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Base curve radius (BCR) or simply base curve (BC) is the measure of an important parameter of a lens in optometry. On a spectacle lens, it is the flatter curvature of the front surface. On a contact lens it is the curvature of the back surface and is sometimes referred to as the back central optic radius (BCOR). Typical values for a contact ...
Vertex distance is the distance between the back surface of a corrective lens, i.e. glasses (spectacles) or contact lenses, and the front of the cornea. Increasing or decreasing the vertex distance changes the optical properties of the system, by moving the focal point forward or backward, effectively changing the power of the lens relative to ...
Astigmatism causes difficulties in seeing fine detail. Astigmatism can be often corrected by glasses with a lens that has different radii of curvature in different planes (a cylindrical lens), contact lenses, or refractive surgery. [5] Astigmatism is quite common. Studies have shown that about one in three people suffers from it.
Base curve: BOZD Back optic zone diameter BOZR Back optic zone radius BVP Back vertex power CLAPC/CLIPC Contact-lens-associated/induced papillary conjunctivitis CLARE Contact-lens-associated red eye CLPU Contact-lens-associated peripheral ulcer Dk Unit of permeability DW Daily wear EW Extended wear FOZD Front optic zone diameter FVP
The term "sphere" comes from the geometry of lenses. Lenses derive their power from curved surfaces. A spherical lens has the same curvature in every direction perpendicular to the optical axis. Spherical lenses are adequate correction when a person has no astigmatism. To correct for astigmatism, the "cylinder" and "axis" components specify how ...
Contact lenses made from these materials are called rigid gas permeable lenses (RGPs). A rigid lens is able to cover the natural shape of the cornea with a new refracting surface. This means that a spherical rigid contact lens can correct corneal astigmatism. Rigid lenses can also be made as a front-toric, back-toric, or bitoric.
The parameters appraised by a lensmeter are the values specified by an ophthalmologist or optometrist on the patient's prescription: sphere, cylinder, axis, add, and in some cases, prism. The lensmeter is also used to check the accuracy of progressive lenses , and is often capable of marking the lens center and various other measurements ...
One of the lens surfaces is shaped like a "cap" from a torus (see figure at right), and the other one is usually spherical. Such a lens behaves like a combination of a spherical lens and a cylindrical lens. Toric lenses are used primarily in eyeglasses, contact lenses and intraocular lenses to correct astigmatism.