Glasses

Glasses, also known as eyeglasses (formal) or spectacles, are frames bearing lenses worn in front of the eyes. They are normally used for vision correction or eye protection. 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 one's eyes against damage from high levels of ultraviolet light. Specialized glasses may be used for viewing specific visual information (such as stereoscopy). Sometimes glasses are worn simply just for aesthetic or fashion purposes.

History

Precursors

The earliest written record of magnification dates back to the 1st century AD, when Seneca the Younger, a tutor of Emperor Nero of Rome, wrote: "Letters, however small and indistinct, are seen enlarged and more clearly through a globe or glass filled with water".^[4] Nero (reigned 54–68 AD) is also said to have watched the gladiatorial games using an emerald as a corrective lens.^[5]

The use of a convex lens to form a magnified image is discussed in Alhazen's Book of Optics (1021). Its translation into Latin from Arabic in the 12th century was instrumental to the invention of eyeglasses in 13th century Italy.^[4]

Englishman 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 is also known to have written on the magnifying properties of lenses.^[6]

Sunglasses, in the form of flat panes of smoky quartz, were used in China in the 12th century.^{[lower-alpha 1]} Similarly, the Inuit have used snow goggles for eye protection. However, while they did not offer any corrective benefits^[7] they did improve visual acuity by narrowing the field of vision^[8] in a way related to the pinhole camera effect.

Invention of eyeglasses

The first eyeglasses were made in Italy at about 1286 originally consisting of thin pieces of glass which were placed directly onto the eye ball. According to a sermon delivered on February 23, 1306, by the Dominican friar Giordano da Pisa (ca. 1255–1311): "It 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."^[9] Giordano's colleague Friar Alessandro della Spina of Pisa (d. 1313) was soon making eyeglasses. The Ancient Chronicle of 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."^[10] By 1301, there were guild regulations in Venice governing the sale of eyeglasses.^[11]

In 1907, Professor Berthold Laufer, who was a German-American anthropologist, stated in his history of spectacles that 'the opinion that spectacles originated in India is of the greatest probability and that spectacles must have been known in India earlier than in Europe'.^[12][13] The German word brille (eyeglasses) is derived from Sanskrit vaidurya.^[14]

However, Joseph Needham showed that the mention of spectacles in the manuscript Laufer used to justify the prior invention of them in Asia did not exist in older versions of that manuscript, and the reference to them in later versions was added during the Ming dynasty^[15]  — after eyeglasses had been invented in Europe.

Although there have been claims that Salvino degli Armati of Florence invented eyeglasses, these claims have been exposed as hoaxes.^[16][17] Furthermore, although there have been claims that Marco Polo encountered eyeglasses during his travels in China in the 13th century, no such statement appears in his accounts.^[18][19] Indeed, the earliest mentions of eyeglasses in China occur in the 15th century and those Chinese sources state that eyeglasses were imported.^[20]

The earliest pictorial evidence for the use of eyeglasses is Tommaso 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 Alps in an altarpiece of the church of Bad Wildungen, Germany, in 1403.

These early spectacles had convex lenses that could correct both hyperopia (farsightedness), and the presbyopia that commonly develops as a symptom of aging. 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.^{[lower-alpha 2]}

Later developments

The American scientist Benjamin Franklin, who suffered from both myopia and presbyopia, invented bifocals. Serious historians have from time to time produced evidence to suggest that others may have preceded him in the invention; however, a correspondence between George Whatley and John Fenno, editor of The Gazette of the United States, suggested that Franklin had indeed invented bifocals, and perhaps 50 years earlier than had been originally thought.^[21]

The first lenses for correcting Astigmatism were designed by the British astronomer George Airy in 1825.^[22]

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 some time before 1727, possibly 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 were also fashionable from the second half of the 18th century 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^[23]), 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, as their technology has improved. 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.

