If you have wondered what all the numbers and letters mean on your contact lens prescription, you're not alone. But we're here to make sense of the seemingly incomprehensible scribbles of your ophthalmologist. There are standard abbreviations that are used in optical prescriptions, and although you don't need to understand all the nuances of each, it is helpful to get a broad grasp of them, especially when you're buying your lenses from an outlet other than the eye care provider who conducted your exam.
To read your contact prescription, you’ll need to identify and reference your eye, power or sphere, base curve, diameter, and brand. A basic contact prescription includes these indicators, usually in a table format. Each of these markers helps the optometrist know key information about your eyes and the correction needed to fix your vision. People with astigmatism may require toric lenses, requiring the prescription to include the cylinder (CYL) and the axis (AXIS) which gives additional markers needed to correct the common condition. Other parts of the table that can be included are Add Power (ADD), used specifically in bi-focal, or multi-focal contact lenses, and the color (COLOR) of the contact lens.
Each of your eyes may have different challenges to correct. You may have a different prescription for each eye, or even need a Toric or Multi-focal lens in one eye and not the other, or require a different brand for each eye. So let's dive into the details of this example prescription. There may be a few more numbers on the example provided here than what you see on your own prescription. See the sample prescription below for an example of what a contact lens prescription looks like:
It's also worth noting that your eyeglass prescription and your contact lens prescription are not the same. A contact lens prescription provides additional information about the size of the lens (not just information about the power.) And the power will also be different because, while a contact lens sits directly on the surface of the eye, eyeglasses sit about 12mm in front of it. So the eyeglass prescription must account for that distance as well.
Shopping around online is an easy way to save money on your contact lenses. However, the law requires a valid prescription to purchase contact lenses. If you're in the United States, you have a right to a copy of your prescription.
OD and OS: Along the top, or side, of any prescription you will find the letters OD and OS. Wouldn't it be easier if they just wrote L and R for Left and Right? Well, OD is short for the Latin term for Right Eye: "oculus dexter", and OS comes from the Latin for Left eye "oculus sinister." So just remember OD is your RIGHT eye and OS is your LEFT eye. Occasionally you may see the letters OU on your prescription, which is short for "oculus uterque" or EACH EYE, meaning the same measurement applies to both.
PWR: PWR refers to refractive power. Sometimes this may also be called SPH (sphere.) This is the amount of correction, measured in diopters (an optical unit of measurement,) that is needed to bring your vision to 20/20 - or as close as possible. If this number is preceded by a minus sign, then you are nearsighted (you have myopia.) If there is a plus sign before the number, you are farsighted (you have hyperopia.) The further the number is from zero, the stronger your prescription. It is common to have a different prescription in each eye.
BC: Base Curve refers to the back curvature of the contact lens, which needs to fit seamlessly against the surface of your eye. The optometrist uses this number to ensure the size of your contacts perfectly integrates with the size of your cornea. The base curve is usually measured in millimeters between 8 and 10. If your prescription does not include a base curve, it's most likely because the brand of your lenses only comes in one base curve.
DIA: Diameter is the distance across the surface of the contact lens, measured in millimeters. This number is usually between 13 and 15 and determines where on your eye the contact lens will sit. If this measurement isn't correct the contact lens will be uncomfortable and may scratch your eye.
A diopter is a unit of measurement used to determine the correction, or focusing power, your eye requires to see clearly. An optician will measure your eye in diopters and use that measurement to determine the proper strength of your lenses. While important for the professional who takes your prescription to know, the diopter count is more of a technical measurement than an important part of the Rx for you to keep in mind.
CYL: Usually a number between -4 and +4, Cylinder value is the amount of power needed to correct your astigmatism.
Axis: Expresses the orientation of the cylinder value on the lens. The Axis number is displayed in degrees, between 0 and 180°.
ADD: Add Power is the added magnifying power in portions of the lens to help with seeing up close (in diopters).
Your contact lens prescription may also include a brand name that carries the lenses your eye care provider felt would be the best fit. If you're choosing to cosmetically enhance your eye color with your lenses, that information will also be on the prescription. In some cases, the prescription also includes how often to replace the lenses (whether they are daily, weekly, monthly etc.).
Most contact lens prescriptions come with an expiration date between one and two years from the date of your last exam. Once that prescription expires, it's time to head back to the eye doctor for a check-up and to make sure nothing has changed. Our eyesight changes as we get older, so regular check-ins with your eye doctor allow you to protect the health of your eyes with the most up-to-date prescription.
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