Corrective
Corrective lenses are used to correct refractive errors of the eye by modifying the effective focal length of the lens in order to alleviate the effects of conditions such as nearsighte
dness (myopia), farsightedness (hyperopia) or astigmatism. Another common condition in older patients 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).
The power of a lens is generally measured in diopters. Glasses correcting for myopia will have negative diopter strengths, and glasses correcting for hypermetropia will have positive diopter strengths. Glasses correcting for astigmatism require two different strengths placed at right angles in the same lens. Prescription lenses, made to conform to the prescription of an ophthalmologist or optometrist, are used to make prescription glasses, which are then verified correct using a professional lensmeter.
Pinhole glasses are a type of corrective glasses which do not use a conventional lens and are claimed to help correct the eye's refractive error without introducing the image distortion of traditional lens-based glasses.
Correcting one's vision is done by using lenses to move the focal point on the retina accordingly with one's particular needs. The depth of the curve, the thickness of the lens, and the precise shape of the lens can all be used to change the focal point. [12] Eyeglasses can normally compensate and correct four types of vision deficiencies. Myopia is a vision disorder that causes far objects to appear blurred but near objects are seen clearly. [13] Individuals suffering from myopia are prescribed eyeglasses with concave lenses which compensate for the refraction error by moving the image of the distant objects that cannot be seen clearly backward onto the retina.
Correcting hyperopia is normally done with eyeglasses with convex lenses. With this disorder, the patients can see distant objects clearly but they have trouble with seeing objects that are close to them. Eyeglasses with convex lenses compensate for the refraction errors by moving the image of a distant object forward onto the retina. [14]
Astigmatism is typically corrected with a cylindrical lens. This disorder is caused by a non-uniform curvature in the refractive surfaces of the eye which leads to an abnormality in focalizing the light rays on the retina. As a result, a part of the light rays are focused on the retina and the other part is focalized behind it or in front of it.
Presbyopia is more frequent in people over 40 years old and it is corrected with convex lenses. These patients need reading or bifocal eyeglasses.
Corrective eyeglasses can significantly improve the life quality of the patient as they are helpful in both correcting vision disorders and reducing problems that appear when such lenses are needed, such as headaches or squinting.
Corrective lenses can also be added to work masks or eyeglasses which are used in sports.
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.
OSHA provides guidance on the type of safety eyewear that should be used for a particular application.
Some safety glasses are designed to fit ove
r 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. Modern safety glasses tend to be given a more stylish design in order to encourage their use. Corrective glasses with plastic lenses can be used in place of safety glasses in many environments; this is one advantage that they have over contact lenses.
There are also safety glasses for welding, w
hich 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.
Worker safety eyewear is available in various lens colors and/or with coatings to protect or enable eyesight in different lighting conditions, particularly when outdoors.
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.
Safety lenses are usually made of polycarbonate, which is basically jet windshield plastic that is very unlikely to break. [15] Safety glasses are also available in prescription form for those persons who need corrective lenses. [16] Depending on the particular area in which the individuals work, they may be required to wear side protectors additionally to safety eyeglasses.
In order to comply with the ANSI Z87.1 requirements, safety eyeglasses must pass the high velocity and high mass tests. Also, the lenses of protective goggles, faceshield windows and welding filters cannot be thinner than 3 mm excepting the high impact lenses which are meant to be installed in prescription frames which cannot be thinner than 2 mm.
Sunglasses
Sunglasses may be made with either prescription or non-prescription lenses that are darkened to provide protection against bright visible light and, possibly, ultraviolet (UV) light. Photochromic lenses, which are photosensitive, darken when struck by UV light.
Light polarization is an added feature that can be applied to sunglass lenses. Polarization filters 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, or instrument readings on liquid crystal displays may be blocked.
Yellow lenses increase color contrast and improve depth perception. Brown lenses are common among golfers, but cause color distortion. Also, green/yellow tinted lens are common when night driving and aid in the removing of glare off on-coming headlights. 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.
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. Many blind people wear opaque glasses to hide their eyes for aesthetic reasons.
3D glasses
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 or cyan lens. 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. Polarized 3D glasses allow for color 3D, while the red-blue lenses produce a dull black-and-white picture with red and blue fringes. Both types have been distributed to audiences at 3D movies.
One kind of electronic 3D spectacles uses electronic
shutters, while
virtual reality glasses and helmets have separate video screens for each eye. A 3D effect can also be produ ced using
LCD shutter glasses.
Reading glasses
Magnifying lenses or generic spectacles that are used to treat mild presbyopia and hyperopia can be bought off the shelf. 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 & visual discomfort. There have also been many cases where people have delayed having a proper eye examinaton with an optometrist or ophthalmologist, preferring to purchase off the shelf glasses, who have put their sight at risk from conditions such as AMD, Glaucoma and complications from Diabetes. It is important to str
ess off the shelf readers are not a replacement for regular eye examinations.
Reading glasses come in two main styles: full frames, in which the entire lens is made in the reading prescription, and half-eyes, the smaller "Ben Franklin" style glasses that sit lower down on the nose. [18]
Full reading glasses are more suitable for people who only need them for close-up reading while half-eye reading glasses can be used to read at smaller or larger distances. The reading glasses are most of the time needed by people who have never worn glasses.
Although specialists recommend individuals who need to wear eyeglasses to have them custom-made according to their own needs, most of the patients prefer buying them at a pharmacy or department store. This type of eyeglass-shopping became very popular in the 1990s when it was estimated that over 30 million pairs were sold per year. These reading glasses are not as expensive as the custom-made ones and they are certainly designed to catch the buyer's eye. Glasses that can be purchased off the shelf are available in a wide variety of colors and designs, suitable for different tastes. As this habit can result in worsening one's vision prob
lems, there is a new alternative to buy eyeglasses easy and cheap from online prescription eyeglasses stores.[19]
The downside of the ready-made eyeglasses is that they are basically made in "one size", meaning that they come with the same prescription in the same lenses and the location of the optical center of the lenses is also the same. Yet, most individuals who need reading glasses need different prescriptions in each eye and wearing such pre-made eyeglasses can result in headaches, eyestrain or nausea. These "side effects" tend to occur when the prescription that the glasses have is too far from the one that the individual needs.
Bifocals, trifocals, and progressive lenses
As people age, their ability to focus is lessened and many decide to use multiple-f ocus lenses, which can be
bifocal or even
trifocal, to cover all the situations in which they use their sight. Traditional multifocal lenses have two or three distinct horizontal 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, unnoticeable by most wearers, while other glasses 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
Extreme magnification (bioptics)
A form of glasses with extreme magnification to improve the distance vision of those with severe eyesight impairment, especially people with
albinism, are known as
bioptics or a
bioptic telescope. They may take the form of self-contained glasses that resemble
goggles or
binoculars, or may be attached to existing glasses.
Fashion
For some celebrities, glasses form part of their identity. United States Senator Barry Goldwater continued to wear lensless horn-rimmed glasses 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. Comedian 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. The rock band Weezer is known for some of the members wearing thick-rimmed glasses, as well as actor Jeff Goldblum who is often seen in the Browline glasses style. Singer Anastacia who dominated the European charts in the early 21st century is noted for wearing odd coloured glasses.
Browline glasses were the standard for men in the 1950s and 1960s.
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.
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.
