6.3 Segment Heights and PAL Fitting Cross

The vertical side of centration

PD controls horizontal placement. Segment height and fitting-cross height control vertical placement. A lens can have a perfect prescription and perfect horizontal PD and still fail if the vertical geometry is wrong. This is especially true for bifocals, trifocals, occupational lenses, and progressive addition lenses.

The first rule is that heights are measured after the frame is properly adjusted on the patient. If the frame is sitting low during measurement but later adjusted upward, the segment will be too high. If the frame is tilted during measurement and later straightened, the effective height can change. Measurement belongs to the final wearing position, not the tray position.

Traditional multifocal height references

Different designs use different height conventions. The exact fitting recommendation can vary by lens design and prescriber preference, but basic dispensing patterns are stable enough for exam preparation.

A flat-top bifocal that is too low can force the patient to lift the chin to read. A flat-top that is too high can place the segment line in distance gaze, causing image jump complaints and distracting blur. A trifocal that is too low may deprive the patient of intermediate vision. A trifocal that is too high may interfere with walking or distance tasks.

PAL fitting cross

A progressive lens does not have a visible segment line. Instead, temporary markings identify the fitting cross, distance reference point, near reference point, prism reference point, and alignment marks. The fitting cross is the primary fitting reference. In many PAL designs it is placed at pupil center with the patient in natural posture, but the manufacturer chart must be followed when available.

The fitting cross is not the optical center in the simple single vision sense. PAL optics change gradually through the corridor, and the lens contains areas of unwanted astigmatism in the periphery. Incorrect fitting height can place the patient too high or too low in the design. If the fitting cross is too low, the patient may struggle to find near or may tip the head back. If it is too high, distance may be soft or the patient may feel crowded by the corridor.

Height measurement sequence

Use the same disciplined sequence for every multifocal or PAL order.

1. Confirm the prescription, lens design, and intended use.
2. Select a frame with enough B size for the design and corridor.
3. Adjust the frame before measuring: bridge, temples, pantoscopic tilt, face-form, and level.
4. Seat or stand the patient in natural posture, looking at a distant target at eye level.
5. Position yourself at the patient's eye level to avoid parallax.
6. Mark pupil centers on demo lenses or use the digital system correctly.
7. Measure from the lowest point of the eyewire or lens shape to the mark.
8. Record right and left heights separately if they differ.
9. Recheck after the patient removes and replaces the frame.
10. For PALs, preserve or verify temporary markings at delivery.

A useful habit is to ask the patient to look naturally, not to stare at the optician's finger or raise the chin. Some patients change posture when being measured. Watch shoulders, head tilt, and whether the patient is peering over the ruler.

Frame selection before height measurement

A frame with insufficient B size can make a good height measurement impossible. PALs require enough vertical depth for distance, corridor, and near zones. Short-corridor designs can help in shallow frames, but they do not remove the need for accurate fitting. A frame that sits too low may need bridge adjustment, different nose pads, or a different bridge design before measurements are meaningful.

For lined bifocals, very shallow frames can produce an inadequate reading area. Very deep frames may place the reading zone too far below the eye if the patient has a low fitting posture. Aesthetic preference should be balanced with lens function.

Troubleshooting cases

Case: A new PAL wearer says the reading area is hard to find and lifts the chin to read. Check whether the fitting crosses were placed too low, whether the frame has slipped down, and whether the corridor design matches the selected B size. Also verify add power and markings before assuming the patient cannot adapt.

Case: A bifocal patient complains that the line blocks distance vision while walking. The segment may be too high, the frame may be riding high, or the patient may be using a lens design unsuited for mobility. Confirm segment height in the worn position and ask where the problem occurs.

Case: A trifocal wearer cannot see the computer clearly. The intermediate segment may be too low, the monitor may be too high, or the lens may not match the working distance. Ask about monitor height and distance instead of treating the complaint as a generic blurry lens issue.

Verification at delivery

For PALs, keep temporary markings until final verification is complete. Place the frame on the patient, confirm the fitting crosses align with pupil centers, and check that the permanent engravings are present for future troubleshooting. For lined multifocals, visually compare segment heights, verify symmetry when appropriate, and remember that facial asymmetry can justify unequal right and left heights.

The NOCE often tests judgment: do not measure heights on an unadjusted frame, do not use one automatic value for every patient, and do not remake before checking whether the frame has slipped or been adjusted after measurement.