How to Fit Fresnel Prisms
Fresnel prisms are often used in the Orthoptic clinic to manage double vision. Fresnels are thin, transparent plastic sheets with a series of parallel lines that bend light to realign the images seen by each eye. Fresnel prisms can be applied directly to an existing pair of glasses, making them ideal for trial use, short-term management or for patients whose prism prescription is likely to change.
As an Orthoptist knowing how to fit Fresnels to a pair of glasses will be an important skill. To do this you will need:
Fresnel prisms
Scissors (preferably sharp and clean)
Wipeable pen/marker
Running water
Tissues
1: Position the Prism
Before starting, grab the correct strength Fresnel for the patient. Take it out of the case and identify the base direction. You want to position the base so that it is right for the patient, with the smooth side of the Fresnel to the glass:
Base in = thick edge towards the nose
Base out = thick edge towards the ear
Base up = thick edge at the top
Base down = thick edge at the bottom
You want to hold the prism sheet over the lens and align the lines as straight as possible.
2: Trace and Cut
With the prism now in the perfect alignment, you should trace the outline of the lens with a wipeable marker. Then cut carefully along the outline with sharp scissors. It’s better to cut slightly larger than the lens first, then trim down for a precise fit.
3: Clean the Glasses
Before you start, make sure that the lens of the glasses is completely clean and free of dust, fingerprints, or grease. You can wash the lens gently with warm water. The Fresnel adheres best to a smooth, clean surface.
4: Apply with Water
Fresnel prisms stick to the lens using water tension. Run the smooth side of the prism under some water to moisten it then, place it directly onto the lens surface. Gently press from the center outward to remove any bubbles and excess water. Personally, I prefer to adhere the prims to the lens under running water. I find that this helps it adhere the quickest. Then remove the lens from the water and use your finger or a tissue to push out any air bubbles.
5: Check the Fit
When the prism feels stuck to the lens, look through the glasses to make sure the prism is sitting properly, with no folds or trapped air bubbles. If needed, peel it off, rinse, and reapply. Once dried, give the glasses back to the patient and ensure that it joins their double vision. Warn the patient that they may notice some mild blur or rainbow-like colours for the first few days but that this will settle.
Cleaning and Maintenance:
The Frensel prism should stay stuck to the lens between appointments, but with daily wear there may be some dirt and grime that builds up between the ridges. To clean the Fresnel, advise the patient that they can gently peel off the prism, wash it in lukewarm soapy water, rinse, and reapply. They should avoid using alcohol-based cleaners, as they can damage the prism material and cause the material to bend. They could also use a small toothbrush to clean between the ridges, rather than removing the whole Fresnel.
