A contact lens prescription is a specific medical document containing all the measurements necessary for correcting vision and ensuring a proper fit. The abbreviations and numbers detail the exact power, curvature, and size of lens you require. Understanding these terms confirms you are receiving the correct medical device.
Understanding Which Eye Is Which
The first step in decoding the prescription is identifying which eye each measurement refers to, as powers often differ between the left and the right eye. OD stands for Oculus Dexter (right eye), and OS stands for Oculus Sinister (left eye). A third, less frequently used abbreviation, OU, stands for Oculus Uterque, signifying both eyes.
Decoding the Vision Correction Numbers
The core of the prescription is the mathematical power used to restore clear vision, measured in units called diopters. This power is represented primarily by the Sphere, Cylinder, and Axis values. The Sphere (SPH or PWR) indicates the main lens power needed to correct nearsightedness or farsightedness. A minus sign (-) signifies correction for myopia, while a plus sign (+) indicates correction for hyperopia. A higher absolute number means a stronger corrective power is required.
If the cornea is irregularly curved, resulting in astigmatism, additional values will be present. The Cylinder (CYL) indicates the extra power required to compensate for this curvature. This measurement is typically a negative number that represents the lens strength needed to make the light focus evenly. If the CYL column is blank, it means the eye has a regular, spherical shape and does not require astigmatism correction.
The Axis (AX) is a number between 1 and 180 that is only present if a CYL value exists. It specifies the orientation or angle on the cornea where the astigmatism correction must be applied. The Add Power (ADD) is included only for multifocal or bifocal lenses, indicating the supplemental magnifying power needed for reading. This value ensures simultaneous clear distance and near vision for individuals with presbyopia.
Physical Fit Measurements
Beyond the vision correction power, a contact lens prescription must contain measurements that dictate how the lens physically sits on the eye’s surface. These parameters are fundamental for comfort, stability, and corneal health. The Base Curve (BC) defines the curvature of the back surface of the contact lens, measured in millimeters. This number must accurately match the curvature of the cornea to ensure the lens rests properly.
A lower BC number corresponds to a steeper corneal curve, while a higher number indicates a flatter curve. If the BC is too flat, the lens moves excessively, causing discomfort; if too steep, it can restrict oxygen flow and lead to eye health issues. The Diameter (DIA) is the measurement, also in millimeters, of the contact lens across its widest point, ensuring the lens correctly covers the cornea and stays centered.
The prescription also specifies the exact Brand or Material of the lens that was successfully fitted. This specification is included because different materials and designs, such as hydrogel versus silicone hydrogel, affect oxygen permeability and how the lens interacts with the tear film.
Key Differences from Eyeglass Prescriptions
The most significant functional difference between a contact lens prescription and an eyeglass prescription relates to proximity to the eye. Eyeglasses sit approximately 12 to 14 millimeters away from the cornea, a measurement known as vertex distance. Contact lenses, however, sit directly on the corneal surface, meaning their vertex distance is zero. This difference in position changes the effective power of the lens.
For higher powers, typically those exceeding plus or minus 4.00 diopters, the vision correction numbers must be mathematically adjusted when moving from glasses to contacts. For example, a high minus lens will require a slightly weaker contact lens power than the glasses power to achieve the same visual result. The doctor performs this vertex distance calculation to ensure the contact lens power is accurate at the corneal plane.
Furthermore, an eyeglass prescription does not include the Base Curve (BC) or Diameter (DIA) because glasses do not touch the eye. Unlike a glasses prescription, a contact lens prescription has a strict expiration date, typically one year. This expiration ensures the patient undergoes regular eye health checks, confirming that the lenses still fit correctly and are not causing any underlying issues.