Bally

What is Index

In lenses, the "index" refers to the refractive index, a number indicating how efficiently a lens material bends light, with higher numbers resulting in thinner and lighter lenses for the same prescription strength. For example, a 1.50 standard index is suitable for weak prescriptions, while higher indices like 1.67 or 1.74 are used for stronger prescriptions to create thinner, more aesthetically pleasing, and lighter lenses.

What is Prism

Unless your prescription shows that you have Prism, there is no need to select it.

Prism correction is used in eyeglasses for some people with diplopia or double vision. This is when someone sees two separate images of one object. The prism helps align the two images so that only one image is seen.

Cylinder and Axis

Some prescriptions do not include cylinder and axis values; if your prescription shows one number for each eye, only make a selection from the sphere lists.

What if my P.D. is written double such as 66/63?

The first number is the P.D. number we require to make your lenses. The second number for example (63) is the near pupil distance, which is for readers and near vision. This number is only needed when reading glasses are required.

Pupillary Distance (PD)

Is the distance (usually in millimeters) between the centers of the pupils in each eye. This measurement is used when making prescription eyeglasses. Positioning lenses correctly in relation to the pupil centers is especially important for higher-powered lenses.

What is an average measurement?

The average adult’s PD is between 54-74 mm; kids' are between 43-58 mm.

OD: OD

stands for oculus dexter, the Latin phrase for “right eye”.

OS: OS

stands for oculus sinister, the Latin phrase for “left eye”.

If both the left and right eyes have the same diagnosis and prescription information, the abbreviation “OU” is used instead, or “ocular uniter,” meaning both eyes.

Sphere (SPH):

This indicates the amount of lens power, measured in diopters (D), prescribed to correct myopia (nearsightedness) or hyperopia (farsightedness). The spherical power of a lens is indicated with a plus sign (+) for hyperopia and a minus sign (-) for myopia.

Cylinder (CYL):

This indicates the amount of lens power for astigmatism. If no cylinder power is noted, either you’ve no astigmatism or your astigmatism is so slight that it is not necessary to correct it with your eyeglasses or lenses.

Axis:

This describes the degree and direction of your astigmatism. The axis is a figure which determines the angle of the correction needed to see clearly, measured in degrees from 1 to 180.