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Generally speaking, the prism correction is usually left empty in most prescriptions since majority of prescription glasses wearers will not have a prism measurement. And many people always feel confused that what the actual function for this correction if the values are added to the prescription glasses.
Prism correction is used to treat eye muscle imbalances or other conditions that cause errors in eye orientation or fixation. And if you have crossed eyes, lazy eye, or some diseases of the eyes, you might found the image looking through the lenses are displaced if you only wear ordinary prescription glasses. The prism correction prescribed by the doctor works by moving the perceived location of what is being seen relative to the other eye.
Occasionally, if the two eyes are not properly aligned when looking directly at the same thing, in order to make them re-aligned, prism should be ground to the lenses. It requires a wedge shaped piece of plastic or glass in their prescription glasses to make their eyes work together as one or both of their eyes deviate away from their correct optical line. Prism correction is measured in “prism diopters”, and the value under the ‘prism’ heading stands for the strength of the prism. Besides, we should always pay the attention for the directions of the prism correction: Base down (BD), Base up (BU), Base out (BO) and Base in (BI).
With the help of the prism correction added to the prescription glasses, eyes that cross can turn in or out, and also one eye can turn up or down. If you have added the prism to the prescription glasses, it will not appear as a strange looking triangle in the lens. At present, the prism is not available for all kinds of lenses, especially for multi-focal lenses. So if you want to add prism for prescription eyeglasses, we would suggest you to choose single vision or regular bifocal (with a line) instead of progressive prescription glasses.
In short, prism is always used for two main purposes: one is to neutralize or compensate for patience with acquired strabismus or significant heterophoria and the other is to affect special awareness and midline awareness. |
