Fresnel prism
Information for patients from the Orthoptic Department
This leaflet has been given to you by your orthoptist. They have suggested that a fresnel prism may be the best treatment for you at this stage.
What is a prism?
A prism is a wedge-shaped piece of glass or other transparent material. It deflects rays of light toward the base.
A prism is routinely used in orthoptic practice. It measures the size of a squint (how much the eye turns in or out). It can also be used to redirect the rays of light onto the correct point of the back of the eye.
What is a fresnel prism?
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A fresnel prism -
A fresnel prism attached to a glasses lens
A fresnel prism is a thin transparent plastic sheet, which alters the direction of the image entering the eye. One side has a series of angular grooves (prisms), and the other smooth side attaches to the lens of your glasses.
Why have I been given a fresnel prism?
There are many reasons for using a fresnel prism. Your orthoptist will discuss with you why they have advised you to use one. The most common reasons are as follows.
To investigate whether your brain has the potential to use your two eyes together as a pair.
To stop double vision (diplopia).
To use alongside eye exercises, which your orthoptist has given you.
To predict the outcome of squint surgery.
How is the fresnel prism fitted?
Your orthoptist will fit the prism to your glasses using water. If you have your glasses with you, they will do this during your orthoptic appointment. The orthoptist will also give you instructions on how to clean and care for your glasses / prism.
How long do I need to wear the prism?
How long you need a prism depends on your individual needs and eye condition. Some patients need a prism for several months. The strength of the prism changes as your eye condition stabilises.
It may be that a fresnel prism is eventually replaced with a prism, that we incorporate into your glasses.
What are the benefits of fresnel prisms?
A prism can relieve double vision.
It is lightweight.
It is easy to clean and remove.
It is a less expensive option, before having the prism strength incorporated into glasses (if possible).
It is easy to change prism strength, as your eye condition changes.
What are the disadvantages of fresnel prisms?
Prisms may cause slight blurring of your vision, due to the lines on the plastic prism.
We need to fit the prism(s) onto a glasses lens. If you do not need glasses to correct your eyesight, we can make a pair with plain glass.
Are there alternatives to fresnel prisms?
To correct the deviation / prevent double vision
We can incorporate small amounts of prism into a pair of glasses. This may make the lenses both thick and heavy.
To prevent the double vision by occlusion of one eye (covering the eye)
Frosted film can be stuck to the glasses lens, this is easily removed and leaves no residue.
Glasses with an opaque (not able to see through) lens from your optometrist.
You can buy an eyepatch from a pharmacy or online.
Your orthoptist will discuss these alternatives with you at your appointment.
How do I clean the prism?
You do not usually need to remove the prism from your glasses to clean it, just wipe it with a lens cloth. If the fresnel prism becomes dirty, remove it and clean with a soft toothbrush and soapy water. Once it is clean you can put it back on the lens.
If your prism falls off
Place the glasses lens under a gentle stream of water, to dampen the surface.
Place the flat side of the prism’s plastic onto the glasses lens, and push out any air bubbles.
Blot away any excess water and allow the lens / prism to dry.
You must refit the prism in the correct position. Your orthoptist will show you how to do this.
What if I have any questions or concerns?
If you have any questions or concerns, please contact the Orthoptic Department who fitted your prism.
Or you can phone the Orthoptic Department at Kent and Canterbury Hospital for advice on 01227 868615.
What do you think of this leaflet?
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Patient Voice Team
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