The measurement units for eyeglasses that your ophthalmologist or optometrist writes on your prescription provide the information required by the laboratory to process your lenses. The numbers show the spherical, cylindrical and axis measurements, which together determine the power and shape of your eyeglass lenses. The need for vision correction occurs as the result of refractive error, which bends light as it enters your eyes, causing it to focus in front of the retina or behind it resulting in blurred vision.
Diopters
The spherical measurement on your eyeglass prescription indicates the extent of your nearsightedness or farsightedness. Diopters are the units of measurement used to express the amount of spherical correction needed. These numbers appear as a plus or minus figure on your prescription and determine the power and curvature of the eyeglass lens needed to correct your refractive error. The curvature of the lens adjusts the way light enters your eyes so that it properly focuses on the retina, according to CNIB, a nonprofit Canadian organization for the blind and partially sighted. A spherical measurement for nearsightedness that reads -1.00 diopters indicates you are able to see clearly up to 1 meter, about 39 inches, but objects appearing farther away begin to blur. A measurement of -2.00 diopters indicates you have clear vision up to a 1/2 meter.
A + sign in front of the number indicates the amount of correction needed for a refractive error that occurs with farsightedness. For example, a +1.00 diopter means you see clearly at one meter, but objects viewed at closer distances appear blurred. A higher diopter measurement for farsightedness indicates a shorter distance of clear vision.
Lens Measurements
The front and back curves of your eyeglass lenses are individually measured in diopters. When combined, the totals from both sides of each lens must equal the spherical measurements specified on your prescription for the respective lenses.
Measurements for Astigmatism
Cylindrical measurements, also written in diopters, measure the extent of astigmatism. Astigmatism, caused by an irregularly shaped cornea, scatters light as it enters your eye, making images appear distorted. This distortion does not affect everything within your range of vision, but appears only when looking in a particular direction, according to CNIB. The spherical and cylindrical measurements added together give the total amount of correction needed.
The axis indicates the location of your astigmatism and is measured in degrees from 0 to 180 along an invisible line running through the center of your pupils. The axis measurement guides the laboratory technician processing your eyeglasses to the exact area requiring correction, while the cylindrical amount informs the technician about the size of the area.
Pupillary Distance (PD)
Pupillary distance, measured in millimeters, represents the distance between your right and left pupils. Because the pupils converge for close-up viewing and are farther apart when viewing objects in the distance, prescriptions for bifocals require two measurements for pupillary distance.