The pupillary or interpupillary distance (generally abbreviated to "DP") is the one that separates the two pupils and is expressed in millimeters. Eye doctors detect it to make sure the lenses are well centered when filling out the prescription for glasses. The average value is 62 mm, although all distances within the range of 54-74 mm are considered normal. You can detect it at home, alone or with the help of a friend; better yet, you can ask an eye doctor or optician to measure it for you.
Steps
Method 1 of 3: Measure it at Home
Step 1. Get a ruler that marks the millimeters
To be able to measure PD at home, you need an instrument that has the sensitivity of one millimeter. If you don't have it, you can print an interpupillary distance ruler after downloading it from the web page of one of the many optical shops that offer this service. As you proceed, set the printer to respect actual size and not scale the image.
Some specialized websites use a program that allows you to take a picture of the face with a credit card next to it as a reference for the scale of size; however, most online eyewear retailers expect DP to be entered manually
Step 2. Stand in front of a mirror
If you have decided to go it alone, you need to reflect your image. Work in a well-lit room, so you can line up the ruler and see the millimeter marks; to ensure a good reading, keep a distance of about 20 cm from the mirror.
- Hold the ruler above the eyes, at the level of the eyebrows.
- Keep your head straight and vertical for proper detection.
Step 3. Close the right eye to align the left pupil
It is easier to proceed with one eye at a time by closing the other. Start from the left by aligning precisely the "0" notch in the center of the pupil; maximum accuracy is required at this stage, otherwise the entire reading will be skewed.
Step 4. Read the corresponding DP value on the right pupil
Don't move your head or ruler, open your right eye and find the notch that is aligned exactly in the center of this pupil. Make sure you keep your gaze straight ahead for an accurate measurement. The number corresponding (in millimeters) to the notch that aligns with the center of the pupil or is very close to it indicates the PD.
It is best to repeat 3-4 measurements to ensure that the result is consistently accurate
Method 2 of 3: Have a Friend Measure it
Step 1. Approach the other person so that they look each other in the face
You should keep a distance of about 20 cm, just like if you were measuring the DP in the mirror; to ensure good detection, do not stand too close or too far.
Step 2. Look over your friend's head
Unlike what happens when you go alone with the mirror (situation in which you cannot avoid looking at your reflection), in this case you have to look "beyond" the head of the person in front of you. Have him crouch or sit in front of you while you stand, so that he is out of your sight; stare at an object that is 3-6m away.
Step 3. Ask the person to take the measurement
You must keep your eyes perfectly still during the measurement. The friend should align the ruler to the pupils just like you would in the mirror; ask him to make the "0" notch coincide with the center of one pupil and measure the horizontal distance to the center of the other.
Method 3 of 3: Have it measured by an ophthalmologist
Step 1. Make an appointment with an eye doctor
An appointment is usually required for a visit and measurement of PD, during which the doctor performs several tests to assess vision and make sure that the prescription of the optical correction is up to date. The tests involve checking the eye muscles, visual acuity, the visual field, the fundus and refraction.
- If you don't have a trusted ophthalmologist, you can find one by searching online or by consulting the yellow pages.
- If you have undergone an exam in the last year, you shouldn't need a new visit. The doctor who tested your visual acuity may have already entered the PD data in your personal file.
Step 2. Get your DP measured
Based on the tests you have undergone, your doctor may decide to evaluate the diameter of your pupils with a digital pupillometer or with an ocular measuring instrument; both can detect the diameter of the pupils and the distance between their centers.
- The pupillometer looks like large binoculars and all you have to do is look through the lenses while the eye doctor measures the data.
- The eye measurement tool resembles a digital camera, based on the specific model the doctor decided to use.
Step 3. Leave the study with a prescription showing PD
The advantage of having this value measured by an ophthalmologist is that you get both an accurate reading and a prescription to buy glasses. Many online optical retailers require an up-to-date prescription and PD value to be entered; having both on hand, you can greatly simplify the process and make sure you have the right glasses for you.
