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Contact lenses are used for vision correction and are placed on the cornea of the eye. They have the same corrective function as conventional glasses or spectacles. However, in comparison they are very lightweight and for all intents and purposes invisible. Contact lenses help shape the image on the retina of the eye by converging or diverging the light rays entering the eye.
Earlier contact lenses were made of glass and were scleral lenses. Scleral lenses are large contact lenses that cover the entire sclera — the white outer layer — of the eye. These unwieldy lenses could only be worn for a short period of time at a time. With the development of PPMA – polymethyl methacrylate – in the 1930s, plastics were first used in contact lenses. These were basically hybrid scleral lenses, made with the combination of both, glass and plastic, in 1936.
By the 1950s, much smaller contact lenses were being developed that covered only the cornea of the eye and not the entire eye.
Types of visual impairments
One of the main uses of contact lenses is to correct visual defects. The common disorders are nearsightedness, farsightedness, astigmatism and presbyopia.
- Myopia – is a visual impairment in which the image of the observed object is formed in front of the retina. During this visual impairment, one can see objects that are close, and not the distant objects, which appear blurry. This defect is also called myopia. This is a common disorder, affecting more than 25 percent of adults in the United States. The defect can be corrected through the use of concave contact lenses.
- Farsightedness – It is also known as hyperopia and the image of the object is formed behind the retina. Objects far away are clearly seen and objects close by appear blurry. Farsightedness is more commonly known as farsightedness, and more than 13 percent of children in the United States, in the age group of 5 to 17, suffer from it. The defect can be corrected through the use of convex contact lenses.
- Astigmatism – This occurs when the lens of the eye has more than one focal point, in different meridians. Astigmatic people cannot see in fine detail and need cylindrical lenses to correct their disorder. Nearly 34 percent of American children in the 5 to 17 age group have this disability.
- Presbyopia – This is a disorder that comes with age, usually after the age of 40. The disorder develops as the lens of the eye loses its elasticity. Bifocal contact lenses are used to correct this vision defect.
In normal vision, light from the object falls on the cornea and focuses on the retina. Due to a refractive error, the light from the object sometimes falls not on the retina, but in front of or behind it. To correct this refractive error, contact lenses are used to focus on the retina.
The type of contact lenses used depends on the type of vision impairment and how many refractive errors are involved. How much the lens bends the light to focus on the retina is measured in diopters (D).
Nearsightedness occurs when light is focused in front of the retina because the eyeball is longer than normal. To correct this disorder, also called nearsightedness, a concave lens is used. This lens is thinner in the center and helps to move the focus forward, towards the retina.
To correct this visual impairment, the curvature in the concave contact lenses is determined by the measurement in diopters. The greater the number of diopters, the greater the vision. In nearsightedness, the diopter number is preceded by a minus sign (-), indicating that the focus is too short for the retina.
In farsightedness, light is focused beyond the retina. Hyperopia is also known as farsightedness, as distant objects are clearly seen with this impairment. The eyeball is shorter than normal and a convex lens is used to correct this vision defect. The contact lens used is thicker in the center and helps move focus back to the retina.
Also in this case, the necessary curvature in the convex contact lenses is determined by the measurement in diopters. The diopter number is preceded by the plus sign (+), indicating that the focus is beyond the retina.
The lenses used for the correction of nearsightedness and farsightedness are categorized as spherical contact lenses.
When the cornea has an irregular shape, the light from the object striking the cornea is focused on more than one point. This distortion of the image is called astigmatism. Special lenses have to be designed, based on the image distortion of the individual. These lenses are known as toric lenses.
While toric lenses are made from the same materials as the spherical lenses, they are specifically designed to meet individual limitations. These lenses have different curvatures, thicker in some places and thinner in others. These lenses are designed to correct astigmatism and nearsightedness or farsightedness when needed.
Correction of presbyopia requires special bifocal lenses, as the person suffering from it needs both nearsightedness and farsightedness correction. With such lenses, either the correction for near deterioration is placed in the center of the lens, with the far correction on the outside, or vice versa.
Types of contact lenses
The first lenses were rigid lenses that did not absorb water. This kept the oxygen from reaching the cornea of the eye, causing eye irritation and other discomfort.
Then came the soft contact lenses made of hydrogel, which allowed oxygen to pass through to the cornea. These lenses became known as “breathable” contact lenses. This allowed contact lenses to be worn comfortably and for extended periods of time. Today there are:
Daily lenses, which are removed at night.
Long-wear lenses that can be worn for extended periods of time without taking them off.
Disposable lenses that can be thrown away after a day, a week or a few weeks.
In addition, there are color contacts, which are for cosmetic purposes.
The physics of contact lenses
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