How can a pair of glasses be called qualified? Not only must there be an accurate diopter, but it must also be processed according to the accurate interpupillary distance. If there is a significant error in the interpupillary distance, the wearer will feel uncomfortable even if the diopter is accurate. So why does inaccurate interpupillary distance cause uncomfortable wearing? With this question, let’s talk about some knowledge about interpupillary distance.
- What is the interpupillary distance?
The distance between the geometric centers of the pupils of both eyes is called the interpupillary distance. In the optometry prescription, the abbreviation is PD, and the unit is mm. Only when the line of sight of both eyes can pass through the optical center of the glasses lens can they be worn comfortably. Therefore, when processing glasses, you should try to make the optical center distance of the glasses close to the interpupillary distance of the eyes.
- Classification of interpupillary distance?
Because the human eye converges inward to different degrees when looking at different distances. The closer the object is viewed, the more the eyes converge inward. Therefore, depending on the gaze distance, the interpupillary distance is roughly divided into far interpupillary distance and near interpupillary distance. The distance interpupillary distance is used for glasses for distance viewing; the near interpupillary distance is used for near glasses, which are also commonly known as flower glasses.
- What are the commonly used interpupillary distance measurement methods?
In optometry, tools such as pupillary distance ruler, pupillary distance meter, and computer refractor are often used for measurement. Taking the most commonly used interpupillary distance ruler method as an example, I will briefly introduce the measurement method of interpupillary distance:
1. The optometrist and the subject sit at the same height and 40cm apart.
2. Place the interpupillary distance ruler horizontally in front of the bridge of the subject’s nose and at a distance equal to the distance between the eyeglasses. Do not tilt it horizontally.
3. Let the subject look at the optometrist’s left eye with both eyes.
4. The optometrist closes his right eye and observes with his left eye so that the 0 mark of the interpupillary scale is tangent to the inner edge of the pupil of the subject’s right eye.
5. Keep the position of the interpupillary distance ruler unchanged, the subject looks at the right eye of the optometrist with both eyes, and the optometrist closes the left eye, and observes with the right eye. The scale where the interpupillary distance ruler aligns with the outer edge of the pupil of the subject’s left eye is Measured interpupillary distance at distance.
- Why does the error in the interpupillary distance during glasses processing cause discomfort?
After understanding some basic common sense about interpupillary distance, let’s return to the opening question. Why does incorrect interpupillary distance cause wearing discomfort?
When two lenses are processed, an error occurs in the interpupillary distance, so there must be one (or two) eye(s) where the light received by the visual axis cannot pass through the optical center of the lens. At this time, due to the prism effect of the lens, the direction of the light entering the eye is changed, and the object images formed in the two eyes do not fall on the corresponding points, resulting in double vision (ghosting). As a result, the brain will immediately produce a correction reflex to adjust the extraocular muscles and eliminate diplopia. If this correction process continues, it will cause discomfort to the wearer, and the larger the error, the more unbearable it will be.
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Post time: Mar-06-2024