Prism Test

Using Trial Frames
  • It is not essential to have trial frames in order to try out small amounts of prism. Neither is it essential to have a big set of refractive lenses (used by opticians to assess long and short sight and astigmatism etc.)
  • It is quite OK to wear your glasses and hold the prism lens in front of the lens of your glasses. Or of course if you don’t need glasses, you can hold the prism in front of your naked eye.
  • If you want to use a trial frame, I suggest you buy one that is good quality and then you will need to buy a set of lenses so that you can set up your prescription. Many sets of lenses you can buy on the internet come with trial frames but in my experience these may not be good enough quality for testing.
  • Once you have your frames and lenses you will need to look at your prescription. There will be a set of values from your right and left eye. There will be a no. for sphere if you are long or short sighted and this will have a minus sign (-) or a plus sign (+). Convex lenses are plus and concave lenses are minus. You will need to carefully select the right power and right sign lens from your set and place it in your frames. When you are using sphere lens it doesn’t matter how you orientate the lens in the frame. For this reason the lens is completely plain with no markings.
  • If you have astigmatism you will need to add a cylinder lens to your left, right eye or both. The same rule applies about finding the no. on your prescription and checking to see if it is plus or minus. You will notice that the cylinder lens does have a marking on it – a small line. This shows you that it is important to orientate this lens correctly. This is not just important – it is crucial to get this right to create you correct prescription. The frames, however, make this easy for you because they have all the numbers of axis written on your frame. And the lens holder is a moving piece and rotates as you turn a knob. You just need to line up the correct number with the little line on the lens Remember that you are not trying to find your astigmatism – just set up the frame as the optician would.

Now all you need is some prisms!

I wish this was the easy bit but it isn’t. If you buy a set of lenses you will likely find you have a set of prisms. Prism strength is measured in diopters. You may have some 0.5 diopter, 1 diopter, and then larger ones.

To test for your phorias, ideally you need: –

4 x 0.25 diopter prisms

2 x 0.50 diopter prisms

(with this combination you will be able to test 0.25, 0.75, 0.50, and 1.00 diopter prism strength in one or both eyes. It is fine to hold more than one prism at a time at a time to add strength of diopter.)

You may find you also need: –

2 x 1.00 diopter

(with these added prisms you will be able to also test 1.25, 1.50 and 2.00 diopter prism strength in one or both eyes.)

I don’t recommend you try any prisms much higher than this because prisms of higher strength can cause chromatic aberration, resulting in colour distortion.

Prism lenses are very hard to find. My best suggestion is that you:
  • Buy a pair of cheap glasses on-line with lenses that are plano (no prescription) but have the strength of prism you require for testing. Buy the glasses in the direction DOWN.
  • Before you remove the lenses from the frames, place a tiny marker (eg. pen mark or sticker) at the bottom of the lens. This will be your BASE.
  • Pop the lenses out of the frames or remove screws to release if there are screws. You will now how have 2 prism lenses. Each prism lens gives you the power that you ordered in any direction. They are identical lenses but you will need to follow the instructions below to know how to use them to test both your right and left eye.

How Do I Use the Prisms?

Don’t forget – the prism is not moving your eye gaze. It is bringing the image to your eye gaze. However when you hold the prism to your eye it will feel as if your eye gaze is moving.

Use the data that you have collected from your other tests. Whichever way your eye-gaze is going, you need to bring the image to your eye.

Testing Both Eyes (Phoria UP)

When your phoria is UP, you want to bring the image UP to your eye so you rotate the prism so that the BASE is DOWN (marker at the bottom of lens.) When you use the prism it will feel like your gaze is going DOWN

Testing Both Eyes (Phoria DOWN)

When your phoria is DOWN, you want to bring the image DOWN to your eye so you rotate the prism so that the BASE is UP (marker at the top of of lens.) When you use the prism it will feel like your gaze is going UP

Testing Right Eye (Phoria OUT)

When your phoria is OUT, you want to bring the image OUT to your eye so you rotate the prism so that the BASE is IN (marker at the left of lens when facing you.) When you use the prism it will feel like your eye is going IN.

Testing Right Eye (Phoria IN)

When your phoria is IN, you want to bring the image IN to your eye so you rotate the prism so that the BASE is OUT (marker at the left of lens.) It will feel like your gaze is going OUT.

Testing Left Eye (Phoria OUT)

When your phoria is OUT, you want to bring the image OUT to your eye so you rotate the prism so that the BASE is IN (marker at the right of lens when facing you.) When you use the prism it will feel like your gaze is going IN.

Testing left Eye (Phoria IN)

When your phoria is IN, you want to bring the image IN to your eye so you rotate the prism so that the BASE is OUT (marker at the the left of lens when facing you.) When you use the prism it will feel like your gaze is going OUT.

Please Note: You can only add trial prisms (usually 38 mm ) to your trial frame. If you have ‘popped’ prisms you need to hold them in front of your glasses or your naked eye.

SUMMARY
  • If your gaze is going OUT you need a prism BASE IN
  • If your gaze is going IN you need a prism BASE OUT
  • If your gaze is going UP you need a prism BASE DOWN
  • If your gaze is going DOWN you need a prism BASE UP

Working with 2 Directions of Gaze (1 eye or both)

  • If your gaze is going OUT and UP you need 1 prism BASE IN and 1 prism BASE DOWN
  • If your gaze is going OUT and DOWN you need 1 prism BASE IN and 1 prism BASE UP
  • If your gaze is going IN and UP you need 1 prism BASE OUT and 1 prism BASE DOWN
  • If your gaze is going IN and DOWN you need 1 prism BASE OUT and 1 prism BASE UP

Finding Required Strength of Diopter

  • Test with your smallest prism and then if you feel your eye needs more, move to a higher one. If your eye becomes uncomfortable or you experience any other unpleasant symptoms, then come back down to the lowest power prism that is comfortable
  • If your tests showed discrepancies on both the horizontal and vertical axis, work with 2 prisms in the directions required. When working with two directions it is very much trial and error when it comes to power. You may want 0.5 IN and 0.25 IN or the other way round for instance.
  • Play. If you feel any negative effects, move the prism from your eye. Only test if your brain and nervous system are happy with you doing it. Usually working the tiny amounts of prism, any negative effects will be minimal.

Collect Your Data

Once you have found a comfortable combination of prism for you, this is more data for your collection. You may feel that this is it – you must have these glasses! You may feel unsure and if so, just collect the data and test again another day.

Working with Changing Phorias

You know you have changing phorias if you: –

  • Test yourself every day and find your phoria is different sometimes.
  • Buy your glasses and then find they don’t feel ‘right’.
What do you do?
  • Test yourself often and when you have the same test result on 2 or 3 separate occasions, or you recognise the feeling of a particular phoria as being a regular occurrence, buy the glasses.
  • If the glasses don’t help you by the time they arrive, just label and keep them for when the phoria you had at the time of testing, might return.
  • If you need several pairs of glasses, buying the same style each time can be easier unless you are OK with changing your appearance often!
  • Having the same case can also help, and good labelling is essential to enable you to find the exact right glasses you need for any moment. I make acronyms for my glasses and I use a system of coloured pen dots on the arms to give me the information that I need about strength and direction of prism. I then also clearly label the cases. As long as the glasses are clearly marked, I can always find the right case again if I have several pairs of glasses in use.

Working with changing phorias is a challenge. But like any skill, with practice you will find you get better at it with some time and patience. And after a while changing glasses to suit your changing phorias will just become second nature.

If you got to bottom of all this information, I am impressed! I wish it could be simpler but if you just give yourself time and work at things very systematically, you will be amazed at the results!

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