Here are some contact lens prescription basics that can come handy:
OD/OS: These are the terms used for right eye and left eye. They are derived from Latin words- Oculus dexter (OD) for “right eye” and oculus sinister (OS) for the “left eye”. They indicate the strength of lens that is prescribed for each eye.
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OU: This is an abbreviation for another Latin term called Oculus uterque, which means “both eyes”. So, if both of your eyes need lenses of the same strength, your doctor may use the term OU instead of OD and OS.
Power (PWR), Sphere (SPH): this number indicates the strength of correction which is needed for that particular eye. If there is a (+) sign before the number, it means hyperopia or far-sightedness. a (-) sign indicates short-sightedness or myopia. If your eye does not need a correction, then you may see the sign PL, or plano, which indicates that the strength required is 0.
The power of the lens is measured in diopters (D), and for this reason lens manufacturers create contact lenses in 0.25 D increments (such as e.g. -4.00 D, -4.25 D, -4.50 D, -4.75 D). The farther away your number is from 0, the stronger is your power prescription.
Base Curve or BC: this is a number which indicates how curved the inside of your lens is. This number usually ranges from 8-10 and it ensures that the lens sits well against the curvature of your eye, thereby making it comfortable. A lower base curve number indicates a steeper curve, and a higher base curve number indicates means a flatter curve.
Diameter or DIA: this number denotes the width of the contact lens from one edge to the other. It is denoted in millimetres and this number ensures that your lens will cover your eye properly.
Brand: your doctor may prescribe you a specific type/brand of contact lens. Since different brands use different materials and may have specific unique features, your doctor will prescribe a type that is best suited for your eyes.
Cylinder or CYL: for those people who have astigmatism, one more value is added to the contact lens prescription for cylindrical lens. It indicates the strength that the lens needs to correct for the astigmatism.
Axis or X: the axis or X is used for astigmatism and it is a value that indicates where in the lens the astigmatism correction should be located.
Add Power or ADD: for people who need contact lenses that have bifocals in them, they will require an additional measurement known as Add power or ADD. This indicates the strength of power that the lens needs to obtain clear vision at close range.