Eyecare Segment Height Measurements
Precise segment and progressive fitting heights, first time, teach patients that you understood their vision needs as well as considered the height in their old glasses, their posture, work, leisure and driving needs. Ask questions to discover what the old glasses didn’t do well because the height was off. One height will meet most but can’t work for all needs so learn the activities for which the glasses will be used most often or where the most comfort is required. Counsel that another pair of glasses, for those other tasks would be better. For example, a pair of progressives for all day wear deliver vision at all needed distances but probably won’t work as well for reading in bed. A pair of single vision readers or near variable focus lenses would work better.A Pantoscopic tilt of about 10 degrees is preferred, especially for progressives.
To measure multifocal fitting heights, sit opposite the patient, adjust the frame so that it is straight and comfortable for the wearer, which is the same procedure when you take monocular PD. Ask the patient to put on the glasses where they are comfortable wearing them. Then, with your eyes at the same height as the patients 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. Next, remove the glasses and draw a straight line (about an inch) across the dot. Place the pen against the table edge, dot against the pen and slide the glasses right and left. See the illustration.
To measure the right height, two things are needed. First, the optician's eyes must be at the same height as the patients. If not, the resulting ink dot will be too low if the optician's eyes are lower. The reverse is also true – sit higher and the dot will be too high. It's simple geometry. Look at this illustration. While it's a bit exaggerated, see where the optician's line of sight crosse the patient's lens. Too low gets a dot that is too low, too high, fitting cross is too high.
Second, once you have a dot for reading seated, ask the patient to stand verify that the dot marked is also OK for walking around.
Ask the patient to put the glasses back on and ask them to stand. From the side view their line of sight so that for progressives 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, it is easy to see if they look below the 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 for reading and out of the way for distance and walking. Watch a patient’s posture as they walk with you around the office. Adjust segment height as needed. Also, a patient’s posture changes as they age so take that into consideration. Never assume that the previous height will be good for the new glasses.