If you wear glasses or contact lenses, you must have seen some numbers along with certain signs and common abbreviations written on your prescription for eyeglasses. Although they look very complex, they are not in reality. However, it does require you to get familiarized with certain basic optical and ophthalmic terminologies.
An example of an eye prescription:
Here is what the traditional abbreviations stand for
OD: - This is a Latin abbreviation of "Oculus Dexter" which means Right Eye.
OS: - This is a Latin abbreviation of "Oculus Sinister" which means Left Eye.
OU: - This is a Latin term for "Oculus Uterque" which means Both Eyes, but is rarely mentioned on a prescription and is usually used pertaining to visual acuity rather than power.
This is the term for Visual Acuity or Best Corrected Visual Acuity with the power mentioned in the prescription. In other words, it means this is the best vision that a person can achieve with the correct glass power.
This denotes the sphere power required to correct your vision. The higher the number, the poorer your eyesight without you wearing them. If it has a minus (-) sign, it means you have short-sightedness or myopia. Myopes can't see clearly for far and need distance vision correction or correction for nearsightedness.
If it has a plus (+) sign, it means you have long-sightedness or hypermetropia and yours is a farsighted prescription. The magnifying power of a regular magnifying glass comes from a plus lens.
Thus it is important to always mention a (+) or (-) sign in this part of the prescriptions for glasses.
This is the amount of astigmatism correction your eye has, i.e. the number that your eye has only in a certain plane due to curvature differences in the front part of your eye,i.e the cornea or in the lens inside the eye. In regular practice, the cylinder power is always prescribed in the minus (-) sign as it is the spectacle and contact lens industry standards. Very rarely you will see a plus cylinder prescribed by an eye doctor. If you do see a plus power it does not mean that your prescription is wrong. Some refer to this plus and minus power as farsighted astigmatism or nearsighted astigmatism. Similar to sphere power, the higher the number, the higher the degree of astigmatism in your eye.
This is the plane at which astigmatism lies, i.e the cylindrical power. If you have cylinder power, your prescription must always have an axis that lies between 1-180. The 90-degree meridian corresponds to the vertical meridian of the eye and the 180-degree meridian corresponds to the horizontal meridian. Rest are simply called oblique axis due to their orientation. The axis measurement is done by retinoscopy or auto refractometer. The cylinder axis and power are refined by a Jackson's Cross Cylinder or JCC.
If you are 40+ and you either wear bifocal or progressive glasses, your prescription will have this "Add" mentioned which basically means, the additional lens power required to read near objects. This is known as presbyopia. Generally, it ranges from +0.75 to +3.00 and usually is symmetrical between both eyes. Rarely due to any other eye problems that severely affect your vision, the correction of presbyopia is asymmetrical.
A very small percentage of eyeglass prescriptions will have prismatic correction mentioned in them. This is basically the prismatic power required to reduce the squint appearance, mitigate double vision or manage the ocular muscles'/eye alignment issues. The unit to specify prism correction in prism diopters. When prescribed, it usually contains the following abbreviations that denote the base position of the prism or the prism direction:
BD - Base down
BU - Base Up
BI - Base In
BO - Base Out
Eye power can be measured Objectively, i.e. without the patient responding, and Subjectively, where the patient's response is required. In objective measurement, your optometrist uses an instrument called "Retinoscope" which shines the light in the eye and based on the reflection from the retina, figures out the number of your eye. Another instrument is one where you see a picture of the house inside which gets blurry for a brief second and measures the power, the "Autorefractor".
The subjective measurement, on the other hand, is where, he asks you questions like, "Is the lens 1 clearer or lens 2?", and "Is it clear with the lens or without?". This subjective measurement is based on the objective measurement which gives the practitioner a starting point for figuring out the accurate power of your eyes. There can be a marginal difference between the subjective and objective measurements which usually occurs because of the working of the eye muscles.
To measure the "Add", there are several textbook-defined techniques, however, Dr. Irvin Borish, who is the father of modern optometry quotes a famous study done by Hofstetter way back in 1944 which suggests that patients' accepted Add (subjective measurement) based on their age is the best method to identify the ideal Add power.
If your glasses prescription has power ≤ ±4.00 Diopters, the same prescription of spectacles can be used as your contact lens prescription. However, beyond ±4.00 Diopters, there are some adjustments, known as "Effective power calculations" that are done to compensate for the distance between your eyes and spectacle to get an appropriate contact lens power.
In minus power, you will see the contact lens power reduces a bit and conversely, increases in case of plus power. This does not mean that your power has changed, but there has been an adjustment made in the power to maintain the clarity you see with your spectacle lens.
Thus your prescription for contact lenses is not always the same as your prescriptions for eyeglasses.
Single Vision Lenses have only one power incorporated in them, either for the distance or for the near, depending on your refractive error. These eyeglasses prescriptions may or may not have prisms mentioned, but definitely will not have any "Add" mentioned in them.
As the name suggests, bifocal eyeglass lenses have two foci, the upper portion contains your distance prescription and the bottom portion which is generally more towards your nose contains the near prescription and has the "Add" power incorporated in them. Similarly, the progressive prescription or multifocal lenses have no special demarcation to identify as distance portion and near portion but have a gradual change in power as we move down the lens to provide clear vision throughout for distance, intermediate (computer), and near distances. This progression depends on the "Add" power and the type of design of progressive lenses. Thus bifocal prescriptions, as well as progressive prescriptions, look the same and the only difference is the lens that is made using those prescriptions.
This abbreviation extends to Interpupillary/pupillary Distance which is the distance between the centre of the pupil of both eyes. This is important as the centre of the lens must lie exactly in front of the centre of the pupil so that the power in the lens appropriately corrects the refractive error. Furthermore, this measurement is also important when deciding on certain types of Vision Therapies, especially in children.