A contact lens prescription is a medical device order that specifies the exact parameters necessary for the lens to fit safely and comfortably on your eye. Obtaining this prescription requires an eye health exam and a fitting to ensure the lenses interact correctly with the eye’s surface. This document is distinct from an eyeglass prescription because contact lenses sit directly on the cornea, requiring unique measurements for proper vision correction and injury prevention. Understanding the various numbers and acronyms is the first step toward safely managing your vision correction needs.
Decoding the Essential Parameters
The contact lens prescription begins by distinguishing between the eyes, typically using the Latin abbreviations OD for the right eye and OS for the left eye, while OU denotes both eyes. The primary vision correction value is listed under PWR or SPH, which stands for Power or Sphere, measured in diopters (D) to correct for nearsightedness (indicated by a minus sign) or farsightedness (indicated by a plus sign). This figure represents the lens strength needed for clear distance vision.
Beyond the vision correction power, the prescription includes parameters for the physical fit of the lens. The BC, or Base Curve, defines the curvature of the back surface of the lens, expressed in millimeters, and must closely match the curve of the patient’s cornea. This measurement ensures the lens rests properly, allowing tears to circulate beneath it and preventing discomfort or corneal damage.
Another physical measurement is the DIA, or Diameter, which specifies the overall width of the lens in millimeters. This parameter ensures the lens covers the cornea and centers correctly over the pupil for stable vision. For patients with astigmatism (an irregular curvature of the cornea), the prescription includes CYL (Cylinder) and AXIS.
The Cylinder value, measured in diopters, indicates the power needed to correct astigmatism. The Axis is a number between 1 and 180 degrees that shows the orientation where the corrective power must be placed to align with the eye’s irregular curvature. For those with presbyopia, a separate value called ADD, or Addition, is included.
The Addition power represents the extra magnifying power needed for reading and close-up tasks, typically used in multifocal or bifocal contact lenses. These specialized values show that a contact lens prescription is highly individualized, accounting for the eye’s unique shape and function in addition to refractive error.
Key Differences from Eyeglass Prescriptions
Contact lens and eyeglass prescriptions are not interchangeable, primarily due to vertex distance. Vertex distance is the physical space between the back surface of a lens and the front surface of the eye (the cornea). Eyeglass lenses sit about 12 to 14 millimeters away, while a contact lens rests directly on the cornea, resulting in a vertex distance of zero.
This difference significantly changes the effective power of the lens, especially for prescriptions stronger than plus or minus 4.00 diopters. An eye doctor must perform vertex compensation to adjust the lens power, resulting in a different SPH number compared to the glasses prescription. For example, a strong minus power for nearsightedness will be a weaker number in the contact lens prescription to compensate for the closer distance.
Furthermore, an eyeglass prescription lacks the physical fitting parameters necessary for a contact lens to function safely. The Base Curve (BC) and Diameter (DIA) measurements, which determine how the lens conforms to the corneal shape, are specific to a contact lens prescription and cannot be derived from glasses. The required separate fitting exam ensures the physical lens parameters are appropriate, preventing issues like corneal abrasion or poor oxygen flow.
Understanding Prescription Validity and Renewal
Contact lens prescriptions have a mandated expiration date, a legal requirement designed to protect consumer eye health. In the United States, federal law establishes a minimum validity period of one year. This minimum ensures that patients return for regular eye examinations to monitor for changes in vision or eye health issues that can arise from contact lens wear.
While the minimum is one year, some state laws may extend the validity period to two years. Once the prescription expires, retailers are legally prohibited from selling new contact lenses until the patient obtains an updated prescription. This requirement necessitates an eye exam and a fitting check to confirm that the lenses are still the correct power and fit the eye properly.
The renewal process is important because contact lenses are medical devices, and an improper fit or outdated prescription can lead to serious complications. Regular check-ups allow the eye care professional to assess the cornea’s health, check for signs of infection, and verify that the current lens material and fit are not causing damage. The expiration date acts as a safety measure to ensure ongoing professional monitoring of eye health.