Merchant Processing Services

Showing the Power

Let’s start with the basics. A prescription for eyeglasses may have some or all of the following: a sphere power, a cylinder power, an axis, a prism with a base direction, and an add power. The following is an example of a common prescription:

Eyeglass prescriptions are written in a standardized format in order to minimize confusion and misinterpretation. The OD, or right eye, is always specified before the OS, or left eye. The sphere, cylinder and axis are always written in that order, and the prism must have a base direction accompanying it.

The Sphere

The sphere primarily represents myopia or hyperopia. Myopia, also known as nearsightedness, is the most common spherical correction and is noted with a minus symbol. Myopia occurs when light rays come into focus in front of the retina. To correct this, a concave surface is ground into a lens, causing the light to diverge and come to rest on the retina. Hyperopia is noted by the plus sign and is known as farsightedness. Hyperopia is when the light rays come into focus behind the retina and is corrected by placing a convex lens in front of the eye. The convex lens converges light, allowing it to come into focus on the retina. In some cases, the abbreviation pl or plano may be noted by the refractionist. This means that there is no spherical correction in the prescription.

When there is no cylinder or axis present on the prescription, the prescription requires a spherical lens. A spherical lens has the same curvature throughout the lens. 

The Cylinder and Axis

The cylinder represents the amount of astigmatism present in the eye. The primary cause of astigmatism is an oval shaped cornea. A normal shaped cornea is spherical, like a baseball. An astigmatic cornea however, is shaped more oblong like a football. An eye with astigmatism has two curves, one steep and one flat, that are typically 90 degrees apart. These two curves represent the strongest and weakest powers of the eye and cause the light to split and come into focus on two separate points in the eye. Astigmatism can be classified based on how the light comes into focus in relation to the cornea.

• compound myopic astigmatism - occurs when both point of light come into focus in front of the retina, ex. -3.00 -2.00 X 45

• compound hyperopic astigmatism – occurs when both points of light come into focus behind the retina, ex. +2.00 +2.00 X 90

• simple myopic astigmatism- occurs when one ray comes into focus in front of the retina and one ray comes into focus on the retina, ex. Pl -1.00 X 135

• simple hyperopic astigmatism – occurs when one ray comes into focus behind the retina and the other ray comes into focus on the retina, ex. Pl +2..00 X 90

• mixed astigmatism- occurs when one ray comes into focus in front of the cornea and the other ray comes into focus behind the retina, ex. +1.00 -2.50 X 180 

The cylinder can also be noted in plus or minus form, depending on who performed the refraction. An optometrist usually writes the prescription in minus form and an ophthalmologist writes the prescription in plus form. Both forms are acceptable and mean the same thing; however, since lenses are ground in minus cylinder form, it is easier to transpose a plus cylinder prescription to a minus cylinder prescription.

The axis also helps in classifying astigmatism. The axis is the orientation of the cylinder in degrees. It gives the direction of the primary meridians, or curves, of the cornea. These meridians are typically 90 degrees apart. The axis can let the ECP know the direction of the steepest and flattest meridians and helps to classify the astigmatism according to those locations. These classifications are:

• with-the-rule astigmatism : axis is between 0 and 30 or 150 and 180 degrees

• against-the-rule astigmatism : axis is between 60 and 120 degrees

• oblique astigmatism : axis is between 30 and 60 or 120 and 150 degrees

Although astigmatism is commonly caused by a corneal imperfection, it can also be caused by a crystalline lens imperfection (lenticular astigmatism) or from scarring (irregular astigmatism). Irregular astigmatism – which is astigmatism with the principle meridians in areas other than 90 degrees apart - is impossible to correct with glasses. This is because the only way to use lenses to correct for astigmatism is by grinding a spherocylinder lens. A spherocylinder lens is usually made by grinding a spherical curve on the front of the spectacle lens and a toric or cylindrical curve on the back of the lens. The toric curve creates two principle meridians on the lens 90 degrees apart.

The Add Power

The add power is the power that needs to be “added” to the other part of the prescription to allow the patient to see clearly at near. Although it can be seen in prescriptions for younger individuals to help with focusing problems, it is more commonly seen in individuals known as presbyopes. Presbyopes are individuals, usually over the age of forty, who have lost the ability to accommodate. Accommodation is the process in which the crystalline lens changes its shape to allow the eye to easily change focus from far to near and back again.
There are two main options to provide the patient with the needed near power. The first way is to prepare reading only glasses. Using the example above, the prescription for reading only glasses would be:

The other option is to use a multifocal. There are many multifocal types and designs available to the ECP, but the main types are flat top bifocals, flat top trifocals, and progressive lenses. Multifocals allow the patient to wear one pair of glasses to meet most of their distance and near visual needs. 

Prism

Prism and base are very rarely seen on a prescription. When one is prescribed, it implies a muscle abnormality that results in double or blurred vision due to a poor eye alignment. Prism refers to a displacement of the image through the lens and the base refers to the direction of displacement. 

Common Abbreviations

• SPH - Sphere power

• DS - Diopters sphere; sphere only

• CYL - cylinder power

• X - axis; X 45 means axis 045

• PL - plano; no power

• O.D. - Oculus dexter; right eye 

• O.S. - Oculus sinister; left eye

• O.U. - Oculi uterque; both eyes

• NV/NVO - near-vision or near-vision-only 

• P.D./PD - pupillary distance; the distance in-between the right and left pupil in millimeters

Beyond the Numbers

There is much debate as to what else needs to be on an eyeglass prescription: Does it need the PD, is a two year expiration date a federal requirement, who can fill a prescription, etc. The answers vary from state to state. It is up to the ECP to check with the state regulatory board to determine what is legally required to be on a prescription. There are some things however, that are usual and customary throughout all the states. These are:

• The doctor’s information such as practice name, license number, address, and contact information

• The doctor’s signature

• The exam date and an expiration date. The expiration date can vary from one to two years unless there is a medical reason that requires more frequent examinations

• Recommendations for lens materials and treatments

The eye care professional’s primary responsibility is to help the patient see better by accurately filling his or her prescription. In order to fulfill this duty, it is important that the ECP learn to analyze the prescription correctly and be able to explain to the patient what the prescription is intended to correct. By doing so, not only does the ECP satisfy the needs of the patient, but he or she also sets themselves apart from the order takers.

Carrie Wilson
BS, LDO, ABOAC, NCLEC