Prisms in eyeglass lenses provide help for those with double vision--when a person sees two separate images while looking with both eyes rather than one image. When the eyes do not work well together, pulling in different directions, prisms installed in eyeglass lenses shift the image in those directions, tricking the brain into believing the eyes work together. Some prisms allow one to see straight ahead while lying back completely. Others expand the vision field by 30 degrees.
Johannes Kepler (1571 to 1630) introduced the term "prism" and "lens" when he invented an astronomical telescope, the Kepler-telescope. Students continue to learn Kepler's concepts of geometric optics today. No one knows who invented the spectacle or eyeglass, but some believe they may have originated in Italy in the late 13th century. Italy became the leader in producing and selling eyeglasses by the mid 1400s, making the highest-quality lenses. In 1999, Eli Peli, O.D., an optometrist, engineer and associate professor of ophthalmology at Harvard Medical School, invented prism eyeglasses called the Peli Lens, also known as the Expansion Prism concept.
Peli first installed a pair of prisms on the top and bottom edges of a lens in a pair of eyeglasses. These prisms picked up objects and directed them into a person's field of vision, much like a car's side-view mirrors. As a result, the prisms helped to restore peripheral vision lost through eye disabilities, such as homonymous hemianopia, which causes blind spots to appear on the left or right side.
Before purchasing eyeglasses with prisms, people first receive temporary prisms they can put on their eyeglasses to see if these will work for them or not. These prisms cost considerable less money, about £195 for a pair. People can also choose to use the stick-on prisms instead of purchasing the more expensive prisms permanently installed on their eyeglasses.
Eyeglass prisms pick up a person's blind spot images no matter what direction the person looks toward. Without the prisms, those with hemianopia can easily run into people, furniture and doors. In a study conducted by Peli in 2008, the prism glasses did help the participants avoid obstacles in many situations. Only a few experienced difficulties with steps or curbs. Prisms may also help people with hemianopia, commonly caused by strokes, to drive vehicles in states that test people with this condition.
Sometimes bifocals can cause double vision because of vastly different prescription lenses for each eye, manufactured in different locations. A prism ground into the lens, called slab-off, evens out the images and eliminates double vision.
Another type of eyeglass with prisms called "bed glasses" allow the wearer to stretch out on a bed and watch television or read. These prisms, also called recumbent eyeglasses, help those who cannot sit upright.