A prism is a piece of glass or plastic that is created to change the direction of light that goes through it. Prisms have many uses in optometry and in glasses. It is not uncommon to prescribe glasses that have a specific prism in the lenses to help with daily activities or improve binocular vision.
Basics of a Prism
Prisms are specially designed to move light in one direction when the light passes through the prism.
A prism will have a thicker end (the base) and a thinner end (the apex). The light will always be deviated towards the thicker end and cause the appearance of things moving towards the thinner end.
The power of a prism is determined by the size and thickness of the prism as well as the material which the prism is made from.
The direction of a prism is determined by where the thinner and thicker ends of the prism are oriented.
Prisms in Glasses
Prisms are frequently used in glasses as a way to move where light appears to enter the eye from.
This is helpful in cases of binocular vision disorders such as convergence insufficiency, in cases of visual field loss such as after a stroke, and for correcting an eye turn or deviation.
Since the prism changes the way light enters the eye, it does not actually affect the alignment or position of the eyes.
Prism for Binocular Vision Disorders
Using binocular vision is a key aspect of many parts of the visual system including depth perception.
There are some cases where the eyes do not work as well when looking inward (convergence insufficiency) or looking outward (divergence insufficiency).
These conditions can cause symptoms including eye strain, headaches, and blurry vision.
Prisms can be used to change the way light enters the eyes and ease the work of the eyes to prevent symptoms in these cases.
Prism for Visual Field Loss
Following a stroke or other neurological event, it is not uncommon to have a large visual field defect.
This means that there is a section of the vision that is not seen when viewing straight ahead.
A prism can be used to help expand the visual field in these individuals by shifting the view towards the direction of the visual field defect.
Prisms used in these cases can reduce injuries from falls or other accidents and improve the function in daily life.
Prisms in Glasses for an Eye Turn
When an eye turn is present, one eye is not directed straight ahead and is either in, out, up, or down.
If an eye turn occurs, there cannot be simultaneous viewing of the same object without the help of a prism over the eye with the eye turn.
By using a prism, the eye that is turned can view the same object as the other eye and depth perception and other binocular vision skills can be restored.
Frequently, prism is used in conjunction with therapy to reduce the reliance on the prism to compensate for the eye turn.