Eyeglasses are devices for aiding, improving or correcting defective sight.
In addition correcting various eye conditions, eyeglasses are also utilized
for protecting eyes from damage caused by sunlight, ultraviolet light or
flying objects. Eyeglasses typically consist of a pair of glass or plastic
lenses mounted in a frame that sits on the bridge of the nose and is held in
place with arms that rest or are hooked behind the ears.
Eyeglasses are made from high-tech materials:
Eyeglass lenses, originally made exclusively from glass, have evolved to
include high-tech polymer materials such as CR39, Transitions® (photochromic
plastic), Polycarbonate, and High Index plastic. Lastly, multifocal lenses
have been developed which contain two or more vision-correcting
prescriptions in one lens (ex. Bifocals = lenses split into two sections;
the upper part is for distance vision and the lower part is for near vision,
and Progressives = lenses that gradually change prescription from normal
distance vision at the top to intermediate vision (arms length vision) in
the middle to a reading prescription at the bottom. Further, eyeglasses have
options for individual needs consisting of lens filters that block the
ultraviolet (UV) light, anti-scratch coatings that increase the surface
hardness of lenses (an important feature when using plastic lenses) and
anti-reflective (AR) coatings that eliminate significant glare and allow
other people to see the eyes of the wearer. There is also a full spectrum of
eyeglass lens tints, from lighter tints to darker tints, for use in
sunglasses.
Eyeglasses correct four main vision problems:
There are four vision deficiencies for which eyeglasses can compensate.
Nearsightedness (myopia) is a vision disorder that causes far objects to
appear blurred while near objects are seen clearly. Corrective eyeglasses
with concave (inwardly curved) lenses compensate for the refraction error by
moving the image of a distant object backward onto the retina Farsightedness
(hyperopia) is a condition that allows far objects to be seen clearly but
presents difficulty with near vision. Corrective eyeglasses with convex
(outwardly curved) lenses compensate for the refraction errors by moving the
image of a distant object forward onto the retina. Astigmatism is a vision
disorder caused by a non-uniform curvature in the refractive surfaces of the
eye. Light rays do not all come to a single focal point on the retina, some
focus on the retina while others focus in front or behind it. This condition
can occur in conjunction with nearsightedness or farsightedness. A special
cylindrical lens is placed in the eyeglasses to correct the condition.
Presbyopia is a condition that exists when the natural crystalline lens of
the eye loses some of its ability to change shape in order to focus on near
objects. Typically, people moving beyond the age of 40 eventually develop
presbyopia, which is indicated by a need for reading or bifocal eyeglasses.
Corrective eyeglasses with convex lenses compensate for these refractive
errors.
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