A spectacle prescription, often called an Rx, is a formal document created by an eye care professional following a comprehensive eye examination. It specifies the lens power required to correct a patient’s refractive errors, such as myopia (nearsightedness), hyperopia (farsightedness), and astigmatism. These errors prevent light from focusing correctly on the retina. The prescription directs the optical laboratory, ensuring lenses are crafted with the exact strength needed for clear and comfortable vision.
Interpreting the Core Vision Correction Data
Understanding a spectacle prescription involves identifying which eye the measurements correspond to, using Latin abbreviations. OD (oculus dexter) refers to the right eye, and OS (oculus sinister) refers to the left eye. Occasionally, OU (oculus uterque) may be used if the correction needed is identical for both eyes.
The Sphere (SPH) value is the primary measurement indicating the lens power needed to correct nearsightedness or farsightedness, measured in diopters (D). A minus sign (-) signifies correction for nearsightedness. A plus sign (+) indicates correction for farsightedness.
Cylinder (CYL) indicates the additional lens power required to correct astigmatism, a condition where the cornea is shaped more like a football, causing blurred or distorted vision. If this column is blank, astigmatism correction is not required. The Axis number, ranging from 1 to 180 degrees, is only present with a CYL value, specifying the precise orientation where the correction must be applied.
Understanding Add Power and Specialty Measurements
Beyond the foundational distance correction values, a prescription may include ADD power, which stands for “Addition.” This measurement is the extra magnifying power needed for near vision tasks, typically due to presbyopia. ADD power is a positive number, usually ranging from +0.75 D to +3.50 D. It is added to the distance SPH value to determine the total power for the reading portion of a multifocal lens.
The ADD power is applied to the lower segment of multifocal lenses, such as bifocals or progressive lenses. This additional power compensates for the natural stiffening of the eye’s crystalline lens, which generally begins to affect near vision around age 40. The ADD power works in conjunction with SPH, CYL, and AXIS to ensure clear vision at an arm’s length or closer.
A specialized measurement is Prism, included to correct alignment issues between the eyes, such as strabismus or phoria, that can cause double vision. Prism does not correct refractive error; instead, it redirects light to ensure the two eyes perceive a single, fused image. The amount of prism is measured in prism diopters and is accompanied by a Base direction. The Base indicates the orientation of the prism’s thickest edge, using abbreviations like BI (Base In) or BU (Base Up).
Prescription Duration and Different Types of Vision Correction
A spectacle prescription has a finite lifespan, typically expiring after one or two years, depending on local regulations. This expiration ensures corrective lenses remain accurate and prompts patients to undergo routine eye health examinations. Regular checks monitor for potential changes in vision and detect developing eye diseases early.
A spectacle prescription is distinctly different from a contact lens prescription, and the two are not interchangeable. Spectacle lenses sit approximately 12 millimeters away from the eye, which significantly influences the final power needed for correction. Contact lenses rest directly on the eye’s surface, requiring a power adjustment.
A contact lens prescription requires specific measurements absent from a spectacle Rx, including the lens’s base curve and diameter. These measurements ensure the lens fits the curvature of the patient’s cornea correctly and remains comfortable. Federal regulations, such as the U.S. Federal Trade Commission’s Eyeglass Rule, require eye care providers to give patients their spectacle prescription immediately after the examination.