Importance of Interpupillary Distance in Spectacles

Globally approximately 145 million people of the total population and approximately 40% of India’s population is having an uncorrected refractive error (means need a pair of glass/spectacle).

Refractive error is one of the most important ocular conditions that are affecting all age groups and a priority under the VISION2020 initiative. Most refractive errors can be easily corrected at the primary care level with the help of spectacles. Uncorrected refractive error is the second leading cause of vision impairment in developing countries (including India).

Interpupillary distance (IPD) is the distance between the centers of your two eyes (center of corneal reflex) which is measured in millimeter. IPD can be measured monocularly and binocularly. Monocular IPD is the distance between the center of each eye and the center bridge of the nose. Monocular IPD may be slightly different for each eye due to anatomical variation and is dependent on your eye placement. An adult IPD usually remains more than kids IPD. IPD measurement is required for all types of spectacles including buying spectacles for distance or near. The near IPD is usually smaller than distance IPD due to convergence of the eyes together to fuse an object at near.

Different ways to measure Interpupillary Distance:
• Ruler and torchlight (old method)
• Pupillometer
• Essilor Visioffice (USA)
• Eye-Ruler 2 (USA)
• Zeiss i-Terminal2

Interpupillary Distance Measurement with Ruler and torchlight
Examiner should be exactly in front of the patient maintaining the same eye level height and holding a ruler at arm's length distance. Examiner closes his right eye and the patient must be looking at the examiner’s left eye with his right eye and put the ruler at the patient’s nose bridge area (for better stability) while aligning ‘0’ at patient’s center of the right eye. And without moving off a ruler examiner should open his right eye (after closing his left eye) and the patient must be looking at the examiner’s right eye with his left eye. Now measure the distance between both eyes center with ruler - this is your distance IPD. Average distance IPD may fall in the range of 56-66mm. The drawback of this method is parallax may skew measurement, examiner PD and patient’s PD must be very similar and the patient’s pupil must have the same size.

Another way to measure IPD is the distance between the right eye’s outer limbus to left eye’s inner limbus. With this method, IPD measurement may be inaccurate.

Monocular IPD

Interpupillary Distance measurement with Pupillometer
Interpupillary Distance measurement with pupillometer is more convenient. A Pupillometer has forehead rest, nose bridge, dial to change the distance to focus, display for monocular and binocular PD. Putting dial at infinite while using pupillometer over patient nose and aligning forehead rest. Now patient must be looking at small light inside the pupillometer and the examiner should align the measuring slide (a line or bar) until vertical lines should coincide with the corneal reflex. And read the reading in the display for monocular and binocular IPD.

Essilor Visioffice2 (USA)
This device is faster than using manual measurement and takes all necessary measurements for personalized progressive lens and provides numerous solutions to select the lenses and frames. This device assesses the different measurement centers of eye rotation, the distance between pupils, the dominance of eye, reading distance, pantoscopic tilt, the fitting height of the lens, and frame parameters ie. wrap angle, A, B, and DBL.

Essilor Visioffice2

Zeiss i-Terminal2
Zeiss i-Terminal2 is faster than using manual measurement and captures and calculates the patient’s required parameters with the click of a button. This device enables the measurement of highly ametropic patients through vergence control technology.  Measurement can be taken for any type of frame including large-size sunglasses and sports frames. It can measurement all height persons (children, taller or wheelchair person) ranging from ~120-208cms.

IPD is used to align the center of eyeglass lenses with the center of your eyes, which will give you a comfortable and relaxed vision. If the centers of eyeglass lenses are not aligned properly as per your eyes centration, then you may experience eyestrain, headaches, distorted vision, double vision, blurred vision, and/or an inability to wear your eyeglasses for a longer duration.

There are so many spectacle users (patients) who complain of eye strain. Many spectacle users are unsatisfactory that is the result of incorrect estimation of the IPD for distance and near. So it is an important While making a pair of glasses the dispensing professional need to measure Interpupillary distance (IPD) of a person.

IPD measurement is not the part of an eye examination but the IPD measurement is the responsibility of the dispensing professional and it is an important part of a spectacle prescription. Other spectacle dispensing services includes selection of an appropriate frame, lens type, lens thickness and coatings, adjustment of the spectacle frame, and proper alignment of the bifocal or progressive lens height.
Sometimes being perfect IPD alignment and proper spectacle lens fitting still patient complain of eye strain or unsatisfied. What could be the next to find for eye strain or unsatisfactory? So it is very useful scientifically to check the front and back curvature of the lens, coating of the lens, design (aspheric or conventional) of the lens of old glasses as sometimes spectacle users may not very satisfy with new glasses because of this. Sometimes new spectacle lens may have different front and back curvature, different coating of the lens, and design (aspheric or conventional) of the lens.

There are some more factors including frame size (bigger or smaller size over face), pantoscopic tilt (the amount of tilt on the front of the frame), wrap angle (the amount the frame is curved towards your face) and frame alignment over the face which can lead eye strain or unsatisfactory.

There are some other measurements that your optician will take when fitting you for progressive or multifocal lenses. There are two important key measurements - the IPD and the segment height must be checked while fitting a multifocal lens. These two measurements will be providing a comfortable vision with lesser adaptation time.

Each spectacle lens manufacturer is having its version of customized progressives. Near-pupillary distance measurement is also important in progressive addition lenses as these lenses are having customization instead of being standard. There are few other measurements including the vertex distance (the distance between the frame and the cornea), the pantascopic tilt, and the wrap angle to be checked while making a selection of a frame. The latest progressive lenses are more user friendly, giving a wider area of the field of vision and are generally easier to adapt to, especially for those with higher power prescriptions.

In my practice, if any patient complains of eye strain, double vision (shadow feeling while watching TV, or reading text at near), headache after wearing new glasses then my first check would be refraction (repeat power) check-up, IPD measurement, vertex distance, pantoscopic tilt, etc. Most patients are satisfied with repeat refraction and IPD measurement and their comfort changes after reassuring these two checks.