Types of Glasses

Glasses come in many types. They can be classified by their primary function, but also appear in combinations such as prescription sunglasses or safety glasses which enhanced magnification.

Corrective

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. Another common condition in patients over forty years old 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). Corrective lenses are made to conform to the prescription of an ophthalmologist or optometrist. A lensmeter can be used to verify the specifications of a pair of glasses.

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 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.

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 appear such as headaches or squinting. A small amount of time is needed to adapt to the new lenses, usually 1–2 weeks.

Single vision

Single vision lenses correct for only one distance. If they correct for far distance, the person must accommodate to see clearly up close. If the person cannot accommodate, the may need a separate pair of single vision glasses for near distances, or else use a multifocal lens (see below).

Over the counter reading glasses

Ready-made reading glasses go by many names, including over the counter glasses, cheaters, magnifiers, non-prescription readers, or generic readers. They offer clearer vision to people with presbyopia and hyperopia. 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 in strengths ranging from +0.75 to +3.50 diopters. These glasses do not take into account the mathematics of the wearer's distance prescription, often causing the distance to become blurry unless they are removed. If the wearer has little to no need for correction in the distance, may work quite well for seeing better during near vision tasks. But if the person has a need for correction in the distance, it is less likely that they will be perfectly effective.

Reading glasses come in two main styles: full frames, in which the entire lens is made in the reading prescription, and half-eyes, style glasses that sit lower down on the nose.^[24] Full frame readers must be removed to see distance clearly, while the distance can be clearly viewed over the top of half-eye readers.

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.

Bifocal

With a bifocal, the upper part of the lens is generally used for distance vision, while 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 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 the glasses, which would be required with single vision correction.

Trifocal

Trifocal lenses are similar to bifocals, except that the two focal areas are separated by a third 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

Progressive addition or varifocal lenses provide a smooth transition from distance correction to near correction, eliminating segment lines and allowing clear vision at all distances. The lack of any abrupt change in power and the uniform appearance of the lens gives rise to the name "no-line bifocal".

Adjustable focus

Adjustable or variable focus dynamically adjusts focal length, typically allowing clear vision at any distance. It is especially useful for treating the loss of accommodation common in presbyopia.

Safety

Safety glasses are worn to protect the eyes during a variety of tasks. They are made with shatter-resistant plastic lenses to protect the eye from flying debris and can shield the eyes from hazardous splatters such as blood or chemicals. 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.

Sunglasses

Sunglasses provide improved comfort and protection against bright light and often against ultraviolet (UV) light. Photochromic lenses, which are photosensitive, darken when struck by UV light. The dark tint of the lenses in a pair of sunglasses blocks the transmission of light through the lens.

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 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 often emit polarized light making them sometimes difficult to view with polarized sunglasses.

Sunglasses may be worn just for aesthetic purposes, or simply to hide the eyes. Examples of sunglasses that were popular for these reasons include teashades and mirrorshades. Many blind people wear nearly opaque glasses to hide their eyes for cosmetic reasons.

Sunglasses may also have corrective lenses. 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 top of another pair of glasses. Otherwise, many people opt to wear contact lenses to correct their vision so that standard sunglasses can be used.

3D glasses

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, 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 the signal electronically and then transmit light directly into the viewers 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.

Magnification (bioptics)

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. They may take the form of self-contained glasses that resemble goggles or binoculars, or may be attached to existing glasses.

Yellow-tinted computer/Gaming glasses

These glasses, worn during computer use, are said to help minimize strain on the eyes and reduce fatigue. These kind of eyewear are generally used by gamers, game designers, graphic artists and multi-media software users.^[25]

Fashion

Many people require glasses for the reasons listed above. There are many shapes, colors, and materials that can be used when designing frames and lenses that can be utilized in various combinations. Oftentimes, the selection of a frame is made based on how it will affect the appearance of the wearer. Some people with good natural eyesight like to wear eyeglasses as a style accessory.

