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Typical values for a contact lens are from 8.0 to 10.0 mm. The base curve is the radius of the sphere of the back of the lens that the prescription describes (the lower the number, the steeper the curve of the cornea and the lens, the higher the number, the flatter the curve of the cornea and the lens).
Diameter and base curve radius. The parameters specified in a contact lens prescription may include: Brand name; Material; Base curve radius (BC, BCR) Diameter (D, OAD) Optical power in diopters (dpt) Center thickness (CT) Prescriptions for contact lenses and glasses may be similar but are not interchangeable.
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 equivalent prescription at the patient's cornea (say, for a contact lens) can be calculated as follows (this example assumes a negative cylinder sign convention): Power 1 is the spherical value, and power 2 is the steeper power of the astigmatic axis:
Thus the base curve defines the front surface of the main part of the lens while the back surface geometry is changed to achieve the desired distance power. The "bifocal" is a third spherical segment, called an add segment , found on the front surface of the lens.
P = front base curve (in 1/metres); h = vertex distance (in metres); F = back vertex power (in 1/metres), (essentially, the prescription for the lens, quoted in diopters). If the difference between the eyes is up to 3 diopters, iseikonic lenses can compensate.