When receiving a contact or eyeglass prescription, the document often contains unfamiliar abbreviations and numbers. Deciphering this technical medical language is necessary to ensure the correct corrective lenses are purchased. Understanding these shorthand terms allows patients to verify their prescription and engage more effectively in their vision care.
Decoding the Latin: OD, OS, and OU
The first step in reading any eye prescription is understanding the traditional Latin abbreviations used to designate the eyes. OD stands for Oculus Dexter, the medical term for the right eye. OS is the abbreviation for Oculus Sinister, which refers to the left eye. These terms are used universally in eye care to ensure clarity regarding which eye a specific measurement applies to.
A less common abbreviation is OU, which stands for Oculus Uterque (both eyes). This term is used when the vision correction needed is the same for both eyes. Although some modern prescriptions use the simpler English abbreviations RE (Right Eye) and LE (Left Eye), the Latin OD and OS remain the standard for organizing the data. They function as organizational headers, mapping the numerical values to the specific eye they correct.
Interpreting Your Prescription Numbers
The numbers following the OD and OS designations represent the precise measurements for lens power. The most common term is Sphere (SPH), which indicates the main lens power required to correct nearsightedness or farsightedness. This power is measured in units called diopters (D). A minus sign (-) next to the SPH value signifies nearsightedness, while a plus sign (+) or no sign indicates farsightedness.
The next field is Cylinder (CYL), which represents the lens power needed to correct for astigmatism. Astigmatism occurs when the cornea is shaped more like a football, causing blurred vision. If this column is blank, no measurable astigmatism correction is required. If a CYL value is present, it is followed by the Axis measurement, a number between 1 and 180 degrees. The Axis specifies the orientation or angle at which the cylindrical power must be applied to align with the eye’s curvature.
Why Contact and Eyeglass Prescriptions Differ
A prescription for eyeglasses is not the same as a prescription for contact lenses, even if the OD and OS values appear similar. The primary reason for this difference is “vertex distance.” Vertex distance is the physical space between the back surface of the eyeglass lens and the front surface of the cornea.
Eyeglasses typically sit 12 to 14 millimeters away from the eye, which affects the effective power of the lens by shifting the focal point. Conversely, a contact lens rests directly on the cornea, making the vertex distance essentially zero. This change in distance requires a mathematical adjustment to the lens power to ensure light focuses identically on the retina.
For higher prescriptions, typically those exceeding plus or minus four diopters, the vertex distance difference necessitates a change in diopter strength. For instance, a nearsighted patient with a strong minus prescription requires a slightly weaker power in their contact lenses than in their glasses for the same visual clarity. This calculation is performed by the eye care professional to ensure the contact lens provides accurate, corrected vision.