The soft contact lens is one of the most common forms of corrective lens. A prescription for soft contacts includes a particular brand, as not all styles and materials are the same, and a number of other measurements per eye such as the curvature and diameter of the lens. Some brands are only available in one curvature and size and these measurements might not be included in the prescription. The prescription also includes:
OD stands for oculus dexter, the Latin phrase for “right eye.” This abbreviation precedes the prescription information for the right eye, which is always listed first. If both the left and right eye has the same diagnosis and prescription information, the abbreviation “OU” is used instead, or “ocular uniter,” meaning both eyes.
OS stands for oculus sinister, the Latin phrase for “left eye.” This abbreviation precedes the prescription information for the left eye, which is always listed second. If both the left and right eye has the same diagnosis and prescription information, the abbreviation “OU” is used instead, or “ocular uniter,” meaning both eyes.
The spherical power, indicated on the prescription as power or sphere, of a soft contact lens is calculated in a unit of measurement equal to the reciprocal of the focal length of a lens called a diopter. The spherical power of a lens is indicated with a plus sign (+) for hyperopia and a minus sign (-) for myopia. A prescription that is -2.00 will allow light waves to converge at half a meter for someone with myopia while a +2.00 diopter lens will do the same for a person who has hyperopia.
Abbreviated “bc,” the base curve of a lens indicates the curvature on the inside of a lens. This measurement is noted in millimeters, usually between 8 and 10, and attempts to come as close to the curve of the eye as possible to ensure the greatest fit and comfort. If the cornea is more bulged or curved, this number will be lower.
Abbreviated “dia,” the diameter of a lens measures the width of a lens in millimeters, from end to end. The average size of a contact lens is between 13.5 and 15 millimeters.
The brand of a contact lens indicates the company (e.g. Acuvue) and make (e.g. Oasys) of a particular lens. Different lenses are used to treat different kinds of refractory errors and different lenses are more appropriate for different kinds of personal and physical conditions.
Although color is generally not used in the treatment of a refractory error, lenses do come in numerous colors or tints. Visibility tints don’t change the way a lens functions but are intended to help the wearer differentiate lenses from a storage case. Enhancers are tinted lenses designed to enhance or embellish an eye’s natural color. Opaque contacts are designed to mask the eye color of someone with dark eyes in a way that keeps the original iris color from showing through. Opaque contacts and enhancers are quite popular but are generally noticeable when compared to natural eye colors.
For individuals with presbyopia alone or in conjunction with another refractory error, bifocal or multifocal lenses are often needed. In this case, an “add” measurement is also used in a prescription. This measurement indicates the additional amount of power needed to ensure clear vision at a close range and is always preceded by a plus (+) sign.
For individuals with astigmatism, the cylinder power of a lens measures the degree to which a lens must account for astigmatism. The cylinder is an oval band on a lens that enables an individual to see all ranges clearly at once and is measured in diopters, like spherical power. The cylinder power of a lens is typically expressed with a minus (-) sign and is always written separately from the sphere power to avoid confusion.
The axis indicates the orientation of the astigmatism, a measurement that designates where the cylinder correction should be located. Generally, the cylinder runs up and down or side to side, at 90 degrees or 180 degrees. The symbol “x” normally precedes the axis.