Contact lenses are used for foresight revising and are placed on the cornea of the eye. They do the same remedial function that accepted spectacles, or glasses, do. However, in comparison, they are very light in weight and are, for all purposes, invisible. Caress lenses help form the image on the retina of the eye by whether converging or diverging the rays of light entering the eye.
[b]Lens[/b]Earlier Caress lenses were made of glass, and were scleral lenses. Scleral lenses are large Caress lenses that cover the unblemished sclera - the white outer coating - of the eye. These unwieldy lenses could only be worn for a short period at a time. With the amelioration of Ppma - polymethyl methacrylate - in the 1930s, plastics were first used in Caress lenses. These were in fact, hybrid scleral lenses, made with the composition of both, glass and plastic, in 1936.
By the 1950s, much smaller Caress lenses were advanced that covered only the cornea of the eye and not the whole eye.
Types of foresight Impairments
One of the major uses of Caress lenses is to spoton optical defects. The normal impairments are Myopia, Hyperopia, Astigmatism, and Presbyopia.
- Myopia - is a optical disability where the image of the object seen is formed in front of the retina. While this optical impairment, one can see objects that are near, and not the distant objects, which appear blurred. This flaw 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 flaw can be corrected by the use of concave Caress 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 flaw can be corrected by the use of convex Caress lenses.
- Astigmatism - This happens when the lens of the eye has more than one focal point, in dissimilar meridians. Astigmatic citizen cannot see in fine detail, and need cylindrical lenses to spoton 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, commonly after the age of 40. The impairment develops as the lens of the eye loses its elasticity. Bifocal Caress lenses are used to spoton this foresight defect.
Lenses Used For foresight 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 whether in front of it, or behind it. To spoton this refractive error, Caress lenses are used to focus on to the retina.
The type of Caress lenses used depends on the type of foresight 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 spoton 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 spoton this foresight impairment, the curvature in the concave Caress lenses is carefully by the determination in diopters. The larger the estimate of diopters, larger is the foresight defect. In myopia, the diopter estimate 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 spoton this foresight defect. The Caress 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 Caress lenses is carefully by the determination in diopters. The diopter estimate is preceded by the plus (+) sign, denoting that the focus is beyond the retina.
The lenses used for the revising of myopia and hyperopia are categorized as spherical Caress 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 personel impairments. These lenses have dissimilar curvatures, thicker in some places, and thinner in others. These lenses are designed to spoton astigmatism and myopia or hyperopia, if required.
For the revising of presbyopia, Special bifocal lenses are required, as the person suffering from it requires both revising for nearsightedness and farsightedness. In such lenses, whether the revising for near impairment is placed in the town of the lens, with the distant revising on the outside, or vice versa.
Types Of Caress Lenses
The initial lenses were rigid lenses that did not discharge water. This kept the oxygen from passing into the cornea of the eye, causing eye irritation and other discomforts.
Then came the soft Caress lenses made from hydrogel, which allowed oxygen to pass straight through them to the cornea. These lenses came to be known as 'breathable' Caress lenses. This made it possible for Caress 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.
The Physics Of touch Lenses
0 comments:
Post a Comment