Personal image

For most of their history, eyeglasses were seen as unfashionable, and carried several potentially negative connotations: wearing glasses caused individuals to be stigmatized and stereotyped as pious clergymen (as those in religious vocation were the most likely to be literate and therefore the most likely to need reading glasses), elderly, or physically weak and passive.^[26][27] The stigma began to fall away in the early 1900s when the popular Theodore Roosevelt was regularly photographed wearing eyeglasses, and in the 1910s when popular comedian Harold Lloyd began wearing a pair of horn-rimmed glasses as the "Glasses" character in his films.^[26][27]

Since, eyeglasses have become an acceptable fashion item and often act as a key component in individuals' personal image. Musicians Buddy Holly and John Lennon became synonymous with the styles of eye-glasses they wore to the point that thick, black horn-rimmed glasses are often called "Buddy Holly glasses" and perfectly round metal eyeglass frames called "John Lennon Glasses." British comedic actor Eric Sykes was known in the United Kingdom for wearing thick, square, horn-rimmed glasses, which were in fact a sophisticated hearing aid that alleviated his deafness by allowing him to "hear" vibrations.^[28] Some celebrities have become so associated with their eyeglasses that they continued to wear them even after taking alternate measures against vision problems: United States Senator Barry Goldwater and comedian Drew Carey continued to wear non-prescription glasses after being fitted for contacts and getting laser eye surgery, respectively.

Other celebrities have used glasses to differentiate themselves from the characters they play, such as Anne Kirkbride, who wears oversized, 1980s-style round horn-rimmed glasses as Deirdre Barlow on the soap opera Coronation Street, and Masaharu Morimoto, who wears glasses to separate his professional persona as a chef from his stage persona as Iron Chef Japanese.

Recently, many NBA players wear lensless glasses with thick plastic frames like horn-rimmed glasses during post-game interviews, geek chic that draws comparisons to Steve Urkel.^[29][30]

In superhero fiction, eyeglasses have become a standard component of various heroes' disguises (as masks), allowing them to adopt a nondescript demeanor when they are not in their superhero persona: Superman is well known for wearing 1950s style horn-rimmed glasses as Clark Kent, while Wonder Woman wears either round, Harold Lloyd style glasses or 1970s style bug-eye glasses as Diana Prince.

Styles

In the 20th century, eyeglasses came to be considered a component of fashion; as such, various different styles have come in and out of popularity. Most are still in regular use, albeit with varying degrees of frequency.

• Browline glasses
• Bug-eye glasses
• Cat eye glasses
• GI glasses
• Horn-rimmed glasses
• Lensless glasses
• Pince nez
• Rimless glasses

A rare and currently noncommercial variation are rimless and frameless glasses attached to a piercing at the bridge of a wearer's nose.^[31] Such glasses have the visual look of the pince-nez.

Spectacle Lens shape

Corrective lenses can be produced in many different shapes from 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 which allows for many trendy and fashionable shapes. Higher power lenses can cause distortion of peripheral vision and may become thick and heavy if a large lens shape is used. However, if the lens becomes too small, the field of view can be drastically reduced. 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.

Vertex Distance

Vertex distance is the space between the front of the eye and the back surface of the lens. In glasses with powers greater than four diopters, the vertex distance can affect the effective power of the glasses. Another consideration is that a smaller vertex distance allows the same field of view through a smaller lens. But there is a limit on how close the lenses can be to the eye, since it can be bothersome if the eyelashes brush against the lens surface while blinking.

Refractive index

In the UK and the US, the refractive index is generally specified with respect to the yellow He-d Fraunhofer line, commonly abbreviated as n_d. Lens materials are classified by their refractive index, as follows:

• Normal index - 1.48 ≤ n_d < 1.54
• Mid-index - 1.54 ≤ n_d < 1.60
• High-index - 1.60 ≤ n_d < 1.74
• Very high index - 1.76 ≤ n_d

This is a general classification. Indexes of n_d values that are ≥ 1.60 can be, often for marketing purposes, referred to as high-index. Likewise, Trivex and other borderline normal/mid-index materials, may be referred to as mid-index.

