What is Pupillary Distance? How to Measure it?

It is difficult for an optician to prescribe the right frames for you by merely looking at your eyes. Like many other procedures, there are some measurement that would assist the optician in recommending the type of glasses for you.

Buying a pair of glasses from any store is a problem if you do not know what a pupillary distance is or what it is used for. What then can be done? Find out how to measure what a pupillary distance is and why it is so important.

The Structure And Importance Of The Pupil

If you stare close into someone’s eye, you would see a black circle in the centre of their eye. This black spot is called the pupil. It is located in the centre of the iris, a muscle that permits light to enter the eye by opening and closing.

Pupils are centrally-located to direct light to the sight where images are formed called the retina. The pupil is just like the bull-eyes in the middle of a dart board.

What Is The Pupillary Distance?

Everyone’s eyes are set at some distance apart from one another. The distance between the centre of your two pupils is known as the pupillary distance (PD). This length, which is also called the interpupillary distance (IPD), is measured in millimetres. The measurement ensures that your prescribed frames are centred in front of your eyes when purchasing a new pair of glasses to avoid unwanted problems. A PD measurement is vital for people with high-power prescriptions.

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You will need your PD to buy a pair of glasses online to ensure they have been made correctly. If you have been to the optician, your PD wont be listed on your prescription. It is a professional measurement that ensure spectacles are made correctly.

How To Tell If Your PD Is Wrong?

PD measurements should not be assumed or guessed from the size of the head or the age of the person. Apart from the distorted information that the optometrist would use in prescribing your frames, a wrong pupillary distance measurement can also cause eye strain. Other symptoms of a wrongly prescribed frame caused by an incorrect PD measurement are blurred and double vision and migraines. An accurate pupillary distance measurement is particularly important when fitting progressive glasses.

Types Of PD Measurement

There are usually two types of PD measurement; Distant and Reading PD measurements. Distant PD measurement is used in bifocal glasses and varifocal glasses. It is also used in prescribing glasses used in seeing far and intermediate distances. As the name suggests, reading PD measurements are only used to prescribe glasses that would be used for reading.

How Your Pupillary Distance Is Measured

To get your PD, visit your optician. He or she would make use of a pupillary distance ruler. This type of ruler has a special gap in the middle, so it can sit comfortably on your nose while the professional takes the reading. Also, it is calibrated in millimetres. Some PD rulers are also adjustable to the positions of the eye. There are sections cut out in either case for you to look through.

A normal ruler cannot be placed on the eyes to determine this quantity as it will fall off. Moreover, a normal ruler would not give a precise and accurate reading that would be vital in prescribing frames. To get the zero point of the ruler to align perfectly with the centre of the pupil is very tasking and would lead to a wrong result of your pupillary distance.

The Steps To Take In Measuring Your Distant PD are;

• Sit upright beside your optician: Your optician would also be seated at an arm’s length from you. If you sit too far, it would be difficult to use the ruler. If your optician stands to take this reading, he or she may get an inaccurate reading due to parallax error.
• Your optician would hold the PD ruler between the thumb and the index finger. He or she would steady their hands by placing them on your head before the measurement is made.
• The optician taking the measurement would close his or her right eye and look into your right eye. It is essential that your eyes are aligned when the reading is being taken.
• Lastly, your optician will repeat the procedure with the other eye.

What Is A Dual PD For Glasses?

There are two types of pupillary distances: Binocular and monocular PD. You can have a single value or two values for your PD. A two-value PD is called a dual or binocular PD. It is popular among people who have irregular distances between the bridge of their noses and the centre of their pupil. Such PD values are written with a slash. For example, 31/33 is a double PD value, while 63 is a single PD value.

It is advisable to use monocular pupillary distances as this type of PD adjusts imbalances if the eye is not equally centred.

Dual PD’s does not induce eye strain, and so is used in people with lower prescriptions.

Does Your PD Matter When Getting A Pair Of Glasses?

Your PD is not just a number. Without this special number, a correct pair of glasses cannot be prescribed for you by any optician. If you use a pair of glasses that was not made specifically from your pupillary distance value, serious damage can be done to your eyes.

The pupillary distance is taken to make sure that the centre of the lens is aligned perfectly with the centre of the pupil. Without this measurement, precise vision cannot be achieved.

Does Your PD Have To Be Exact?

Your pupillary distance is to be exact for your pair of glasses to fit you perfectly and not cause eyestrains or pains. The PD used in constructing it must be exact. Any slight error in millimetres would not make the pair of glasses to be evenly positioned on the centre of the pupils.

What Other Ways Can You Use To Know Your PD?

Your PD can be taken from any of your old glasses. However, instead of measuring this distance, all you need to do is to send it to the manufacturer. From this old pair of glasses, they would be able to determine your PD.

Another way to get a far more accurate PD measurement than what you would have received if you used a pupillary ruler is by using a pupillometer. A corneal reflex pupillometer is a machine that is calibrated and designed specifically for measuring PD.

Why Not To Take Your Own PD

Your pupillary distance is not a value that you should take by yourself. Firstly, there is a high probability of getting a wrong pupillary distance: To measure your PD value by yourself, you would need to sit in front of a mirror. But all you see when taking the reading is only a reflection of the objects you are using. Errors due to parallax are very likely to occur when reading off the pupillary ruler.

Also, if the mirror is convex or concave, then it would give a larger ruler image, and the measured pupillary distance would be inconsistent. Furthermore, it is nearly impossible to read the pupillary ruler by yourself. Most of these types of rulers are opaque.

Does The Average Pupillary Distance Change With Age?

The change in PD is greatly affected by changes in the skull. As the body grows and develops, the skull and eye socket should also increase in size. This would lead to a slight increase in the PD. However, this increment is only noticeable in children. People who have reached the peak of physical development rarely experience any change in their PD with time.

So, always take a new PD measurement for children before getting them a new pair of glasses. Growth spurts would likely alter the measurement.

In Conclusion

Doctors are not obliged to give you your PD. Therefore, it is important to know what your pupillary distance is and how it is used in prescribing a pair of glasses for you. Your PD is essential in fitting a pair of glasses and should be performed by a professional optician or support staff.