple, and green lenses offer no real benefits to vision enhancement and are mainly cosmetic. Some sunglasses with interchangeable lenses have optional clear lenses to protect the eyes during low light or night time activities and a colored lens with UV protection for times where sun protection is needed. Debate exists as to whether "blue blocking" or amber tinted lenses have a protective effect.

Sunglasses are often worn just for aesthetic purposes, or simply to hide the eyes. Examples of sunglasses that were popular for these reasons include teashades and mirrorshades.

Special

The illusion of three dimensions on a two dimensional surface can be created by providing each eye with different visual information. Classic 3D glasses create the illusion of three dimensions when viewing specially prepared images. The classic 3D glasses have one red lens and one blue lens. 3D glasses made of cardboard and plastic are distributed at 3D movies. Another kind of 3D glasses uses polarized filters, with one lens polarized vertically and the other horizontally, with the two images required for stereo vision polarized the same way. The polarized 3D specs allow for color 3D, while the red-blue lenses produce a dull black-and-white picture with red and blue fringes.

One kind of electronic 3D spectacles uses electronic shutters, while virtual reality glasses and helmets have separate video screens for each eye.

Variations

Glasses can be very simple, such as magnifying lenses which are used to treat mild hyperopia and presbyopia can be bought off the shelf, normally referred to as reading glasses. Most glasses are made to a particular prescription, based on degree of myopia or hyperopia combined with astigmatism. Lenses can be ground to specific prescriptions, but in some cases standard off-the-shelf prescriptions suffice, but require custom fitting to particular frames.

As people age, their ability to focus is lessened and many decide to use multiple-focus lenses, bifocal or even trifocal to cover all the situations in which they use their sight. Traditional multifocal lenses have two or three distinct viewing areas, each requiring a conscious effort of refocusing. Some modern multifocal lenses, such as Progressive lenses (known as "no-line bifocals"), give a smooth transition between these different focal points and is unnoticeable by most wearers, while others have lenses specifically intended for use with computer monitors at a fixed distance. People may have several pairs of glasses, one for each task or distance, with specific glasses for reading, computer use, television watching, and writing.

Rimless

Three-piece rimless and semi-rimless glasses are common variations that differ from regular glasses in that their frames do not completely encircle the lenses. Three-piece rimless glasses have no frame around the lenses, and the bridge and temples are mounted directly onto the lenses. Semi-rimless (or half-rimless) glasses have a frame that only partially encircles the lenses (commonly the top portion), which are held in place most often by high strength nylon wire. A rare and currently non commercial variation are rimless and frameless glasses attached to a piercing at the bridge of a wearers nose. Such glasses have the visual look of the pince-nez.

Glazing

Spectacle lenses are edged into the frame's rim using glazing machines operated by ophthalmic technicians. The edging process begins with a trace being taken of the frame's eye shape. In earlier days the trace was replicated onto a plastic pattern called a Former. Nowadays the process is patternless and the shape is sent to the edger electronically.

The lens, in the form of a round uncut, is positioned in the correct manner to match the prescription and a block is stuck to the lens and that block fits into a chuck in the edging machine. A diamond coated wheel spins as the edger replicates the frame's eye-shape to the uncut lens. A 'v' bevel is applied to allow the edge of the lens to fit into the frame rim.

Fashion

Glasses can be a major part of personal expression, from the extravagance of Elton John and Dame Edna Everage, from Groucho Marx to Buddy Holly.

For some celebrities, glasses form part of their identity. American Senator Barry Goldwater continued to wear lensless horn-rimmed spectacles after being fitted with contact lenses because he was not recognizable without his trademark glasses. British soap star Anne Kirkbride had the same problem: her character on Coronation Street, Deirdre Barlow, became so well-known for her big frames that she was expected to wear them at social gatherings and in international tours, even though Kirkbride has always worn contact lenses. Drew Carey continued to wear glasses for the same reason after getting corrective laser eye surgery. British comedic actor Eric Sykes, who became profoundly deaf as an adult, wears glasses that contain no lenses; they are actually a bone-conducting hearing aid. Masaharu Morimoto wears glasses to separate his professional persona as a chef from his stage persona as Iron Chef Japanese. John Lennon wore his round-lens 'Windsor' spectacles from some of his time with the Beatles to his murder in 1980. Rock band Weezer are known for some of the members wearing thick-rimmed glasses.

In popular culture, glasses were all the disguise Superman and Wonder Woman needed to hide in plain view as alter egos Clark Kent and Diana Prince, respectively. An example of halo effect is seen in the stereotype that those who wear glasses are intelligent or, especially in teen culture, even geeks and nerds. Some people who find that wearing glasses may look nerdy turn to contact lenses instead, especially under peer pressure. Others turn to laser eye surgery, as do some would-be pilots.

Another unpopular aspect of glasses is their inconvenience. Even through the creation of light frames, such as those made of titanium, very flexible frames, and new lens materials and optical coatings, glasses can still cause problems during rigorous sports. The lenses can become greasy or trap vapour when eating hot food, swimming, walking in rain or rapid temperature changes (such as walking into a warm building from cold temperatures outside), reducing visibility significantly. Scraping, fracturing, or breakage of the lenses require time-consuming and costly professional repair, though modern plastic lenses are almost indestructible and very scratch-resistant.

Apple, Inc. co-founder Stephen Wozniak had a pair of eyeglasses made with lenses in the shape of the well-known Apple logo. The lenses were made from a block of acrylic, laminated from layers in the usual rainbow colors, and machined into the appropriate outline, with a custom-made frame in the same shape. They were made by a Silicon Valley optician.

See also

• Eye examination
• Eyeglass prescription
• Corrective lens
• Geek chic
• History of optics
• Meganekko
• X-ray vision
• Stereoscopy

References

External links

• Antique Spectacles, extensive history and pictures of spectacles.
• British Optical Association Museum, Spectacles Gallery