Advantages of higher indices

• Thinner, sometimes lighter lenses (See below).
• Improved UV protection over CR-39 and glass lenses.

Disadvantages of increased indices

• Lower Abbe number meaning, amongst other things, increased chromatic aberration.
• Poorer light transmission and increased backside and inner-surface reflections (see Fresnel reflection equation) increasing importance of anti-reflective coating.
• Manufacturing defects have more impact on optical quality^{[citation needed]}.
• Theoretically, off-axis optical quality degrades (oblique astigmatic error). In practice this degradation should not be perceptible - current frame styles are much smaller than they would have to be for these aberrations to be noticeable to the patient, the aberration occurring some distance away from the optical centre of the lens (off-axis).

Optical quality

Abbe number

Of all of the properties of a particular lens material, the one that most closely relates to its optical performance is its dispersion, which is specified by the Abbe number. 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 (±4D or greater). Generally, lower Abbe numbers are 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, a change from 30 to 32 Abbe 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 optical center of the lens. Note that 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 ±4D. At ±8D even glass (V_d≤58) produces chromatic aberration that can be noticed by 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:

• Moves to keep the visual axis close to its achromatic axis, which is completely free of dispersion (i.e., to see the dispersion one would have to concentrate on points in the periphery of vision, where visual clarity is quite poor)
• Is very insensitive, especially to color, in the periphery (i.e., at retinal points distant from the achromatic axis and thus not falling on the fovea, where the cone cells responsible for color vision are concentrated. See: Anatomy and Physiology of the Retina.)

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 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:

• Try to use the smallest vertical lens size that is comfortable. Generally, chromatic aberrations are more noticeable as the pupil moves vertically below the optical center of the lens (e.g., reading or looking at the ground while standing or walking). Keep in mind that a smaller vertical lens size will result in a greater amount of vertical head movement, especially while performing activities that involve short and intermediate distance viewing, which could lead to an increase in neck strain, especially in occupations involving a large vertical field of view.
• Restrict the choice of lens material to the highest Abbe value at acceptable thickness. The oldest most basic commonly used lens materials also happen to have the best optical characteristics at the expense of corrective lens thickness (i.e., cosmetics). Newer materials have focused on improved cosmetics and increased impact safety, at the expense of optical quality. Lenses sold in the USA must pass the Food and Drug Administration ball-drop impact test, and depending on needed index these seem to currently have ‘best in class’ Abbe vs Index (N_d): Glass (2x weight of plastics) or CR-39 (2 mm vs. 1.5 mm thickness typical on newer materials) 58 @ 1.5, Sola Spectralite (47@1.53), Sola Finalite (43@1.6), and Hoya Eyry (36 @ 1.7). For impact resistance safety glass is offered at a variety of indexes at high Abbe number, but is still 2x the weight of plastics. Polycarbonate (V_d=30-32) is very dispersive, but has excellent shatter resistance. Trivex (V_d=43 @ 1.53), is also heavily marketed as an impact resistant alternative to Polycarbonate, for individuals who don’t need polycarbonate’s index. Trivex is also one of the lightest materials available.
• Use contact lenses in place of eyeglasses. A contact lens rests directly on the surface of the cornea and moves in sync with all eye movements. Consequently the contact lens is always directly aligned on center with the pupil and there is never any off-axis misalignment between the pupil and the optical center of the lens.

Power error (-D corrections for myopia)

Power error is the change in the optical power of 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 the lens. Generally, best spherical form lenses attempt to keep the ocular curve between four and seven diopters.

Lens induced oblique astigmatism (+D corrections for presbyopia)

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 the clarity of vision in the periphery.

Minimizing power error and lens induced astigmatism

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 the lens then moves with the eye.

