PD Meters (Digital Pupilometers)

How a pupilometer works

The customer looks into the instrument while the operator lines up a hairline with the light reflex on each cornea. That reflex sits close to where the line of sight crosses the cornea, which is why pupilometers are more repeatable than a ruler. A near setting gives the reading PD.

Monocular PD

Faces are rarely symmetrical, so progressives, strong prescriptions and aspheric lenses should be ordered with a PD for each eye. For progressives, 1 mm of monocular PD error narrows the reading area by about 2 mm.

Good practice

1. Check the instrument against a known reference.
2. Seat the forehead rest evenly.
3. Set far fixation for distance PD, the working distance for near.
4. Align on each corneal reflex; occlude for monocular readings.
5. Take every reading twice.

Accuracy evidence

A study of nine corneal-reflection pupilometers found measurable errors, mainly from poor eye relief and scale friction, and recommended periodic accuracy checks, especially at very small and very large PDs. Another study found an examiner’s PD ruler within about 0.6 mm of a pupilometer on average, and self-measurement far less repeatable.