Contact lenses are worn on the surface of the eye. The Ohio State University Medical Center says that contacts correct vision problems by using magnification in the lens to add or subtract power, also referred to as strength. Power is the amount of correction needed to correct vision to an acceptable level, usually 20/20. Eye care practitioners use tools to measure and verify the power of a contact lens before the lenses are dispensed to a patient.
Significance
All prescription contact lenses are designed to fit the front of the eye. An eye care practitioner measures a person's corneal curvature with an optical machine called a keratometer. The curves are written as numbers called K-readings and are used to help determine the size and fit of the contact lenses. In addition to the K-readings, an eye doctor uses a person's visual error correction, transcribed as power or strength of the contact lenses, to design the final contact lens prescription.
Features
Contact lens powers are written and expressed in a unit of measurement called a diopter, written as D. Lens powers that correct for nearsightedness are written in minus form, with a minus sign before the number; for example, -2.00 D. A farsighted prescription is written in plus power, such as +2.00 D.
Lensometer
Lens power is measured on an optical machine called a lensometer. Lensometers are the type of optical equipment used most often to measure the strength of lenses, especially eyeglass lenses. Specific types of lensometers have additional equipment that make reading the power of a contact lens easier, but it is possible to use a regular lensometer to read the prescription strength of a contact lens.
Radiuscope
A radiuscope is designed to measure the base curve of a contact lens. According to the April, 2005 issue of Contact Lens Spectrum, using the radiuscope is one of the most important parameters of measuring a contact lens because it affects the lens-to-cornea fitting relationship. The curves of the contact correspond to the K-readings done in the contact lens fitting examination. There are two images in a radiuscope, a steeper curve and a flatter curve.
Lens Thickness and Diameter
Lens thickness and diameter affect the contact lens prescription. Lens calipers are used to measure the contact lens thickness, which impacts oxygen transmission to the eye, lens position and comfort. Diameter is measured by observing the lens in the lensometer or by hand with a small optical ruler called a PD ruler.