Barring contacts, a good lens designer doesn’t have many parameters which can be traded off to improve vision. Index has little effect on error. Note that, chromatic aberration is often perceived as ‘blurry vision’ in the lens periphery giving the impression of power error, although this is actually due to color shifting. Chromatic aberration can be improved by using 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 sometime selected for cosmetic reasons. Aspheric or atoric design can reduce errors induced by using 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 a spherically designed center area for improved vision and fit.^[32]

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 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 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.

Cosmetics and weight

Reducing lens thickness

Note that the greatest cosmetic improvement on lens thickness (and weight) is had from choosing a frame which holds physically small lenses. The smallest of the popular adult lens sizes available in retail outlets is about 50mm across. There are a few adult sizes of 40mm and although they are quite rare, can reduce lens weight to about half of the 50mm versions. See the diagram opposite, for a simplified graphical explanation of how smaller sizes with the same radius of curvature can greatly reduce thickness. 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 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 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.

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 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)^[33] 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.

Weight

Material density typically increases as lens thickness is reduced by increasing index. There is also a minimum lens thickness required to support the lens shape. These factors results 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 a given material. Ways to reduce the weight and thickness of spectacle lenses, in approximate order of importance are these:

• Choose spectacle frames with small lenses; that is to say, so that the longest measurement across the lens at any angle is as short as possible. This gives the greatest advantage of all.
• Choose a frame that allows the pupil to occupy the exact middle point of the lens.
• Choose a lens as near round as possible. These are less commonly found than other shapes.
• Choose as high a refractive index for the lens material as cost permits.

It is not always possible to follow the above points, because of the rarity of such frames, and the need for more pleasing appearance. However, these are the main factors to consider if ever it should become necessary and possible to do so.

Facial distortion and social stigma

Eyeglasses for a high-diopter nearsighted or farsighted person cause 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.

• For extreme nearsightedness the eyes appear small and sunken into the face, and the sides of the skull can be visible through the lens. This gives the wearer the appearance of having a very large or fat head in contrast with their eyes.
• For extreme farsightedness the eyes appear very large on the face, making the wearer's head seem too small.

Either situation can result in social stigma^[34] for children and adults due to apparent unattractiveness or ugliness of the wearer caused by these facial distortions. This can result in 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 is normal. Aspheric/atoric eyeglass design can also reduce minification and magnification of the eye for observers at some angles.

Lens materials

Optical crown glass (B270)

• Refractive index (n_d): 1.52288
• Abbe value (V_d): 58.5
• Density: 2.55 g/cm³ (the heaviest corrective lens material in common use, today)
• UV cutoff: 320 nm
  • Please note: Schott B270 is an optical glass used in precision optics. It is NOT an ophthalmic glass. Schott ophthalmic glass types are S-1 and S-3. The issue here is an incorrect value for UVA and UVB transmission, as well as other related product type issues. ***

Glass lenses have become less common in recent years due 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 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 value 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 is commonly used in telescopes and binoculars, and fluorite crown glasses such as Schott N-FK51A (n_d=1.48656 / V_d=84.47 / D=3.675 g/cm³), which is 16.2 times the price of a comparable amount of BK7, and are commonly used in high-end camera lenses). However, one would be very hard pressed to find a laboratory that would be willing to acquire or shape custom eyeglass lenses, considering that the order would most likely consist of just two different lenses, out of these materials. Generally, V_d values above 60 are of dubious value, except in combinations of extreme prescriptions, large lens sizes, a high wearer sensitivity to dispersion, and occupations that involve work with high contrast elements (e.g., reading dark print on very bright white paper, construction involving contrast of building elements against a cloudy white sky, a workplace with recessed can or other concentrated small area lighting, etc.).

