A glasses prescription acts as a precise optical map that guides the creation of corrective lenses. This document contains standardized measurements necessary to grind lenses that focus light correctly onto the retina. The arrangement of letters and numbers is a universal language used by eye care professionals and lab technicians. Understanding these values helps a wearer comprehend the specific nature of their vision needs and the exact corrections being applied.
Decoding the Primary Refractive Values
The first step in reading a prescription involves identifying which eye is being referenced, a distinction made using Latin abbreviations. OD (Oculus Dexter) designates the measurement for the right eye, while OS (Oculus Sinister) refers to the left eye. Occasionally, OU (Oculus Uterque) may be used to indicate a value that applies to both eyes.
The fundamental measurement of lens power is found under the column labeled Sphere (SPH), which corrects for nearsightedness or farsightedness. This value is measured in diopters, the standard unit that quantifies the optical power of a lens. A negative sign indicates correction for nearsightedness (myopia), which requires a diverging lens to push the focus back onto the retina.
A positive sign in the SPH column signifies farsightedness (hyperopia), where a converging lens is used to bring the focal point forward. The higher the numerical value, regardless of the sign, the stronger the lens power needed for clear distance vision. These values are measured in increments of 0.25 diopters.
The Cylinder (CYL) value and the Axis work together to correct for astigmatism, a condition where the cornea or lens is shaped more like a football than a sphere. The CYL number indicates the amount of extra lens power required to compensate for this curvature. If this column is left blank, no astigmatism correction is necessary for that eye.
The Axis is a number between 1 and 180 degrees that specifies the orientation of the astigmatism on the eye. This value is only present when a CYL value is included, as it tells the lab where to place the cylindrical correction on the lens. The Axis ensures the corrective power is aligned precisely with the steepest or flattest curve of the eye.
Understanding the Near Vision Addition (ADD)
The Addition power (ADD) is a separate measurement that addresses presbyopia, the age-related difficulty in focusing on close objects. This condition occurs as the eye’s natural lens loses flexibility over time, usually after the age of 40. The ADD value provides the extra magnifying power needed for tasks like reading or working on a computer.
This power is measured in diopters and is always a positive number. The ADD power is supplemental, meaning it is added to the SPH power to determine the final strength for near vision. This value is the same for both the right and left eyes.
The ADD measurement is used exclusively when making multifocal lenses, such as bifocals, trifocals, or progressive lenses. In these lens types, the additional power is incorporated into the lower portion of the lens. This design allows the wearer to look through the top part of the lens for distance and the bottom part for near-vision tasks.
Essential Measurements: Pupillary Distance and Prism
Beyond the lens powers, a prescription also includes measurements that relate to how the lens must be positioned within the frame. The Pupillary Distance (PD) is the measurement in millimeters between the centers of the pupils. An accurate PD is important because it dictates the location of the optical center of the lens.
Proper alignment ensures the wearer is looking through the clearest part of the lens, preventing discomfort, strain, or distortion. The PD may be listed as a single number for both eyes, known as a binocular PD, or as two separate numbers, representing the distance from the center of the nose to each pupil, called a monocular PD.
Another measurement that appears less frequently is Prism, which is a correction used to manage double vision or binocular alignment issues. This value is expressed in prism diopters and works by shifting the image to compensate for a misalignment between the two eyes. The Prism correction is accompanied by a direction, such as Base Up (BU), Base Down (BD), Base In (BI), or Base Out (BO).