If you want to fit a fresnel prism foil to your glasses you need a few things to prepare before you can apply the prism foil to your glasses. There are two ways to do this and I will explain both to you in this article.
In my opinion, as an optician, this task should be done by a professional. However, a lot of people asked for it. This is why I wrote this article here.
So let us talk about what you need in preparation for this task.
- little screwdriver if you have a metal frame.
- a non-permanent marker
- a large bowl filled with water
- already cleaned glasses
- Your prescription
Identify Where To Put the Foil and How To Position It.
Usually, the prism will be fitted on the side of the lenses facing your eyes (aka back surface, inner surface). Your eye care professional gave you a prescription that tells you where to position the foil on your lens. The Fresnel prism foil works right if the base is applied correctly.
In some cases, you will get the base written in words (up, down, in, out) for a specific eye or you will get the axis noted down. We will cover both options here. Before anything is done we need to think for a moment about how we need to position the foil.
If you look at the foil you can see one side where “base” is written on the foil. This base needs to be positioned in the direction that is written down in your prescription. Let us assume it says for your right eye 16D is needed with base in.
Then what you need to do is to place the base in. In this case, you need to rotate the prism foil. The smooth surface of the foil will touch the surface of your lenses.
If you are set now and know where the base should be pointing in your glasses you can jump to the next step.
In some cases instead of the written words where to position the base of the foil, you will see the axis written down. In such a case the following picture will come in handy.
If the axis is written down the easy case would be if it would be 0°, 90°, 180°, or 270°. Because then you can position the base pretty easily in exactly the same fashion as we did before. The reason therefor is:
- 90° means base up
- 0° means base in for your right eye
- 180° means base out for your right eye
- 270° means base down
But in some cases, the foil needs to be positioned in an askew way like 70° in our next example. In such a case I recommend printing out this PDF and position it under your glasses. This way it will be easier to see how to position the fresnel prism foil on your glasses.
Now position your right lens (yes that is the one you are using with your right eye) over the printed-out schema on the right. Take your non-permanent marker and make a dot on the lens outer surface right over the middle of the schema. Do this again with the 70° mark.
Now connect the two dots.
After you connected the dots on the outer surface we can now make sense of how to position the foil on the inner surface of your lens. Therefore make sure the base now points in the needed direction of the axis. The lines on the fresnel foil must cross your marked-up line at exactly 90°.
Congratulations, now you can cut out the foil in the next step.
Mark the Shape of Your Lens and Cut It Out
I highly recommend you get the lens out of the frame on which the fresnel foil should be placed. You could use a screwdriver for this purpose or if you have a plastic frame you need to push the lens out. The technique for this is presented in the video down below. Make sure to watch it carefully.
when the lens is outside of the frame you can place the smooth surface on the inner side of the lenses. After you placed the foil on the lens you can mark the shape of the lens on the foil. I recommend you mark the shape on the smooth surface of the foil.
After you cut it out you can place the smooth part on the inner side of your lens. A lot of people just press it on. However this can produce air bubbles between your lens and the foil. This is why I recommend to place a large bowl of water near you.
Put the lens back in the frame. Now you can place the fresnel prism foil and your glasses underwater and press the foil on the lens. This way you will probably have fewer air bubbles under your foil. If you see some just press them in the direction of the end of the foil until they disappear.
The only thing left to do now for you is to dry your lenses with the foil. I recommend Kleenex for this purpose. Just speckle the foil dry. If you rub it dry it will wear out faster. Usually, a fresnel prism foil needs to be replaced after a few months. Depending on the environmental influences and how it is cleaned this means three up to eight months.
In the picture above you can see the mounted fresnel prism foil. Especially if you wear this type of correction for the first time you will notice the visible lines. An alternative is to fit the prismatic power directly into the lens. But talk about this case with your eye care professional.
How To Clean a Fresnel Prism Foil?
You can clean a fresnel prism foil best under look warm water in combination with soap. The soap will clean the grease on the surface of the foil. But make sure to get a soap that does not include skincare ingredients. Usually, they lead to smearing on the foils’ surface.
I recommend avoiding dry cloths. Because with them you will need to rub the surface more to get rid of the dirt on the foil. This way the foil needs to be replaced faster. The reason is the surface of the Fresnel prism foil will turn dull over time.
I wish you a great day.