An eye prescription slip represents a standardized medical document containing the specific measurements needed to correct a person’s vision. To an untrained eye, the abbreviations and numbers often resemble a complex code. These forms must be precise, as even tiny variations in measurement can impact visual clarity and comfort. Understanding the fundamental terminology on the prescription is the first step toward knowing what your corrective lenses will do.
Designating the Right and Left Eye
While some modern prescription forms simplify the labels, using RT for Right and LT for Left, the standardized abbreviations in optometry and ophthalmology have a traditional Latin origin. The term OD, which stands for Oculus Dexter, is universally used to denote the measurement values for the right eye. Similarly, the abbreviation OS, or Oculus Sinister, is reserved for the measurements specific to the left eye. Occasionally, you may encounter the abbreviation OU, which is short for Oculus Uterque, meaning both eyes. This term is typically used when a specific measurement, such as a reading addition, is identical for both eyes.
Decoding the Main Correction Values
The core of any distance vision prescription lies in three interlinked measurements: Sphere, Cylinder, and Axis. These values determine the primary correction needed for clear distance vision.
Sphere (SPH)
SPH, or Sphere, indicates the primary lens power, measured in diopters, that is needed to correct nearsightedness or farsightedness. A minus sign (-) signifies myopia, or nearsightedness. Conversely, a plus sign (+) indicates hyperopia, or farsightedness. The value under the SPH column is the same across all meridians of the lens, meaning the correction is uniform and spherical in shape. If this column contains the word “Plano” or “PL,” it means no spherical power is needed for distance vision correction.
Cylinder (CYL)
The CYL, or Cylinder, value is included only if a person has astigmatism, a common condition where the cornea is shaped more like a football than a perfect sphere. This measurement specifies the extra power needed to correct the difference in curvature between the two principal meridians of the eye. If this column is blank, it means no significant astigmatism is present or requires correction.
Axis (AXIS)
The third value, AXIS, defines the precise orientation of the cylinder correction, always listed as a number between 1 and 180 degrees. This number does not indicate the power of the correction but rather the angle at which the corrective cylindrical power must be placed on the lens. This angular measure is essential for accurately positioning the lens to neutralize the uneven focusing caused by astigmatism.
Understanding Near Vision and Alignment Measures
Beyond the primary distance correction, an eye prescription may include supplemental measurements for specific visual needs, such as the ADD, or Addition, power. This value represents the extra magnifying power, always a plus number, required for reading and other close-up tasks. The ADD power is needed to correct presbyopia, the natural loss of near focusing ability that occurs with age. The ADD value is applied to the lower portion of multifocal lenses, such as bifocals or progressives, and is typically the same for both eyes.
A separate measurement, Prism, is included for a small percentage of prescriptions to correct eye alignment issues that may cause double vision, known as diplopia. Prism correction is measured in prism diopters and is accompanied by a base direction, such as BU (Base Up), BD (Base Down), BI (Base In), or BO (Base Out). The base indicates the orientation of the prism’s thickest edge.
Finally, PD, or Pupillary Distance, measures the distance in millimeters between the centers of the two pupils. This mechanical measurement is crucial for the optical lab to align the center of each corrective lens precisely in front of the corresponding pupil for comfortable and clear vision.