Plastic (CR-39)

• Refractive index (n_d): 1.498 (standard)
• Abbe value (V_d): 59.3
• Density: 1.31 g/cm³
• UV cutoff: 355 nm

Plastic lenses are currently the most commonly prescribed lens, due to their relative safety, low cost, ease of production, and outstanding optical quality. The main drawbacks of many types of plastic lenses are the ease by which a lens can be scratched, and the limitations and costs of producing higher index lenses. CR-39 lenses are the exception to the plastics in that they have inherent scratch resistance.

Trivex

• Refractive index (n_d): 1.532
• Abbe value (V_d): 43–45 (depending on licensing manufacturer)
• Density: 1.1 g/cm³ (the lightest corrective lens material in common use)
• UV cutoff: 380 nm

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 value) and a slightly lower density. Its lower refractive index of 1.532 vs. polycarbonate's 1.586, however, may result in slightly thicker lenses. Along with polycarbonate and the various high-index plastics, Trivex is a lab favorite for use in rimless frames, due to the ease with which it can be drilled as well as its resistance to cracking around the drill holes. One other advantage that Trivex has over polycarbonate is that it can be easily tinted.

Polycarbonate

• Refractive index (n_d): 1.586
• Abbe value (V_d): 30
• Density: 1.2 g/cm³
• UV cutoff: 385 nm

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, scratch resistant coating is typically applied after shaping and polishing the lens. Standard polycarbonate with an Abbe value of 30 is one of the worst materials optically, if chromatic aberration intolerance 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 value which may be bothersome to individuals sensitive to chromatic aberrations.

High-index plastics (thiourethanes)

• Refractive index (n_d): 1.600–1.740
• Abbe value (V_d): 42–32 (higher indexes generally result in lower Abbe values)
• Density: 1.3–1.5 (g/cm³)
• UV cutoff: 380–400 nm

High-index plastics allow for thinner lenses. The lenses may not be lighter, however, due to the increase in density vs. mid- and normal index materials. A disadvantage is that high-index plastic lenses suffer from a much higher level of chromatic aberrations, which can be seen from their lower Abbe value. Aside from thinness of the lens, another advantage of high-index plastics is their strength and shatter resistance, although not as shatter resistant as polycarbonate. This makes them particularly suitable for rimless eyeglasses.

These high-refractive-index plastics are typically thiourethanes, with the sulfur atoms in the polymer being responsible for the high refractive index.^[35] The sulfur content can be up to 60 percent by weight for an n=1.74 material.^[35]

Ophthalmic material property tables

Material, Plastic	Index (Nd)	Abbe (Vd)	Specific Gravity (g/cm3)	UVB/ UVA	Reflected light (%)	Minimum thickness typ/min (mm)	Note
CR-39 Hard Resin	1.49	59	1.31	100% / 90%	7.97	?/2.0
PPG Trivex (Average)	1.53	44	1.11	100% / 100%	8.70	?/1.0	PPG, Augen, HOYA, Thai Optical, X-cel, Younger
SOLA Spectralite	1.54	47	1.21	100% / 98%	8.96		(also Vision 3456 (Kodak)?)
Essilor Ormex	1.56	37	1.23	100% / 100%	9.52
Polycarbonate	1.586	30	1.20	100% / 100%	10.27	?/1.5	Tegra (Vision-Ease) Airwear (Essilor) FeatherWates (LensCrafters)
MR-8 1.6 Plastic	1.6	41	1.30	100% / 100%	10.43
MR-6 1.6 Plastic	1.6	36	1.34	100% / 100%	10.57
MR-20 1.6 Plastic	1.60	42	1.30	100% / 100%
SOLA Finalite	1.60	42	1.22	100% / 100%	10.65
MR-7 1.67 Plastic	1.67	32	1.35	100% / 100%	12.26
MR-10 1.67 Plastic	1.67	32	1.37	100% / 100%	12.34
Nikon 4 Plastic NL4	1.67	32	1.35	100% / 100%
Hoya EYRY	1.70	36	1.41	100% / 100%	13.44	?/1.5
MR-174 1.74 Plastic	1.74	33	1.47	100% / 100%	14.36		Hyperindex 174 (Optima)
Nikon 5 Plastic NL5	1.74	33	1.46	100% / 100%
Tokai	1.76	30	1.49	100% / 100%

