How to Measure PD: Pupilometer, PD Ruler and Near PD

The short answer

Pupillary distance (PD) is the distance between the centers of the pupils, in millimeters. The most repeatable way to take it is with a corneal-reflection pupilometer: the customer looks into the instrument, you line up a hairline on the small light reflex in each eye, and you read the total PD and the PD for each eye. Set the fixation distance to infinity for distance glasses, or to the customer’s working distance for reading glasses. A careful examiner with a PD ruler comes close on binocular PD, but a pupilometer gives monocular and near values directly.

Binocular, monocular and near PD

One customer has several PDs. Which one the lab needs depends on the lens and on where the customer will look through it.

1. Binocular PD: pupil center to pupil center. For single-vision distance glasses this one figure is often enough.
2. Right monocular PD: from the center of the nose bridge to the right pupil.
3. Left monocular PD: the same for the left eye. Faces are rarely symmetrical, so the two halves often differ. Progressives, strong prescriptions and aspheric lenses should be ordered with monocular values.
4. Corneal reflex: the small reflection a pupilometer lines up on. It sits closer to where the line of sight crosses the cornea than the pupil center does, typically about 1 mm toward the nose.
5. Distance PD: when the eyes look far away, the lines of sight are parallel.
6. Near PD: for a near target the eyes turn in, so the lines of sight cross the lens plane closer together. Near PD is always shorter than distance PD.
7. Working distance: how far away the customer reads or works. It is the fixation distance you set for a near PD.

How to measure PD with a pupilometer

The steps below follow a digital pupilometer such as the CP-32A1, which reads binocular PD from 45 to 82 mm and monocular PD from 22.5 to 41 mm.

1. Check the instrument. The display should be bright; replace the batteries if it dims. Check it against a known reference from time to time (see below).
2. Sit at the customer’s eye level, facing them. Seat the forehead and nose rests evenly, so the instrument is neither tilted nor turned on the face.
3. Set the fixation distance. Use infinity for a distance PD. For a near PD, set the customer’s working distance: the CP-32A1 offers 30, 35, 40, 50 and 65 cm, 1 m, 2 m and infinity.
4. Ask the customer to look at the light inside the instrument with both eyes open.
5. Line up each hairline on the corneal reflex with the slider for that eye. For a monocular reading, use the occluder to cover the other eye.
6. Read right, total and left PD from the display. Take each reading at least twice, and compare the result with any facial asymmetry you can see.
7. Write down both monocular values and the fixation distance on the job ticket, so the lab knows whether it is a distance or a near PD.

The PD ruler method and its limits

A millimeter PD ruler is the traditional backup. For a distance PD:

1. Sit at arm’s length, about 40 cm, with your eyes level with the customer’s. Rest the ruler on the bridge of the nose.
2. Close your right eye. Ask the customer to look at your open left eye, and set the zero mark on the center of their right pupil.
3. Open your right eye and close your left. Ask the customer to look at your right eye, and read the scale at the center of their left pupil.
4. If the pupils are hard to see, use the same edge on both eyes instead, for example the left-hand edge of each iris as you see it.

Swapping eyes keeps the customer’s lines of sight roughly parallel, which is what makes the result a distance PD. The method depends on your eyes staying at the same distance and height, and on the customer holding fixation. Monocular PD by ruler also depends on where you judge the center of the nose to be. In a 2012 study, an examiner’s PD ruler differed from a pupilometer by about 0.6 mm on average, while self-measurement and an early phone app were far less repeatable.

Near PD: measure it or work it out

Near PD is shorter because the eyes converge on a near target. The best way to get it is to measure it at the customer’s real working distance with the pupilometer’s near setting. If you only have a distance PD, a common estimate is:

near PD ≈ distance PD × working distance ÷ (working distance + 27 mm)

Here 27 mm stands for a typical distance from the spectacle lens to the eye’s center of rotation; a longer vertex distance makes the near PD slightly smaller. For a 70 mm distance PD this gives about 65.5 mm at 40 cm, about 64 mm at 30 cm and about 63 mm at 25 cm. That is why a 70 mm distance PD usually becomes a near PD somewhere around 63 to 65.5 mm.

Errors that move a PD reading

• Head turned or the instrument seated unevenly: total PD may look right while the two monocular values shift in opposite directions.
• Parallax: with a ruler, your eye must be straight in front of the mark you read.
• The wrong fixation distance: a near PD on a distance job, or the other way round.
• Fixation drifting: if the customer looks at your nose instead of your open eye, the reading changes.
• Halving the binocular PD for progressives: on an asymmetric face this puts one optical center in the wrong place. For progressives, each 1 mm of monocular PD error narrows the reading area by about 2 mm.
• Mixing pupil centers and iris edges between the two eyes.
• Taking one reading only: always take two and compare.

Check the pupilometer, then the finished glasses

A study of nine corneal-reflection pupilometers found measurable errors, mainly from poor eye relief and friction in the scale, and recommended periodic accuracy checks, especially at very small and very large PDs. A simple routine is to measure a reference you trust, such as a colleague whose PD has been measured several times, every so often, and note the result. Keep the viewing optics clean, and take the batteries out if the instrument will be stored.

After edging, check that the PD went into the glasses. Mark the optical centers with a lensmeter such as the HV-600, then measure from the center of the bridge to each dot and compare with the ordered monocular PDs. The lensometer guide shows how to find and mark the optical center.