Pupil distance(PD) Definition

Pupil distance: Two horizontal distance from the geometric center of the pupil to another.

Monocular Pupil distance: Horizontal distance from the geometric center of the pupil to the bridge of the nose.

Distance PD: PD when see far.

Near PD: PD when see near.

Pupil height, PH: Vertical distance from the geometric center of the pupil to the bottom of the spectacle.

Distance PD measurement methods:


1. Ruler Measurement

Measurement methods can be divided into the corneal limbus measurement method, pupil brim measurement method, pupil center measurement method and corneal central reflection point measurement method.

Corneal limbus measurement method: To determine the inner edge of the cornea of an eye to another outer edge of the cornea.

Pupil brim measurement method: Determination of an inner edge of the pupil to to another outer edge of the pupil.

Pupil center measurement method: Determination of an eye of the pupil center to another pupil center.

Corneal central reflection point measurement method: Measurement of two horizontal corneal reflections.

Repeated measurements three times averaged.

Factors: 1. Pupil distance difference between examiner and patient. Examiners Pupil is large, the measurement results is too large, on the contrary, is small. 2. Whether the level of attention.

2. Centrometer Measurement

Using this instrument, you can easily use the measurement of distance, near, and monocular PD.

3. Visual line measure

Could you image how to determine? If you are clever enough.

Near PD measurement methods:

Methods are basically the same.

The difference are: 1. With the patient sitting in the same horizontal line, call the patient to watch your nose or the middle point of your both eyes. 2. Adjusted the Centrometer’s distance scale to 33cm.
Calculation of measurement: We can use similar triangles principle to calculate near PD if we know the distance PD.

Next article I will introduce how to measure the special patient’s PD such as strabism and nystagmus.