A contact lens prescription is a precise medical document detailing the specifications required for corrective vision devices that sit directly on the eye. Deciphering this specialized terminology is necessary, particularly when a person has astigmatism, which requires a specific type of lens known as a toric lens. This lens type introduces additional parameters to the prescription slip. Understanding these codes is essential for ordering the correct product.
Understanding the Structure of the Prescription
A contact lens prescription identifies the specific eye using Latin abbreviations. OD (Oculus Dexter) refers to the right eye, and OS (Oculus Sinister) refers to the left eye. OU (Oculus Uterque) indicates a parameter that applies to both eyes.
A contact lens prescription is fundamentally different from an eyeglass prescription. Eyeglasses sit approximately 12 millimeters away from the eye, a distance known as the vertex distance. Since contact lenses rest directly on the eye’s surface, the required power changes significantly, especially for stronger corrections. Using eyeglass values to order contacts will result in an incorrect product.
Deciphering Spherical Power and Astigmatism Correction
The primary correction is found under the SPH (Sphere) column, which measures the main focusing power of the lens in diopters. This value corrects for either nearsightedness or farsightedness, also known as myopia or hyperopia. A minus sign (-) indicates correction for nearsightedness, while a plus sign (+) denotes correction for farsightedness.
For those with astigmatism, the prescription includes additional values to account for the eye’s irregularly curved shape. Astigmatism occurs when the cornea or lens is shaped like a football, causing light to focus unevenly onto the retina. This requires a toric lens to provide differentiated correction.
The CYL (Cylinder) value indicates the precise amount of extra lens power required to correct this irregular curvature. If this column is blank or reads “DS” (Diopters Sphere), astigmatism correction is not required. The CYL value, measured in diopters, is always a minus number, representing the additional power needed to align the out-of-focus meridian.
The AXIS number defines the exact orientation or angle (1 to 180 degrees) where the cylindrical power must be applied. For example, 90 degrees indicates vertical correction, while 180 degrees means the correction is horizontal.
Since a toric lens must maintain a specific orientation, the AXIS value ensures the lens does not rotate during wear. Toric lens design includes features that stabilize it at the prescribed angle, ensuring the corrective power aligns with the eye’s curve. Both the CYL and AXIS fields must be accurate for the lens to provide clear, stable vision.
Key Measurements and Lens Specifications
Beyond the corrective powers, a prescription includes physical measurements to ensure the lens fits safely and comfortably onto the cornea. The Base Curve (BC) defines the curvature of the back surface of the contact lens. This value is measured in millimeters, typically falling between 8.0 and 9.5 mm for soft lenses.
The BC must closely match the curvature of the wearer’s cornea; a smaller number indicates a steeper lens curve, while a larger number indicates a flatter lens. If the BC is incorrect, the lens may fit too tightly, restricting oxygen flow, or too loosely, causing discomfort and unstable vision. Improper fit can lead to irritation, redness, and compromise eye health.
The Diameter (DIA) measures the total width of the lens from edge to edge in millimeters. This measurement ensures the lens properly covers the cornea. For soft lenses, the DIA commonly ranges from 13.8 to 14.5 mm.
Every contact lens prescription must specify the exact Brand and Material of the lens prescribed. Different manufacturers use distinct materials that affect oxygen permeability, water content, and how the lens physically interacts with the eye. Even if the SPH, CYL, and AXIS values are identical, switching brands without a new fitting can compromise comfort and safety, making this specification non-negotiable.