Pupillary distance, in simple terms, is the distance between the pupils of the two eyes. When normal people look at the same object with both eyes, the object is imaged on the retinas of both eyes respectively and overlapped in the visual center of the brain to form a complete, three - dimensional single object. This function is called binocular single vision. However, during the formation of binocular single vision in infants and young children, it is very easy to be affected by external factors, causing one eye to focus on the target while the other eye deviates and cannot look at the same target, thus leading to strabismus. In medicine, when the eyeball turns inward when looking at an object, it is called esotropia, which is commonly known as "cross - eyed".

We all know that when getting glasses, we generally need to know the size of the pupillary distance. Pupillary distances are divided into: distance pupillary distance, near pupillary distance, and common pupillary distance. Pupillary Distance in English, is abbreviated as PD in the optometry and glasses - fitting prescription, with the unit of mm. When going to an optical store for optometry, the machine can usually automatically measure the pupillary distance. So, what is the pupillary distance of an average person? Generally, the pupillary distance of adult men is between 60mm and 73mm, and that of adult women is between 53mm and 68mm. For Asians, the pupillary distance is generally between 57mm and 67mm, and the most common pupillary distance is 62mm.

 

Note that if the measured value is not within this range, it may be caused by measurement errors, and multiple measurements are required.

 

The pupillary distance can also be measured by oneself. The following introduces the most commonly used method for measuring pupillary distance and a method for measuring special pupillary distances.

 

(1) Commonly used method for measuring pupillary distance:
The measurer and the measured person sit face - to - face. The measured person looks at a fixed object 5 meters or more behind the measurer.
The measurer holds the right end of the ruler with the right hand, aligns the "0" scale on the left of the ruler with the center point (pupil) of the black part of the measured person's right eye. Here, it's important to know how to read a ruler correctly. Most rulers have both centimeter and millimeter markings. The longer lines represent centimeters, and the shorter lines between them are millimeters. Each centimeter is divided into 10 equal parts, corresponding to 10 millimeters. Then observe the scale on the ruler corresponding to the center point (pupil) of the black part of the measured person's left eye, accurate to millimeters. This scale is the measured person's pupillary distance.
Repeat step 2. Measure a total of 3 times and take the average value. Thus, the routine distance pupillary distance measurement is completed.

(2) Measurement of pupillary distance in special cases:
Unequal pupil sizes
Measure the distances from the inner and outer edges of the right pupil to the outer and inner edges of the left pupil respectively, and then take the average value of the three readings. That is, PD=(AB + CD)/3.


Asymmetric pupil positions
That is, the pupils of one or both eyes are not at the center of the iris, which is common after trauma or elderly cataract surgery. In this case, the pupillary distance is difficult to measure, and glasses - fitting trial can be used to determine its value.
Measurement of pupillary distance for strabismus eyes


⑴ The examiner and the patient sit face - to - face at a distance of 40cm, keeping their lines of sight at the same height.
⑵ The examiner holds the pupillary distance ruler or straight ruler with the thumb and index finger of the right hand, and the other fingers rest on the patient's cheek. Then place the pupillary distance ruler at the lowest point of the bridge of the nose and tilt it slightly along the angle of the bridge of the nose.
⑶ The examiner closes the right eye, asks the patient's right eye to look at the examiner's left eye, and the examiner covers the patient's left eye with the left hand and aligns the "zero position" of the pupillary distance ruler with the center of the patient's right pupil.
⑷ The examiner opens the right eye and closes the left eye, asks the patient's left eye to look at the examiner's right eye, and the examiner covers the patient's right eye with the left hand and reads the value on the pupillary distance ruler at the center of the patient's left pupil, which is the patient's pupillary distance.


4. Measurement of monocular pupillary distance
When the patient's bridge of the nose is significantly deviated from the mid - line, monocular pupillary distance measurement is required.
⑴ The examiner and the patient sit face - to - face at a distance of 40cm, keeping their lines of sight at the same height.
⑵ The examiner should measure from the center of the pupil of a certain eye to the mid - line of the deviated bridge of the nose respectively to obtain the monocular pupillary distance.
⑶ Precise monocular pupillary distance measurement requires the use of a pupillometer.