About Binocular Prisms
Sometimes called yoked prisms, binocular prisms are used by developmental optometrists to treat some types of visual problems. Not only do the prisms work to shift where an image seems to be located, they force the person wearing them to make a visual adjustment to cope with the distortion. Often the binocular prism has an effect on the person’s posture and balance. That is why they may be a helpful tool for persons on the autism spectrum who tend to hold their eyes in a downcast posture. The binocular prism can be oriented so that the eyes look upward.
A prism is a transparent object that admits light but causes the rays to get bent or shifted. The most obvious example is a piece of quartz or other clear crystal. When placed towards the light just so, a person can maneuver the stone to cause a rainbow of color to be projected on a white wall or piece of clothing. Prisms can be made by grinding glass or plastic into a lens that functions to bend the light in specific predictable angles. The angle of displacement is measured in prismatic diopters. This potential to manipulate light makes prisms important optical tools.
One prism diopter shifts light 1 centimeter when the light source is 1 meter away. Think of a prism as a triangular pyramid. The narrowest part is at the top (the apex). The fattest part is at the bottom (the base). The direction that a prism moves the light is away from its base. It is correct to refer to a prism by its direction so a base down prism shifts light up. A base up prism shifts light down. A base right prism shifts light to the left, and a base left prism shifts light to the right.
When a person has a strabismus, an eye condition where one eye turns in or out of alignment with the other eye, their eye doctor may add some prism to their glasses. This makes the person’s eyes look aligned cosmetically evenly, though they may need to enroll in a program of optometric vision therapy to learn to fuse the images taken in by each of the eyes. Prism may be prescribed for one or both lenses of a person’s glasses. When prism is added to both lenses, it is called binocular prism.
Binocular prism is also used in optometric vision therapy with patients whose mid-line may have shifted after a stroke or brain injury. It is also used to provide a visual distortion that a patient must overcome. In therapy, this can be a powerful tool that forces shifts in posture, balance, visual attention, and visual flexibility. When binocular prism is used in therapy, the prisms are oriented in the same direction and called yoked prisms. So, for instance, a base right pair of six diopter yoked prism glasses will shift, where things one meter away seem to be to the left 12 centimeters. Add the prism power of each lens to arrive at the total measurement of the prismatic distortion.
Dr. Melvin Kaplan is a developmental optometrist whose work with binocular prism in vision therapy has been effective in treating patients with autism. His book, “Seeing With New Eyes,” is written for parents and optometrists using accessible language. In it, Kaplan explains how he uses binocular prism as a diagnostic assessment tool as well as what he does with it in the vision therapy context. When he deduces that a patient responds positively to binocular prism, he prescribes it in their glasses. Sometimes the prescription is therapeutic and meant to be temporary until the full benefit of the vision therapy can be accomplished. Sometimes the prescription is corrective and will continue to be added to the glasses on a more long-term basis.