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!