What is IPD?
Your inter-pupillary distance (IPD) is the distance between the centre of both of your pupils, measured in millimetres. Eye doctors will always measure this distance to ensure an accurate fit when they write prescriptions for eyeglasses and lenses. However, it is also important to know this for when using a VR headset or AR glasses too.
Why is is important to know my IPD?
It’s important to know your IPD when it comes to using VR and AR headsets because each headset can be adjusted to match your IPD level, for optimal image quality and comfort. If we know our IPD measurement, we will have a better understanding which headsets are most suitable for our eyes. Each headset is different. Some have adjustable sliders, whilst others, such as the Quest 2, have fixed settings at set distances.
If your IPD isn’t accurate, it can cause eye strain, fatigue, or simply not allow you to see properly. The higher your prescription, the bigger the effect an incorrect IPD can have.
If the IPD of your particular headset is incorrectly set to your individual measurement, then the scale of the virtual world wont be as accurate as possible. Setting the correct IPD is also very important for maximizing the image quality in VR headsets as well as AR glasses.
Not all headsets allow full adjustment of IPD, so you may have to get it as close to the settings that are fixed on your headset. The Quest 2 for example only has 3 settings; under 61mm, 61-66mm or above 66mm. Unfortunately, this is why some people don’t see things as crisp as others in the headset.
In order too achieve maximum clarity and field of view, when you look through the ‘optical center’ of the lens, often known as the ‘sweet spot’, you want the center of your eyes to align with the optical center of the lenses. Depending on the lens, a misalignment can cause a significant reduction in visual quality. It can also decrease the field of view (FOV), causing a ‘screen door’ or tunnel effect.
Even if a given headset doesn’t have a physical IPD adjustment, most headsets have a software IPD adjustment which can correct the sense of scale. In both cases you’ll need to know your own IPD measurement to set this properly.
How do I measure my IPD?
Using an App
If you have an iPhone, or iPad or touch, you can do this with an app. EyeMeasure works on all iOS versions 12.0 or later. EyeMeasure enables you to measure your inter-pupillary distance instantly and it’s accurate to within 0.5mm.
You can also get a measuring tool on android called GlassesOn.
Manually
If you dont want to use an app, you can measure your IPD manually yourself, or get someone to do it for you.
Ask Your Eye Doctor or optician: The most accurate IPD measurement you’ll be able to get is from an eye-doctor or optician. If you are an adult who is fully grown and already uses prescription lenses, they should have an accurate measurement on file. You can call and ask if they can provide your IPD measurement (in millimeters). If you’re still younger than 20 years old and it’s also been a year or more since you last saw the eye-doctor or optician, you could go back to get make sure you have an up-to-date measurement as your body may have changed since your last visit.
With a mirror and ruler: Start by standing in front of a mirror and with your ruler, and hold it up to your nose, so that the measuring edge runs directly underneath both of your pupils. Now close your left eye and look directly at your right eye. Move the ruler so that the ‘0’ or first marking appears directly underneath the center of your right pupil. To make sure you are as accurate as possible, keep the ruler as still as you can, otherwise it will affect the measurement you are taking. Now, with the ruler steady, close your right eye and look directly at your left eye. You should now be able to see the mark directly underneath the center of your left pupil. This is your inter-pupillary distance.
Ask someone to measure for you: This is the following the same principle as doing it yourself in the mirror. But if you struggling to keep the ruler steady whilst doing it, consider asking a friend or family member to do it for you. You can keep both eyes open and stare straight ahead. The other person should be able to accurate place the ruler for you and record the measurement. You should stand approximately 8 inches (20 centimeters) away from your friend. Do not stand too close or too far away to ensure an accurate reading.
With the headset: This is not the most scientific approach, however, you can use the headset to see what appears to be the best image for you. This method only works with headsets that have a physical IPD adjustment. Put the headset on the first setting and turn the headset on. See if the image is crisp and clear. If it is blurred, consider the next setting up and try again. Repeat until you are happy with the setting. The best way to do this while inside the headset is to close your non-dominant eye. Use your dominant eye to look at a some text. Now begin adjusting the IPD setting to find the position that gives maximum sharpness. It works best if you do not try this with both eyes open as you can easily misalign your IPD when using both eyes.
What is the average IPD?
The average adult IPD is 63 millimeters, although the normal range for most people is between 54 and 74 millimeters. For the average child however, this comes in at around 51 millimetres. Therefore a child should remeasure theirs on a yearly basis, until adulthood, to get the best results.