Optical Prescriptions RX
How to read your Rx
A prescription for contact lenses is different to a prescription for spectacles. The contact lens prescription contains additional information, not required in the spectacle prescription, like Brand of Contact Lenses, Base Curve and Diameter.
A prescription is broken down into RIGHT eye (OD) and LEFT eye (OS).
OD stands of “oculus dexter” which is latin for right eye. This abbreviation precedes the Rx information for the right eye, which is always listed first. When you have the same prescription is both eyes you may see the abbreviation OU “ocular uniter” meaning “both eyes”
OS stands for “oculus sinister” which is latin for “left eye”. This abbreviation precedes the Rx information for the left eye, which is always listed second.
Base Curve (BC) – used only in contact lens prescriptions
The base curve of a lens indicates the curvature on the inside of a lens. Measured in millimetres between the numbers 8 and 10, as close as possible to the curve of the eyeball so the contact lens fits the eye nicely and is comfortable. The more the cornea is curved the lower this number will be.
Diameter- (DIA) – used only in contact lens prescriptions
The diameter is the measurement of the width of the lens. The average size of a contact lens is between 13.5 and 15 millimetres from end to end.
Sphere (SPH) – used in both spectacle and contact lens prescriptions
Spherical Power (which may be written as either sphere or power) is calculated as a unit of measurement equal to the reciprocal of the focal length of a lens called a diopter. This is indicated with a plus sign (+) for hypermetropia and a minus sign (-) for myopia.
Brand – used only in contact lens prescriptions
The brand of a contact lens indicates the company (e.g. Acuvue) and make (e.g. Oasys) of a particular lens. Different lenses are prescribed depending on refraction, physical and personal conditions.
Colour – used only in contact lens prescriptions
Coloured lenses are used for cosmetic eye colour change and are available in a wide range of colours and tints.
ADD– used in both spectacle and contact lens prescriptions
Add is used when you require additional correction to ensure clear vision at close range and it is always preceded by a plus (+) sign.
Cylinder (CYL)– used in both spectacle and contact lens prescriptions
This measurement is used when there is an uneven curvature of the eye (astigmatism) measured in diopters. The cyl is an oval band on a lens to enables an individual to see all ranges clearly at once. The cylinder power of a lens is written with a minus (-) sign and is separate to the power/sphere.
Axis– used in both spectacle and contact lens prescriptions
The axis indicates the orientation of the astigmatism (uneven curvature of the eye). This measurement designates the degree where the cyl correction must be. This is a number between 5 and 180, often preceded by an “X” on the prescription.
What do the numbers mean?
All prescriptions are written in dioptres in 0.25 steps. The diopter is a metric unit, and is related to meters and centimeters.
Myopic prescriptions are written preceded by a minus sign. e.g. -3.25. A person who is short sighted by -1.00 can see clearly up to one meter, whereas a person who is short sighted by -2.00 can only see clearly up to 50 centimeters.
Hypermetropic prescriptions are written preceded by a plus sign. e.g. +3.25, or +3.75.
Astigmatic prescriptions are written as a combination of a sphere and a cylinder. e.g. A person who is short sighted by -3.25, with -1.75 of astigmatism, would have a prescription of -3.25/-1.75. Astigmatism also has an axis component, which indicates the orientation of the cylinder, and is always from 0 to 180 degrees. e.g. -3.25/-1.75X150.
Presbyopic prescriptions are usually written as an add onto the distance prescription, and is always preceded by a plus sign. e.g. +2.00. A prescription for reading glasses can however be written simply as a plus prescription.
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