January 10, 2010 | POSTED BY | Articles, Clinical Optometry
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Distance PD is the distance between the pupils when viewing a distant target. Near PD is the distance between the pupils when viewing a near target. Near Centration Distance (CD) is the distance between the visual axes in a spectacle plane when viewing a near target. Near PD is actually impossible to measure unless you remove the patient’s cornea, but distance and near cd can easily be obtained using a small metric ruler. Measurements are generally recorded in mm. There is not usually a large difference between the two measurements, with distance PD usually 3 to 4 mm greater than near CD, and as such, measurements are recorded as distance over near (ie. 64mm/60mm).

What is the whole point of this measurement? When fitting a patient for glasses it is imperative that the patient’s pupils are aligned with the optical centers of the lenses. This will ensure the patient does not experience any unwanted prism by decentration, a concept you will become extremely familiar with in your first year of study.

Our assessment required us to measure the distance pd and near cd of four professors. We were graded on accuracy and technique (ie: sitting at correct distance and position). The task itself is quite simple. It is important to have confidence in your technique and to give proper instructions to the patient. Here is a bonus tip to all the future first year SUNY students! Introduce yourself to every patient as “intern …..” We were not told we had to do this and I along with a vast majority of the entire class lost precious points because of this. You are now one step ahead as you prepare for first year! Also, wash your hands before interacting with each patient and make sure the patient knows you have washed your hands. Remember these tips and you will be well on your way to a solid score.

That just about covers pd. The next and final assessment is the retinoscopy and subjective refraction assessment worth a staggering 10% of your grade. You do not want to miss this coming attraction!

Antonio Chirumbolo

  • jhue

    nicely done. how come no one ever comments??! i can’t be the first….

  • Dr. Joel Kestenbaum

    In my opinion, it is very difficult to align center pupil to center pupil without having an illuminated target directly in front of the examiner’s open eye to create a corneal light reflex. An easier way is to measure the outside of one pupil to the inside of the other…… In cases of patients with a very dark iris, where it is impossible to see the pupil, use the inside of one visible iris to the outside of the other. Good luck.. Remember practice makes almost perfect.

  • http://www.ophthalmetry.com doo

    I am a doctor. I am going to write an article about ways to determine PD. Hope you like. and could you exchange links to my blog.Thanks.http://www.ophthalmetry.com/pupil-distance-measurement-methods/

  • http://www.ophthalmetry.com doo

    You can see your link at the bottom of my site.

  • Thai Nguyen

    Here’s the method that I learned to measure interpupillary distances:

    a) Near PD

    1. Close your right eye (OD), and tell the patient to look at your other eye that is opened.
    2. Place the PD ruler gently on the nose and align the zero with the limbus of the patient’s OD furthest from the nose (EDGE of the temporal portion of the limbus of the pt’s RIGHT eye).
    3. Read off the patient’s NEAR PD with your left eye by measuring the corresponding point on the patient’s LEFT eye.

    NOTE: You should be reading off the measurement at the EDGE of the Nasal portion of the limbus of the pt’s LEFT eye.

    b) Distance PD

    4. From step #3, without moving your PD ruler, close your LEFT eye, and tell your pt to look at your RIGHT eye.
    5. Read off the patient’s DISTANCE PD with your RIGHT eye by measuring the corresponding point on the pt’s LEFT eye.

    NOTE: You should be reading off the measurement at the EDGE of the Nasal portion of the limbus of the pt’s LEFT eye.

    Hopefully, you can visualize my explanations.

  • aaaaaaa

    Whilst measuring PDs you need to close one eye, I measure limbus to limbus. I can’t close 1 eye and see comfortable, and I think placing my hand over my eye looks unprofessional and can confuse some patients on where to look. Besides using a pupillometer what else can I do?