More than 24 million people in the U.S. wear contact lenses, notes the University of Michigan's Kellogg Eye Center. Contact lens exams are different than examinations for glasses. During a contact exam, the eye is measured and other tests are done to determine the best type and fit of contact lenses. There is a wide range in the numbers in contact prescriptions, but some measurements have an average in the power, curve and size of the contact lenses.
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Power is the strength of the contact lens, the part that magnifies and helps correct vision. According to the Ohio State University Medical Center, although contact lenses are similar to a spectacle prescription, contact lens measurements usually have different numbers. Power is written as a unit of measurement called a diopter. Diopters can range from 0.00 to 20.00 and be written as a plus number or a minus number. Plus is for farsighted and minus is for nearsighted. The average number for both far and nearsighted is 3.00. The additional numbers almost always include the curve and size of a contact lens, but may contain other information as well.
The base curve, or BC, is the radius of the back of the contact lens, the part that sits on your eye. According to Laser Eye Surgery Review , the average measurement of a base curve is between 8.0 and 10.0. An example of a common base curve would be BC 8.7mm.
All contact lenses have a measurement called the diameter. This measurement is in millimeters and is the size of the lens. The August, 2005 issue of "Contact Lens Spectrum" notes that the average contact lens diameter is 14.0mm.
Contact lenses are considered a medical device and just like prescriptions for medications, contact lens prescriptions expire. The measurements of the eye change and must be updated with eye examinations every one to two years. Most doctors write an expiration date of one year on a contact lens prescription.
The Federal Trade Commission enforces the Contact Lens and Eyeglass Rules, a law that provides patients the right to have a copy of their contact lens and eyeglass prescription. Eye doctors must give patients a copy of their final prescription for glasses and contact lenses.