There are two types of lenses prescribed for correcting or improving vision. These include:
Eyeglasses (also called spectacles). Eyeglasses, the most common form of eyewear used to correct or improve many types of vision problems, are a frame that holds two pieces of glass or plastic. These have been ground into lenses to correct refractive errors. Refractive errors can include nearsightedness or myopia (difficulty seeing far away), farsightedness or hyperopia (difficulty seeing close up), and astigmatism (blurring due to an irregularly shaped cornea). Eyeglasses perform this function by adding or subtracting focusing power to the eye's cornea and lens.
Contact lenses. Contact lenses are worn directly on the cornea of the eye. Like eyeglasses, contact lenses help to correct refractive errors. They perform this function by adding or subtracting focusing power to the eye's cornea and lens.
The lens power of eyeglasses is measured in diopters. This measurement reflects the amount of power necessary to focus images directly onto the retina. When looking at an eyeglasses prescription, you will see the following abbreviations:
O.D. Oculus dextrous simply refers to the right eye (sometimes the abbreviation RE is used).
O.S. Oculus sinister refers to the left eye (sometimes the abbreviation LE is used).
In addition, the eyeglass prescription may also contain the following measurements.
This number measurement shows the extent of the nearsightedness or farsightedness.
This number measurement refers to the amount of astigmatism in the eye. Astigmatism is an irregularly shaped cornea that causes blurring.
This number measurement describes the astigmatism in degrees from the horizontal axis. Most left and right eyes have a similar axis of astigmatism.
Bifocal prescriptions usually have an additional measurement listed on the prescription as add to indicate the strength of the lens. Bifocals are used to correct both nearsightedness and farsightedness.
The type of lenses used in eyeglasses depends on the type of vision problem and may include:
Concave lenses. These are thinnest in the center. They are used to correct nearsightedness (myopia), the numerical prescription in diopters is always marked with a minus (-) symbol.
Convex lenses. These are thickest in the center, like a magnifying glass. They are used to correct farsightedness (hyperopia), the numerical prescription in diopters is always marked with a plus (+) symbol.
Cylindrical lenses. These curve more in one direction than in the other and are used to correct astigmatism.
Over 30 million Americans wear contact lenses, 80% of whom wear daily wear soft lenses. Currently, there are four types of contact lenses in use:
The soft, water-absorbing lens
The rigid, gas-permeable lens
Other rigid lenses
Other flexible, nonwater absorbing lenses
The prescription for contact lenses includes more information than what is available on the prescription for eyeglasses. Special measurements will need to be taken of the curvature of the eye. In addition, the healthcare provider will determine if the eyes are too dry for contact lenses. Also, if there are any corneal problems that may prevent a person from wearing contact lenses. Trial lenses are usually tested on the eyes for a period of time to make sure of proper fit.
The contact lens prescription usually includes the following information:
Contact lens power (measured in diopters, like eyeglasses)
Contact lens base curve
Diameter of the lens
Contact lens manufacturer
Eye care specialists are required by federal law to give you a copy of your contact lens specifications.