Fresnel Prisms

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This leaflet is intended to answer some of the questions patients or carers of patients, who have been given a Fresnel prism under the care of University Hospitals Sussex NHS Foundation Trust.

What will a Fresnel Prism do?

The Fresnel prism is a plastic membrane that fits onto the inside of your glasses. The prism will join together double vision by moving the images closer so that the brain can recognise it as a single image. The prism can also be used to relieve symptoms caused by the effort to maintain comfortable single vision.

Fresnel prisms come in different strengths and your need for the prism may change during the coming weeks or months. The Fresnel prism is a temporary measure and so the strength can be adjusted for maximum comfort. If your eye condition has not changed for 6 months or more it may be possible for glasses to be prescribed with the prism strength incorporated.

Sometimes prisms may be used before surgery to give the orthoptist an idea of what might happen if you were to have an operation to try and correct double vision or a strabismus.

How is it fitted?

The prism is fitted in a particular direction to join your double vision.

It is stuck on using water, which holds it on using suction as it dries. This means that your glasses will not be damaged at all when the prism is removed.

Your orthoptist will test you to get the best strength of prism for you. The prism is cut to fit inside the spectacle frame, usually to the back surface of the lens.

How do I clean it?

You should not remove the prism from the glasses when cleaning, nor put it under a running tap. Use a damp cloth with a tiny drop of liquid soap if necessary and rub the prism along the direction of the lines. Try not to get it too wet as the water may seep between the prism and the lens. This may make the prism edges curl or the prism may come off. Gently pat dry with a soft cloth.

You may also use an old soft toothbrush following along the lines of the prism to clean in the grooves.

What if it comes off?

If the prism does come off place both the glasses and the prism in a bowl of water. Fit the prism onto the lens in the correct position, smooth side against the lens, hold the prism onto the lens and take out of the water. Squeeze the excess water out from under the prism out using a soft cloth.

Why is my vision blurred?

You may notice a slight distortion or blurring of vision caused by the prism ridges. If you persist with the prism you should adapt to this over time.

Even with the prism on sometimes I see double, why?

The prism works best when you look straight ahead. You may still notice some double vision in some positions. It will help if you turn your head more than usual when you look into these positions.

You will still be aware of double vision when not wearing your glasses and prism.

Is the prism curing my double vision or stopping it from getting better?

The prism works to relieve the symptoms of double vision. It is not a cure nor will it hinder your recovery. If a Fresnel prism is unsuitable for any reason your orthoptist will not prescribe one.

The prism is not working anymore, why?

Many conditions which cause double vision will change over time. They may get better or, in some cases, worse. The strength of your Fresnel prism may need to be adjusted to suit your condition as it changes

If you find that you are no longer comfortable with the prism or are having problems with it please contact your orthoptist for advice.