With the advent of online eyeglass sales, there’s a lot of buzz on the internet about the “PD” measurement spectacle manufacturers require in order to make glasses remotely without seeing the patient.
Just what is this measurement? How is it obtained? Is it really that important?
Let’s take some of the mystery out of all this. And like most things involving precise optics, the devil is in the details.
The “PD” measurement is an abbreviation of the term “pupillary distance”. Over simplified, it’s the distance that your eyes are set apart measured in millimeters. Contrary to popular belief, it’s not just one number like your shoe size. Measuring it precisely can make a huge difference in your vision depending on your prescription. The higher your prescription, the more critical the PD measurement has to be. This explains how one person ordering glasses online got lucky using a “close-enough PD” and another got nothing but frustration.
When you look at something far away (further than 20 feet, or what is referred to in optics as “optical infinity”) Your eyes will have a fixed distance apart. This distance is used by an optician to cut lenses into your frame so that both eyes are looking through the “optical centers” of the lenses. Because your eyes are not moving with the lenses (as is the case with contact lenses) the positioning of the lenses in the frame is critical to reducing distortions.
“So what the big deal? Can’t I just pull out a ruler and measure this distance?”
Well the thing is, the distance varies depending on where you are looking. As you might imagine, your eyes pull in when you are looking at something up close. And of course the closer an object is to you, the more your eyes “converge”
So now we have TWO measurements. Your “distance PD” and your “near PD”. The near PD is dependent on the “working distance” that your glasses were designed for. The “near PD” measurement for a pair of glasses designed for computer work 3 feet away will be different than one for near work designed for 14 inches.
But wait: It gets even more complicated….
What an optician REALLY needs to make your gasses accurately is a thing called your “monocular PD”
Specifically: this is the distance between each individual eye and the center of the bridge of your nose.
Believe it or not, it’s pretty rare for both eyes to be equidistant with the center of the bridge of your nose. Mother nature is often not symmetrical.
In order for the optician to cut a lens precisely so that the center of each of the lenses in the frame are in the right place, he/she has to enter into the lens edger these monocular PD numbers. So now we don’t have one measurement…we have 4.
“Ok. So now I have distance and near monocular PDs? Am I done?”
Well…you’re getting more accurate…but unfortunately we aren’t done. Here’s where the “art” aspect of optometry comes in.
It turns out that the muscles that turn your eyes in and out (the lateral and medial rectus muscles for you anatomy people) have a certain “tonus” that varies to a degree from patient to patient (even with the exact same monocular distance and near PDs). This “tonus factor” will also effect the PD measurement. Quite simply, everyone’s eyes have a tendency to more or less drift from fixation. This “drift” sometimes has to be compensated for. Optometrists and Ophthalmologists call this “drift” from fixation “exophoria” and “esophoria”. When an eye permanently turns in or out without both eyes maintaining a fixation, this is called a “tropia” (as in exotropia and esotropia). You can see this drift yourself using a mirror. Simply look at yourself in the mirror and cover one eye. After a few seconds remove your hand. You can watch the covered eye move back to the fixed position from its resting state as it attempt “fusion” This phenomenon will often occur even when the eyes are looking at something at optical infinity. Look at something far away and cover your eyes alternately and you will see the object “jump” because both eyes are not precisely aligned absent a fixation stimulus.
I’m getting a little complicated here…but suffice it to say that the tonus of your eyes sometimes has to be addressed by compensating the PD measurements in the order given to the optician. It’s not an exact science. A patient’s phoria can vary from year to year due to changes in working demand and health issues. A good optometrist often compensates the PD based on ocular motility data.
Finally, I should mention that a thing called prism can complicate the PD measurement. These are measurements given to the optician to permanently compensate for motility problems a patient might have. These motility problems can be congenital (such as a child born with a crossed eye) or can can occur later in life due to vascular or neural episodes (such as diabetes, hypertension, cholesterol, neural disorders etc…) These instructions are often added to a prescription along with the PD measurement. Suffice it to say that the prism number and the PD number are dependent on one another.
Another factor that occasionally comes into play when determining PD involves making consideration for what the PD was in the old glasses the patient walked in with. Simply put: a patient will present at my office wearing glasses that were made using a pupillary distance that was determined incorrectly, or cut in error by the optician. After a few months of difficulty, the patient’s eyes adapted to this wrong measurement. The doctor now is forced with the task of manipulating the PD numbers to compensate for the fact that the patient’s tonus was affected by this prescription made in error, usually by leaving some of the error in place when making to new glasses to wean the person away from the incorrect measurement.
What’s the best way to measure for the PD?
Forget about using a ruler. The best way is to measure with a “pupilometer” This is an electronic device that uses a reflection off the cornea to make a precise measurement independent of parallax. It rests on the patient’s nose, measures monocular and binocular PDs, and can compensate for different working distances.
If the patient has a mild prescription, he/she might get by with a PD measurement that’s a little off. But as a patient’s prescription gets larger, these errors become more significant. It’s one of the many reasons one person has little difficulty having mass produced eyeglasses work for them, while another doesn’t.
Determining the PD measurements (along with other measurements like the seg height, effective diameter, vertex distance etc…) is usually done by the optician in the course of making glasses for a patient. As you can see, there’s a lot of training and consideration that goes into these measurements, and it’s a bit more involved that measuring someone’s shoe size. While an optician may make these measurements in the course of making a pair of glasses for someone, to ask him to make these measurements for a third party lab to sell the patient eyeglasses amounts to asking him to do highly trained task free of charge. These measurements require a face to face visit with an eye care professional. Online labs can not make these measurements accurately and as such will sometimes ask patients to figure these issues out on their own. Things might work out… or not, and any liability is lost when dealing with online labs.
To assure yourself the most precise optics, have your eyeglasses and prescription determined and manufactured by a licensed eye care professional.
Jason Eugenides O.D.