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4 Things to Know About Pupillary Distance (PD)

Author: Premier Eye Associates

The pupillary distance, or PD, is a measurement of the distance between the center of one pupil to the center of the other pupil. This measurement is commonly used when fitting eyeglasses to ensure that the glasses align with the eyes. In addition to maintaining alignment, the pupillary distance is used to reduce any unwanted effects of the prism in the glasses. The pupillary distance is also a guide to what size frames would be suitable for an individual based on the size of the head and PD. It is important to have the pupillary distance measured and accounted for before ordering any glasses.

 

How the Pupillary Distance is Measured

In an optical, glasses shop, or optometry office, the pupillary distance is measured using specialized equipment to make the reading as accurate as possible.

A pupillometer is a handheld device that allows the pupillary distance to be measured by resting the device against the forehead and aligning two view pieces to each pupil.

A pupillometer can measure the PD of each eye separately or combine them for the binocular pupillary distance.

Many offices will utilize instruments to measure the pupillary distance electronically. These instruments may have additional functions such as measuring the prescription, the curve of the front of the eye, or the quality of the tear film.

Another method of determining the pupillary distance is to use a pupil measuring stick to measure the pupillary distance by hand. This is the least precise method of measuring the pupillary distance.

How the Pupillary Distance is Used

When making glasses, the pupillary distance is used to determine where to center the lenses of the glasses.

All prescription glasses have an optical center point which provides the best clarity and the least unwanted distortion.

By using the pupillary distance, the optical center can be aligned with the pupils and line of sight.

Prism from Glasses

If the line of sight is not directly through the optical center of a lens, the prescription lens will induce prism power.

Prism is the term for when a lens or other refracting device bends light in a direction.

This prism power can be very bothersome if the prescription is high or the amount of deviation between the line of sight and the optical center is significant.

When there is prism power induced, the glasses will refract light and bend the light as well, making it seem like the world is shifted or misaligned.

An unneeded prism can cause headaches, eye strain, and double vision.

Using the PD to Create Prism

While in most cases any prism that is created from misalignment is unwanted and unnecessary, it is possible to use the optical properties of the lenses to help create a small amount of prism that is needed in the glasses.

If there are complaints of double vision or eye fatigue, sometimes prism can be prescribed into glasses to help with these symptoms.

The pupillary distance can be used to calculate how to adjust the lenses to induce the appropriate amount and direction of prism in these cases.

 

Dr. Anthony Spina and the staff of Premier Eye Associates specialize in glasses, soft contact lenses, hard contact lenses, and medical eye exams. Call our eye doctor in Auburn, AL today at (334) 539-5391 or schedule an appointment online if you are interested in the pupillary distance.  Our optometrist provides only the highest quality eye care services amongst eye doctors in the Auburn Alabama area.

 

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