Prism in Eyeglasses

You may have heard of the term ‘prism’ in your science classes or maybe just floating around in conversation. You may have seen pictures of a prism triangle with a stream of light entering one side and streams of different colors appearing on the other. Prisms are commonly known to be shapes of glass or plastic that disperse light into its spectral colors, but they also alter the direction of light coming through from the other side of the prism. This ability to change the light’s pathway is important in optometry and is one of the solutions that help patients to see clearer.

Strabismus is a misalignment of the eyes and affects up to 12 million people in the United States. Normal, healthy eyes work together to send similar images of objects in focus to the brain, where the images are then combined into one clear picture. This is known as binocular vision. If one of the eyes is misaligned, then the images sent to the brain from both eyes are too different for the brain to successfully combine into one. Double vision will result and can greatly hinder one’s life. This is where prisms can help.

Prisms are usually a triangular shape with a gradual increase in thickness between the apex and the base. They’re different from prescription lenses because they don’t have any focusing power, so it won’t correct any refractive errors, like nearsightedness or farsightedness. Instead, this difference in thickness, with the apex being the thinner portion and the base being the thicker, allows light rays to enter the prism and bend towards the base. This then displaces the image of objects viewed through the prism towards the apex. In other words, objects viewed through a prism are perceived to be in a different location than their actual location when viewed with the naked eye. This main characteristic of prisms is beyond beneficial for patients with strabismus.

Strabismic patients have at least one eye that is either turned inward, outward, upward or downward. This misaligned eye will send an image of an object that is in a slightly different location than where it actually is to the brain while the normal eye will send an image of the object’s correct location. This can lead to double vision and can cause issues for the patient. Optometrists can alleviate these problems by prescribing prism in his or her glasses. Since prisms displace images of objects, the prescribed lenses can effectively shift objects perceived by the turned eye, so that the image sent to the brain is more in alignment with the images sent from the other eye. For example, if the patient’s right eye is exotropic (the eye is turned outwards), a base out prism will be prescribed for the right lens. As stated earlier, objects viewed through a prism are shifted towards the apex, so if the base of the prism is facing outwards, the apex will be facing inwards. The images that have been turned outward by the exotropic eye will be shifted more inwards and become centered. This reestablishes the patient’s binocular vision and reduces the impression of two images. Prisms can be prescribed as base out, base in, base up or base down, depending on which way the strabismic eye is turned.

Having prisms prescribed in one’s glasses is a great temporary solution for a turned eye. They can alleviate some of the symptoms of strabismus, like double vision, headaches, difficulty with reading, etc., but once the glasses are taken off, there is nothing to trick the brain into thinking that it is seeing normally. These issues will continue to occur or, in some cases, worsen. A great and more permanent solution for strabismus is vision therapy. A misaligned eye is due to uncontrolled movements of the eye muscles. Vision therapy can help patients to regain control of their eye and to be able to use both eyes together to produce a single, clear image by having them perform a variety of visual activities created specifically for them. Here at Insight Vision Center, we create a vision therapy program specific to each patient. We know that each person’s personality, lifestyle, likes and dislikes are all completely different and want to provide the best results as possible for each patient. Living with strabismus is very difficult and we want to do everything we can to make sure everybody loves the way they see!