Material, Glass	Index (Nd)	ABBE (Vd)	Specific Gravity	UVB/ UVA	Reflected light (%)	Minimum thickness typ/min (mm)	Note
Crown Glass	1.525	59	2.54	79% / 20%	8.59
PhotoGray Extra	1.523	57	2.41	100% / 97%	8.59
1.6 Glass	1.604	40	2.62	100% / 61%	10.68		Zeiss Uropal, VisionEase, X-Cel
1.7 Glass	1.706	30	2.93	100% / 76%	13.47		Zeiss Tital, X-Cel, VisionEase, Phillips
1.8 Glass	1.800	25	3.37	100% / 81%	16.47		Zeiss Tital, X-Cell, Phillips, VisionEase,Zhong Chuan Optical(China)
1.9 Glass	1.893	31	4.02	100% / 76%	18.85		Zeiss Lantal, Zhong Chuan Optical(China) not FDA-approved for sale in USA

Reflected light calculated using Fresnel reflection equation for normal waves against air on two interfaces. This is reflection without an AR coating.

Compilations of manufacturer material data can be found at opticampus, firstvisionmedia, and eyecarecontacts. Additional information on branding can be found at eyetopics.

Indices of refraction for a range of materials can be found in the List of indices of refraction.

Lens coatings

Anti-reflective

Anti-reflective coatings help to make the eye behind the lens more visible. They also help lessen back reflections of the white of the eye as well as bright objects behind the eyeglasses wearer (e.g., windows, lamps). Such reduction of back reflections increases the apparent contrast of surroundings. At night, anti-reflective coatings help to reduce headlight glare from oncoming cars, street lamps and heavily lit or neon signs.

One problem with anti-reflective coatings is that historically they have been very easy to scratch. Newer coatings, such as Crizal Alizé with its 5.0 rating and Hoya's Super HiVision with its 10.9 rating on the COLTS Bayer Abrasion Test (glass averages 12–14), try to address this problem by combining scratch resistance with the anti-reflective coating. They also offer a measure of dirt and smudge resistance, due to their hydrophobic properties (110° water drop contact angle for Super HiVision and 112° for Crizal Alizé).

Ultraviolet protection

A UV coating is used to reduce the transmission of light in the ultraviolet spectrum. UV-B radiation increases the likelihood of cataracts, while long term exposure to UV-A radiation can damage the retina. DNA damage from UV light is cumulative and irreversible. Some materials, such as Trivex and Polycarbonate naturally block most UV light and do not benefit from the application of a UV coating.^{[citation needed]}

Scratch resistance

Resists damage to lens surfaces from minor scratches.

Redistribution

Some organizations like Lions Clubs International,^[36] Unite For Sight^[37] and New Eyes for the Needy provide a way to donate glasses and sunglasses. Unite For Sight has redistributed more than 200,000 pairs.^[38]

See also

Notes

References

Bibliography

• Ilardi, Vincent (2007), Renaissance Vision from Spectacles to Telescopes, Philadelphia, PA: American Philosophical Society .
• Needham, Joseph (1962), "part 1", Science & Civilisation in China IV, Cambridge, ENG, UK: Cambridge University Press .

External links

Wikiquote has a collection of quotations related to: Glasses

Wikimedia Commons has media related to Glasses.

Look up spectacles or glasses in Wiktionary, the free dictionary.

• "Spectacles Gallery", Museum, British Optical Association .
• "Spectacles", The Medieval Technology, NYU .
• "Are Your Eyes Right", Popular Science (article), February 1944 , on eyes and how eyeglasses correct vision (page 120).

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