| 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.
Glasses are also known as eyeglasses (especially in North America), spectacles
(an old-fashioned term, often shortened to specs), and (rarely)
frames or lenses. See Other names.
Glasses were originally made from glass, but many are now made from
plastic (often
polycarbonate or CR-39) because of the
danger of breakage and the greater weight of 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 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.
Some glasses are not designed for vision correction. 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
The first recorded use of a corrective lens was by the emperor Nero, who was known to watch the gladiatorial
games using an emerald[1].
Glasses first began to appear in common use in northern Italy late in the 13th century;
most likely in the late 1280s. It is not clear when the technology was invented.
It has been said that Marco Polo reported seeing
many pairs of glasses in China as early as 1275[citation needed].
In 1676, Franciscus
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, and
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.
In 1738, a Florentine historian named Domenico
Manni reported that a tombstone in Florence credited one Salvino
d'Armato (died 1317) with the invention of glasses. Other stories, possibly
legendary, credit Roger Bacon with the
invention. Bacon's published writings describe the magnifying glass (which
he did not invent), but make no mention of glasses. 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".
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.
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 1827.
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[1]),
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.
Types
Corrective
-
Corrective lenses modify the focal length of the eye to alleviate the effects
of shortsightedness (myopia), longsightedness (hyperopia) or
astigmatism. As most people age the crystalline lens of the
eye loses elasticity resulting in presbyopia, which limits
their ability to focus on nearby objects.
The power of a lens is generally measured in diopters. Over-the-counter
reading glasses are typically rated at +1.00 to +3.00 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 that vary in 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.
Some safety glasses are designed to fit over corrective glasses or
sunglasses. They may provide less eye protection than goggles, face shields 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).
Sunglasses
Main article: Sunglasses
Sunglasses may be made with either prescription or non-prescription lenses
that are darkened to provide protection against bright visible light. Good
sunglasses should also protect against ultraviolet light. Because of changes in
the atmosphere, ultraviolet levels are much higher than in the past and
ultraviolet protection for eyes and skin is even more important. It is possible
to have lenses that look very dark and yet offer little ultraviolet protection.
Sunglasses vary greatly and many offer more style than protection.
Glasses with photosensitive lenses,
called photochromic lenses, become
darker in the presence of UV light. Unfortunately, many car windshields protect
the passengers from UV light while not shielding from bright visible light,
making photochromic lenses ineffective where they are most needed. 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.
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.
Virtual reality glasses
and helmets
have separate video screens for each eye and a method for determining the
direction the head is turned.
Variations
Glasses can be very simple. Magnifying lenses for reading that are used to
treat mild hypermetropia and presbyopia can be bought off the shelf, but most
glasses are made to a particular prescription, based on degree of myopia or
hypermetropia combined with astigmatism. Lenses can be ground to specific eyes,
but in most 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. These were originally separate lenses, as invented by Benjamin
Franklin.
Some modern multifocal lenses give a smooth transition between these lenses,
unnoticeable by most wearers. Others have lenses specifically intended for use
with computer monitors at a fixed distance. Many people have several pairs of
glasses, one for each task or distance, with specific glasses for reading,
computer use, television watching, and writing.
Fashion
Glasses are often regarded as unattractive, and many people prefer to wear
contact lenses for that reason. Contact lenses also provide much improved peripheral vision.
On the other hand, many people are attracted to people who wear glasses, and
glasses are available in a wide range of styles, materials, and even designer
labels.
Glasses can be a major part of personal expression, from the extravagance of
Elton
John and Dame Edna Everage,
from Groucho Marx to John
Denver to Drew Carey to Lisa Loeb
all the way to the varied professional personas of eyeglass-wearing knowledge workers.
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 "granny glasses" from some of his time with the Beatles to his
assassination in 1980.
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. This conception probably
comes from an era when most people were illiterate and the first
people to wear glasses were those who did a lot of reading. Some people who find
that wearing glasses may look nerdy turn to contact lenses instead, especially
under peer pressure.
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.
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