For patients suffering from double vision due to a variety of conditions, one possible correction is to tilt all the light that comes into one eye. Then the image coming into one eye is shifted to line up with the unshifted image in the other eye. A glass wedge or prism does exactly that kind of shifting. It’s impractical to put a glass wedge in front of one eye, but there’s another type of prism called a Fresnel prism that does the same job in a thin sheet of plastic. This Fresnel prism film is designed to be adhered directly to the lens of a pair of eyeglasses.
Remove the lens from the frame.
Align the prism film correctly, lay it on the lens, and cut the film to match the lens outline. The ridged side will point towards the wearer’s eye, while the smooth side will adhere to the inside surface of the lens.
Trim the edges of the prism to fit inside the beveled edge of the lens. Slide the razor blade against the bevel of the lens, at the angle of the bevel, so that the film is trimmed inside the beveled edge.
Clean the lens and the prism in warm soapy water. Rinse.
Submerge the lens and prism in a bucket or tub of clean water. Brush any bubbles off the surfaces to be mated. Place the bottom edges together and press the prism onto the lens in a kind of rolling motion, as if laying a pie crust on a cutting board.
Remove the lens assembly from the water and push any bubbles out by gently pushing on the prism film with your thumb, shepherding the bubbles out. Slide the prism to position it properly, if necessary.
Replace the lens in the frame. Be careful not to catch any of the Fresnel prism film between the lens and frame.
You can also do this with the lens in the frame. Cut the prism small enough to fit, and submerge the glasses completely when applying the prism.
If the prism is not positioned properly and you cannot slide it, peel it off and submerge and apply again.