When you receive a prescription for new glasses, the document often looks like a coded table filled with abbreviations and numbers. This array of letters and figures can be confusing, making it difficult to understand the specific measurements that define your vision correction. The prescription is a precise map of your refractive error, and understanding its components is key to ensuring you receive the correct lenses. This guide will decode the most common abbreviations and numerical values to demystify the prescription (Rx) process.
What OD, OS, and OU Mean
The first step in deciphering your prescription is understanding which eye is being measured. The abbreviation OD stands for oculus dexter, which is Latin for “right eye.” OS stands for oculus sinister, the Latin term for “left eye.” These Latin designations are used worldwide in optometry and ophthalmology to ensure clarity for the optical lab.
Occasionally, you may see the abbreviation OU, which is short for oculus uterque, meaning “both eyes.” This designation is often used when the vision correction power is identical for both the right and left eyes. These abbreviations establish the specific row or column of measurements that applies to each individual eye.
The Numerical Components of Your Prescription
Following the OD and OS designations, the prescription lists several columns of numbers, all measured in diopters, the unit of lens power. The first major column is SPH, or Sphere, which indicates the main corrective power for nearsightedness or farsightedness. A negative sign (-) signifies nearsightedness (myopia), meaning difficulty seeing distant objects clearly. Conversely, a positive sign (+) indicates farsightedness (hyperopia), a challenge with close-up vision.
The next two columns, CYL (Cylinder) and AXIS, correct for astigmatism, a common condition where the cornea or lens has an irregular shape. The CYL value measures the amount of lens power needed to correct this irregular curve. This correction is achieved by adding a cylindrical shape to the lens, which focuses light to a single point.
The AXIS is a number between 0 and 180 degrees, specifying the exact angle where the cylindrical correction must be applied to the lens. The CYL value indicates the magnitude of the correction, while the AXIS dictates its rotational placement. If astigmatism is absent, the CYL and AXIS columns may be left blank or contain abbreviations like DS (Diopter Sphere).
For middle-aged or older patients, an ADD (Addition) value may be present, which is the extra magnifying power required for reading or near tasks. This positive value is added to the SPH value for the distance prescription to create the power needed for multifocal lenses, such as bifocals or progressives.
A small percentage of prescriptions may also include a PRISM value, which corrects for eye alignment issues that can cause double vision. This correction is measured in prism diopters and is accompanied by a base direction (e.g., BU for Base Up) to indicate the direction of the prism’s thickest edge.
Ensuring Accuracy and Proper Interpretation
The correct interpretation of the OD and OS designations is paramount, as mixing them up results in the wrong correction being applied to the wrong eye. This error can cause severe headaches and eye strain. When ordering new eyewear, especially online, ensure the values listed under OD are correctly entered for the right lens and OS for the left lens.
A frequent oversight involves misreading the plus (+) and minus (-) signs in the SPH and CYL columns. A difference of a single sign completely changes the type of vision problem being corrected, switching correction between nearsightedness and farsightedness.
The prescription does not always include the Pupillary Distance (PD), which is the measurement in millimeters between the centers of your pupils. The PD is necessary for centering the lenses in the frame, as misalignment can distort vision even with the correct powers. Opticians verify these values and ensure the lenses are properly manufactured and aligned according to the specific OD and OS instructions.