Contact lenses are used for vision correction and are placed on the cornea of the eye˳ They do the same corrective function that conventional spectacles, or glasses, do˳ However, in comparison, they are very light in weight and are, for all purposes, invisible˳ Contact lenses help form the image on the retina of the eye by either converging or diverging the rays of light entering the eye˳
Earlier contact lenses were made of glass, and were scleral lenses˳ Scleral lenses are large contact lenses that cover the complete sclera – the white outer coating – of the eye˳ These unwieldy lenses could only be worn for a short period at a time˳ With the development of PPMA – polymethyl methacrylate – in the 1930s, plastics were first used in contact lenses˳ These were in fact, hybrid scleral lenses, made with the combination of both, glass and plastic, in 1936˳
By the 1950s, much smaller contact lenses were developed that covered only the cornea of the eye and not the whole eye˳
Types of Vision Impairments
One of the major uses of contact lenses is to correct visual defects˳ The general impairments are Myopia, Hyperopia, Astigmatism, and Presbyopia˳
- Myopia – is a visual disability where the image of the object seen is formed in front of the retina˳ During this visual impairment, one can see objects that are near, and not the distant objects, which appear blurred˳ This defect is also known as nearsightedness˳ This is a very common impairment, with over 25 percent of the adults in the United States suffering from it˳ The defect can be corrected by the use of concave contact lenses˳
- Hyperopia – It is also known as Hypermetropia, and the image of the object is formed behind the retina˳ Far objects can be seen clearly, and the near objects appear to be blurred˳ Hyperopia is more commonly known as farsightedness, and more than 13 percent of the children in the United States, in the age group 5 to 17, suffer from it˳ The defect can be corrected by the use of convex contact lenses˳
- Astigmatism – This happens 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 impairment˳ Nearly 34 percent of American children in the age group 5 to 17 have this impairment˳
- Presbyopia – This is an impairment, which comes 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 For Vision Correction
In the case of normal vision, the light from the object hits the cornea and focuses on the retina˳ Due to some refractive error, at times the light from the object does not focus on the retina, but either in front of it, or behind it˳ To correct this refractive error, contact lenses are used to focus on to the retina˳
The type of contact lenses used depends on the type of vision impairment, and how much refractive error is involved˳ How much the lens bends the light to focus on the retina is measured in diopters (D)˳
Myopia occurs when the light is focused in front of the retina, as the eyeball is longer than normal˳ To correct this impairment, which is also known as nearsightedness, a concave lens is used˳ This lens is thinner at the center, and helps move the focus ahead, towards the retina˳
To correct this vision impairment, the curvature in the concave contact lenses is determined by the measurement in diopters˳ The larger the number of diopters, larger is the vision defect˳ In myopia, the diopter number is preceded by a minus (-) sign, denoting that the focus is short of the retina˳
In the case of hyperopia, the light is focused beyond the retina˳ Hyperopia is also known as farsightedness, as distant objects are seen clearly in 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 the focus back onto the retina˳
In this case, too, the curvature required in the convex contact lenses is determined by the measurement in diopters˳ The diopter number is preceded by the plus (+) sign, denoting that the focus is beyond the retina˳
The lenses used for the correction of myopia and hyperopia are categorized as spherical contact lenses˳
When the cornea is irregularly shaped, the light from the object falling on the cornea focuses on more than one point˳ This distortion of the image is called astigmatism˳ Special lenses need to be designed, based on the individual’s distortion of image˳ These lenses are known as toric lenses˳
Though toric lenses are made of the same materials as the spherical lenses, they are specifically designed to suit individual impairments˳ These lenses have different curvatures, thicker in some places, and thinner in others˳ These lenses are designed to correct astigmatism and myopia or hyperopia, if required˳
For the correction of presbyopia, special bifocal lenses are required, as the person suffering from it requires both correction for nearsightedness and farsightedness˳ In such lenses, either the correction for near impairment is placed in the center of the lens, with the distant correction on the outside, or vice versa˳
Types Of Contact Lenses
The initial lenses were rigid lenses that did not absorb water˳ This kept the oxygen from passing into the cornea of the eye, causing eye irritation and other discomforts˳
Then came the soft contact lenses made from hydrogel, which allowed oxygen to pass through them to the cornea˳ These lenses came to be known as ‘breathable’ contact lenses˳ This made it possible for contact lenses to be worn comfortably and for longer periods˳ Today, there are:
Daily wear lenses, which are removed at night˳
Extended wear lenses that can be worn for extended periods without removing˳
Disposable lenses that can be discarded after a day, a week, or a few weeks˳
In addition, there are color contacts, which are for cosmetic purpose˳
Source by https://ezinearticles˳com/?The-Physics-Of-Contact-Lenses&id=266267
