Pupillary distance (PD) is the distance between the centre of pupils of the eye, measured in millimetres. An adult PD generally falls within the range of 54 mm to 68 mm and for children within the range of 41 mm to 55 mm.
When and why PD measurement is important?
Measured to align your spectacles with the line of sight (centre of the pupil ) for proper quality of the view. Distance PD is required when you buy Distance, Bifocal or Progressive lenses whereas Near PD is required only for the reading glasses.
There are two types of PD
- Distance PD (DPD)
- Binocular PD
- Monocular PD
- Right PD (RPD)
- Left PD (LPD)
- Near PD (NPD)
How to measure your patients PD?
- Explain the test to the patient: ‘I am going to measure the distance between your eyes so that I can put your lenses in the correct position for your eyes’.
- Adjust the seating chair and maintain proper eye level with patients eye so that lines of sight is parallel.
- Place the PD ruler on the bridge of the nose or on the forehead. Sturdy your hand by placing your remaining fingers on the patient’s temple.
Binocular Distance Pupillary Distance
- Close your right eye and ask your patients to look at your open left eye.
- Choose a point of reference on the patient’s right eye. I personally use temporal limbus margin, although the centre of the pupil or the temporal pupil margin may also be used. Align the zero point on the ruler with this reference point.
- Close your left eye, open your right and ask the patient to change fixation to your open right eye, taking care not to move the ruler or your head position. By sighting again to the appropriate reference point on the patient’s left eye, you will obtain a reading for the distance PD. This would be the left nasal pupil margin if you used the temporal pupil margin of the right eye.
Monocular pupillary distance
- Close your right eye and ask to look at your open eye.
- Align zero points of the PD ruler at the centre of the right pupil and record the reading.
- Similarly, do the same for the eye, ask the patient to look at your open right eye while closing your left, record the scale reading.
Or you can just measure right monocular PD (RPD) as mentioned above and subtract this reading with binocular distance PD to obtain left monocular PD (LPD). For example, Binocular PD is 66 mm and the RPD is 32 mm then subtracting RPD with Binocular PD we get LPD, 34 mm (66-32=34mm).
Near pupillary distance
- Close your right eye and ask the patients to look at your open left eye.
- Align the zero point on the ruler with the reference point of patients right eye and read scale marking at the reference point of the fellow eye. This is your Near PD.
Common causes for error
- There will be an error in measurement if the examiner is not aligned proper eye level with patients eye.
- PD ruler is not tilted on the patient’s nose so that scale is not in an approximate position where the glasses are worn.
- An error can also result if the examiner or patient moves the head.
- Most importantly error will occur if the examiner does not close or occlude one eye ensure directly seeing opposite eye during measurement. This reduces parallax error and gives a precise measurement.