Contact lenses are used to correct vision and are placed on the cornea of the eye. They perform the same corrective function that conventional glasses or spectacles do. However, in comparison, they are very light and, for all intents and purposes, invisible. Contact lenses help form the image on the retina of the eye by converging or diverging the rays of light entering the eye.
Previous 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 can only be worn for a short period at a time. With the development of PPMA (polymethylmethacrylate) in the 1930s, plastics were first used in contact lenses. In fact, they were hybrid scleral lenses, made with the combination of glass and plastic, in 1936.
In the 1950s, much smaller contact lenses were developed that covered only the cornea of the eye and not the entire eye.
Types of visual impairment
One of the main uses of contact lenses is to correct visual defects. The general deficiencies are myopia, hyperopia, astigmatism and presbyopia.
- Myopia: is a visual disability in which the image of the object seen is formed in front of the retina. During this visual impairment, objects that are close can be seen, and not remote objects, which appear blurry. This defect is also known as myopia. This is a very common impairment, with more than 25 percent of adults in the United States suffering from it. The defect can be corrected by wearing concave contact lenses.
- Farsightedness – Also known as farsightedness, the image of the object is formed behind the retina. Distant objects can be seen clearly and nearby objects appear to be blurry. Farsightedness is more commonly known as farsightedness, and more than 13 percent of children in the United States, in the age group 5 to 17, have it. The defect can be corrected by wearing convex contact lenses.
- Astigmatism: this occurs when the lens of the eye has more than one focal point, on different meridians. Astigmatic people cannot see in fine detail and need cylindrical lenses to correct their disability. Almost 34 percent of American children in the 5 to 17 age group have this disability.
- Presbyopia: This is an impairment that occurs with age, generally after the age of 40. The impairment develops as the lens of the eye loses its elasticity. Bifocal contact lenses are used to correct this vision defect.
Lenses used to correct vision
In the case of normal vision, the light from the object hits the cornea and is focused on the retina. Due to some refractive error, sometimes the light from the object is not focused on the retina, but in front of or behind it. To correct this refractive error, contact lenses are used to focus the retina.
The type of contact lenses used depends on the type of visual impairment and the degree of refractive error involved. The extent to which the lens bends light to focus the retina is measured in diopters (D).
Myopia occurs when light is focused in front of the retina, since the eyeball is longer than normal. To correct this deficiency, which is also known as myopia, a concave lens is used. This lens is thinner in the center and helps move focus to the retina.
To correct this visual impairment, the curvature of concave contact lenses is determined by measuring in diopters. The higher the number of diopters, the greater the vision defect. In myopia, the number of diopters is preceded by a minus sign (-), indicating that the focus is below the retina.
In the case of hyperopia, light is focused beyond the retina. Farsightedness is also known as farsightedness, as remote objects are clearly seen in this disability. 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 to move focus back to the retina.
Also in this case, the required curvature in convex contact lenses is determined by measuring in diopters. The number of diopters is preceded by the plus sign (+), which indicates that the focus is beyond the retina.
Lenses used for the correction of myopia and hyperopia are classified as spherical contact lenses.
When the cornea is irregularly shaped, light from the object falling on the cornea is focused on more than one point. This distortion of the image is called astigmatism. It is necessary to design special lenses, based on the distortion of the individual’s image. These lenses are known as toric lenses.
Although toric lenses are made from the same materials as spherical lenses, they are specifically designed to accommodate individual disabilities. These lenses have different curvatures, thicker in some places and thinner in others. These lenses are designed to correct astigmatism and nearsightedness or farsightedness, if necessary.
For the correction of presbyopia, special bifocal lenses are required, since the person suffering from it requires both correction of myopia and hyperopia. In such lenses, the near deterioration correction is placed in the center of the lens, with the remote correction on the outside, or vice versa.
Types of contact lenses
The initial lenses were rigid lenses that did not absorb water. This prevented oxygen from entering the cornea of the eye, causing eye irritation and other discomfort.
Then came soft contact lenses made of hydrogel, which allowed oxygen to pass through them to the cornea. These lenses became known as “breathable” contact lenses. This made it possible for contact lenses to be worn comfortably and for longer periods. Today there is:
Contact lenses for daily use, which are removed at night.
Extended wear lenses that can be worn for long periods without taking off.
Disposable lenses that can be thrown away after a day, a week, or a few weeks.
In addition, there are colored lenses, which are for cosmetic use.