<unk>  can be marked or found by their primary function, but also appear in combinations such as prescription sunglasses or safety<unk>  with enhanced magnification. Corrective A skyline seen through a corrective lens, showing the effect of refraction 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. The ability of one's eyes to accommodate their focus to near and distant focus alters over time. A common condition in people 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). Few people have a pair of eyes that show exactly equal refractive characteristics; one eye may need a "stronger" (i.e. more refracting) lens than the other. Corrective lenses bring the image back into focus on the retina. They are made to conform to the prescription of an ophthalmologist or optometrist. A lensmeter can be used to verify the specifications of an existing pair of<unk> . 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 result from eye strain, such as headaches or squinting. The most common type of corrective lens is "single vision", which has a uniform refractive index. For people with presbyopia and hyperopia, bifocal and trifocal<unk>  provide two or three different refractive indices, respectively, and progressive lenses have a continuous gradient. Lenses can also be manufactured with high refractive indices, which allow them to be more lightweight and thinner than their counterparts with "low" refractive indices. Reading<unk>  provide a separate set of<unk>  for focusing on close-by objects. Reading<unk>  are available without prescription from drugstores, and offer a cheap, practical solution, though these have a pair of simple lenses of equal power, and so will not correct refraction problems like astigmatism or refractive or prismatic variations between the left and right eye. For the total correction of the individual's sight,<unk>  complying to a recent ophthalmic prescription are required. People who need<unk>  to see often have corrective lens restrictions on their driver's licenses that require them to wear their<unk>  every time they drive or risk fines or jail time. Some militaries issue prescription<unk>  to servicemen and women. These are typically GI<unk> . Many state prisons in the United States issue<unk>  to inmates, often in the form of clear plastic aviators. Adjustable-focus eyeglasses might be used to replace bifocals or trifocals, or might be used to produce cheaper single-vision<unk>  (since they don't have to be custom-manufactured for every person). Pinhole<unk>  are a type of corrective<unk>  that do not use a lens. Pinhole<unk>  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. Pinhole<unk>  can be made in a DIY fashion by making small holes in a piece of card which is then held in front of the eyes with a strap or cardboard arms. Safety Safety<unk>  with side shields Safety<unk>  are worn to protect the eyes in various situations. They are made with break-proof plastic lenses to protect the eye from flying debris or other matter. Construction workers, factory workers, machinists and lab technicians are often required to wear safety<unk>  to shield the eyes from flying debris or hazardous splatters such as blood or chemicals. As of 2017, dentists and surgeons in Canada and other countries are required to wear safety<unk>  to protect against infection from patients' blood or other body fluids. There are also safety<unk>  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 protective eyewear for sports because of their lightweight and flexible properties. Unlike most regular<unk> , safety<unk>  often include protection beside the eyes as well as in front of the eyes. Sunglasses Woman wearing sunglasses Sunglasses provide more comfort and protection against bright light and often against ultraviolet (UV) light. To properly protect the eyes from the dangers of UV light, sunglasses should have UV-400 blocker to provide good coverage against the entire light spectrum that poses a danger. 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 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 tea shades and mirrorshades. Many blind people wear nearly opaque<unk>  to hide their eyes for cosmetic reasons. Many people with light sensitivity conditions wear sunglasses or other tinted<unk>  to make the light more tolerable. Sunglasses may also have corrective lenses, which requires a prescription. Clip-on sunglasses or sunglass clips can be attached to another pair of<unk> . Some wrap-around sunglasses are large enough to be worn over top of another pair of<unk> . Otherwise, many people opt to wear contact lenses to correct their vision so that standard sunglasses can be used. Mixed doubleframe Doubleframe eyewear with one set of lenses on the moving frame and another pair of lenses on a fixed frame (optional). The double frame uplifting<unk>  have one moving frame with one pair of lenses and the basic fixed frame with another pair of lenses (optional), that are connected by four-bar linkage. For example, sun lenses could be easily lifted up and down while mixed with myopia lenses that always stay on. Presbyopia lenses could be also combined and easily removed from the field of view if needed without taking off<unk> . 3D<unk>  The illusion of three dimensions on a two-dimensional surface can be created by providing each eye with different visual information. 3D<unk>  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<unk>  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<unk>  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 viewer's eyes. Anaglyph and polarized<unk>  are distributed to audiences at 3D movies. Polarized and active shutter<unk>  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)<unk>  can also provide magnification that is useful for people with vision impairments or specific occupational demands. An example would be bioptics or bioptic telescopes which have small telescopes mounted on, in, or behind their regular lenses. Newer designs use smaller lightweight telescopes, which can be embedded into the corrective glass and improve aesthetic appearance (mini telescopic spectacles). They may take the form of self-contained<unk>  that resemble goggles or binoculars, or may be attached to existing<unk> . Yellow-tinted computer/gaming<unk>  Yellow tinted<unk>  are a type of<unk>  with a minor yellow tint. They perform minor color correction, on top of reducing eyestrain due to lack of blinking. They may also be considered minor corrective unprescribed<unk> . Depending on the company, these computer or gaming<unk>  can also filter out high energy blue and ultra-violet light from LCD screens, fluorescent lighting, and other sources of light. This allows for reduced eye-strain. These<unk>  can be ordered as standard or prescription lenses that fit into standard optical frames. Blue-light blocking<unk>  Eyeglasses that filter out blue light from computers, smartphones and tablets are becoming increasingly popular in response to concerns about problems caused by blue light overexposure. The problems claimed range from dry eyes to eye strain, sleep cycle disruption, up to macular degeneration which can cause partial blindness. But research shows no measurable ultraviolet radiation from computer monitors. Long hours of computer use may cause eye strain,
Glasses
Bioactive glass
Mustard gas
Chlorofluorocarbon
Methanol
Polychlorinated naphthalene
Alkylphenol
Soylent 