PDs & Segment Heights
08-2008
 Interpupillary Distance and Binocularity
The prescription lists the power needed; PD and height teach positioning. Proper lens centration also delivers binocularity i.e., both eyes working together to see one image. This is dependent on the combination of good PDs and good lab practice. Therefore, taking accurate and consistent PDs is critical.
Using a pupillometer (Fig. 1) conveniently and accurately measures interpupillary distance (PD), the distance between the visual axes of the eyes. This is better than using a ruler since it takes more accurate PD’s more consistently. To take a PD place the nose pads on the patient’s nose with the forehead bar in place helping to center the instrument on the nose. Ask the patient to hold the pupillometer as if they were holding a pair of binoculars and look at the lighted circle.
- Set distance to infinity (∞) for distance PD or the correct near distance for patient near task preference; 35cm=~14 inches, 40cm=~16 inches, 45cm=~18 inches.
- Record monocular or binocular PDs as needed (refer to the chart).
- Record the PD. To be sure, you might take it again.
- Keep the paddle centered unless the patient has difficulty with both eyes focusing on the ring target, otherwise occlude each eye separately and take the monocular PD.
Measuring Multifocals
To measure multifocal fitting heights, sit opposite the patient and adjust the frame so that it is straight and comfortable for the wearer. Ask the patient to put on the glasses where they are comfortable wearing them. Then, with your eyes at the same height as the patient’s dot the fitting height with a marking pen. For bifocals and trifocals, the starting points are top of lower lid for bifocals and top of lower pupil margin for trifocals. For progressives it is pupil center. Refer to the chart on page 20.
Next, remove the glasses and draw a straight line (about an inch) across the dot. Place the pen against the table edge, pen against the dot and slide the glasses right and left. Ask the patient to put the glasses back on and ask them to stand. From the side, view their line of sight. For progressives, confirm that they are looking through the line, for bifocals and trifocals, above it. For bifocals and trifocals, hand the patient a reading card and, as they look down, see if they look below the segment line. Check it again while sitting. In bifocals and trifocals, it can also be confirmed by placing a piece of scotch tape across the line. The tape should be completely in the way of reading and out of the way for distance and walking.
Eyes are at usually different heights, so for progressives it's okay to order dissimilar fitting heights when needed.
Watch a patient’s posture as they walk with you around the office. Adjust segment height as needed. Adjust fitting height as posture changes with age. Never assume that the previous height will be good for the new glasses.
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