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How to measure PD?

Peter Torma, PhD - April 9, 2018 - 0 comments

Assume a patient is given the level and type of correction her eye needs and decides to use lenses for correction. The optician should then design and position lenses in a selected eye-wear frame so that the patient can enjoy perfect vision while wearing it. In theory, one should measure at least three types of parameters that are needed for fitting glasses, such as

  • the accurate horizontal positioning (pupillary distancePupillary distance:
    The distance between the center of the pupils. Also knowned as PD.
    or PD)
  • the vertical positioning (fitting heightFitting height:
    The distance between the center of the pupils and the bottom-most point of the eyewear frame.
  • the distance of the lens from the eye (vertex distanceVertex distance:
    The distance between the front of the eye and the back of the lens.

Fortunately in many cases the vertex distance is not of direct interest since it changes the required lens power only very slightly. Furthermore glass-wearers tend to be fairly flexible about the vertical positioning, possibly because most eye-wears can be adjusted a bit on the nose bridge in this regard. This is why by far the most interesting measure is the PD.

There are several traditional ways to measure PD, the simplest among them is a ruler (a.c.a PD-ruler).

Illustrating a PD ruler

When measuring PD with a ruler one has to be careful about eye convergence. In fact, it is commonly done that the optician or doctor asks the patient to look on her forehead while measuring PD in this eye position. Although in this way the optician measures a smaller PD, this usually comes with no problem, since the human eye tolerates glasses made with smaller PD better than larger PD, because of the eye convergence is a natural motion.

A slightly more sophisticated tool shown in the picture below. In this case, the eye convergence is taken care of by simple rule of thumbs, using the maths formulas discussed also in a related post.

A usual PD measure device

An even more traditional way is to sit down exactly opposite to the patient, who is wearing glasses. Cover either eye and mark the glass surface at the other eye’s lookout point. In this case, the distance between the two dots is the PD. The problem with this method, besides that it needs experience and careful implementation is that it does not give the PD in case of the natural gazing pose of the user.

Is it possible to measure my own PD?

Measuring someone’s own PD is tricky, but not impossible. Many webshops show videos where you either have to take a ruler and go in front of a mirror to see your pupillary distance or ask your friends doing it. The tricky part is certainly the accuracy especially because of the eye convergence. On several of these videos the actor is actually focusing on the ruler of the mirrored object that is typically only a half meter from the mirror, so the eyes are focused only 1 meter instead of the suggested 2-5 meters. This causes measuring smaller PDs. Furthermore when reading numbers on the front and left pupils the focusing point changes from the virtual left eye to the virtual right eye which adds even more error to these methods. Additionally, in many cases when the customer needs to use glasses to see clearly, the ruler has to be positioned on the glasses which tremendously can increase inaccuracies.

Is it possible to measure PD online?

There are several applications in the AppStore that measures PD using an online app. Most of these require the user to hold a credit card (an object with uniformly well-known dimensions) at the forehead or at the cheeks. In these cases, the PD is measured by calculating the width of the credit card in pixels and the distance of the pupils in pixels. A simple ratio then gives the PD. Since these methods measure only in the images plane and do not consider the depth dimension, you can get highly inaccurate results. Usual errors are:

  • The customer is not perpendicular to the camera. Problem: The depth dimension between the pupils are not measured.
  • The credit card is not parallel to either the face or the camera. Problem: The two compared planes have different depth dimensions.
  • The customer does not focus on a far object (e.g. because the whole experiments need to be done in a mirror). The PD will be under-measured because of eye convergence.

To our best knowledge, Mira is the only online app that offers online accurate PD measurement without compromise. Furthermore, it is not only capable of measuring PD, but also all other parameters that are needed for varifocal lens fitting, including monocular PD, fitting height, vertex distance, pantoscopic anglePantoscopic angle:
The vertical angle between the gaze vector and the lens direction.
, wrap angleWrap angle:
The horizontal angle between the gaze vector and the lens direction.
and box dimensions.

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