Overview of Refractive Errors Correction
Several factors are considered when choosing a corrective option:
- Individual preference
- Other health conditions
- Severity and type of refractive error
Before making a selection, the pros and cons of all options should be discussed with an eye care practitioner.
Eyeglass lenses correct refractive errors by focusing light directly on the retina. The type of lens depends on the type and severity of the refractive error.
The strength of a lens (i.e., its refractive power) is measured in optical units called diopters. This measurement indicates how much refractive power the lens must have to focus images directly on the retina. The higher the diopter, the stronger the lens.
The type of refractive error determines the lens's shape. A concave (minus) lens is used to correct myopia (nearsightedness). In myopia, light rays fall in front of the retina rather than on it. Because a concave lens is thin in the center and thicker on the edges, it diverges (spreads out) light rays so that the eye's lens focuses them directly on the retina.
A convex (plus) lens is used to correct hyperopia (farsightedness). In hyperopia, light rays fall behind the retina. The lens is thickest in the center and thinnest on the outer edges. The convex lens converges (concentrates) light rays so that the eye's lens focuses them on the retina.
To correct astigmatism, which is caused by distortions in the shape of the lens or cornea, a cylinder lens is frequently used. The cylinder lens has two refractive powers on one lens. One power is placed over the entire lens and the other is oriented in one direction. This corrects the scattered pattern in which light enters the eye and creates one focal point on the retina.
People that have more than one refractive error may require two pairs of eyeglasses or glasses with multifocal lenses. Multifocal lenses contain two or more vision-correcting prescriptions.
- Bifocals are the most common type of multifocal lenses. The lens is split in two sections; the upper part is for distance vision and the lower part for near vision. They are usually prescribed for people over the age of 40 whose focusing ability has declined due to presbyopia.
- Trifocals have a third section used for middle distance vision (i.e., objects within arm's reach, such as a computer screen).
- Progressive lenses are sometimes referred to as "no-line" bifocals because there is no noticeable line dividing the different prescriptions. There is a continuous change in magnification from the distance, or upper, portion of the lens, to the near, or lower, portion of the lens. They therefore provide a smoother transition between distance and near vision that some feel is closer to natural vision than other multifocal lenses. However, not everyone adapts well to them.
The choice of frames usually depends on personal preference, fashion, comfort, and cost. Frames are made from metals, plastic, nylon, and other synthetics. Each material has its advantages.
Pure titanium, for example, is lightweight, strong, and durable and frames made from titanium are very light, long lasting, and hypoallergenic. They are also expensive. A titanium alloy called Flexon, or the "memory metal," has recently been developed. Flexon frames return to their original shape after being damaged or bent. This metal is very light and resists corrosion.
Traditionally, lenses have been made from glass, but today, they are more commonly made from plastic. Glass lenses are breakable and are about twice as heavy as plastic ones; however, they are more resistant to scratches. Plastic lenses scratch more easily, even with scratch-resistant coatings, but they are much lighter, less likely to break, and can be treated with ultraviolet filters and antiglare coatings.
High-index and aspheric. High-index and aspheric lenses are recommended by opticians for very strong prescriptions that often require very thick lenses. High-index plastics make it possible for strong prescription lenses to be thinner and lighter. The materials used in high-index lenses are dense, using less lens material to achieve the same degree of vision correction. High-index plastic can be used to correct nearsightedness, farsightedness, and astigmatism.
An aspheric lens, unlike a spherical lens, which has constant continuous curvature, has varying degrees of curvature over its surface. This design makes the lens flatter and appear thinner. An aspheric lens's optical properties provide the wearer with a larger, more usable portion of the lens and a clearer view throughout the lens than a conventional spherical design. They are ideal for strong prescriptions to correct farsightedness and in those patients who had cataract removal without replacing the eye's lens with an intraocular lens implant.
Photochromic. This type of lens changes from colorless to dark, depending on the amount of ultraviolet exposure. The lenses are clear, but in sunlight a tint appears, eliminating the need for prescription sunglasses. Photochromic lenses are available in plastic and glass and for nearly every type of refractive error.
Polycarbonate. This is the most impact-resistant material available and is 10 times less likely to break than glass or plastic. They are the lenses of choice for children and adults who engage in activities (e.g., sports) or occupations in which eyeglasses can be easily broken. They are also recommended for those who are monocular (have only one eye) and those who have one functioning eye. Polycarbonate lenses are lighter and thinner than other types of lenses and absorb ultraviolet light, thus negating the need to treat eyeglasses with ultraviolet filters.