I have had several requests for the following test for prism correction.
It is not hard to measure the amount of prism it would take for full correction. All it takes is some adding machine or cash register tape, a marking pen, some painters or masking tape (ideally with very weak "stick-um" for easy removal) and something to measure distances.
It is a little easier to work with metric measurements, but you can do it also with inches and feet. You just have to do a little more math for conversions between the two.
This test is based on the definition of 1 prism diopter as: "That amount of prism that will deflect a ray of light 1 cm at a distance of 1 meter (100 cm)".
Ideally, this test is done without any prism correction in your glasses, but you need to be able to see some calibration marks on the adding machine tape with reasonable clarity. If you can't see the marks without glasses, you can still do the test, but you must account for the prism in the glasses.
1. Select a fairly blank wall that you can attach the calibrated adding machine tape to, using the painters or masking tape.
2. Decide where you will stand or sit while doing the test. Measure the distance from that location to the wall selected in Step 1.
3. Calculate how much displacement 1 prism diopter represents at the distance measured in Step 2.
4. Using the marker, mark the adding machine tape with major divisions 5x the distance calculated in Step 3 and optional minor tick marks at 1 prism diopter intervals. The marks need to be big enough to see easily from the distance in Step 2. You might want to identify the major divisions as 0, 5, 10 etc. Note: Some large bold markers will bleed through the adding machine tape and permanently mark the surface you are using as temporary backing for the adding machine tape. Test and take appropriate precautions to prevent damage by the marker ink.
5. Attach the adding machine tape, stretched out horizontally, to the wall selected in Step 1.
6. On another short piece of adding machine tape mark an arrow lengthwise and attach that piece of tape to the wall, vertically, so the arrow is pointing at 0.
You are ready to do the test.
7 Place yourself at the location selected in Step 2, let your eyes relax so you see double and note where the "0" arrow appears to point in the displaced image. Try this test several times during the day and at varying degrees of fatigue. Make a note of your results.
8. If you are wearing glasses with prism, adjust the readings in Step 7 for the total prism in the glasses.
This test will work with horizontal prism (Base Out or Base In) or vertical prism (Base Up or Base Down) by the placement of the long tape and short tape. Often both horizontal and vertical prism exist at the same time.
Note: It is sometimes difficult, if small amounts of prism are involved, to tell if the prism correction needs to be Base Out or In, Up or Down. You may be able to tell by noticing which way the images are displaced when you block the eyes alternately. For example, if you cover the right eye and the image from the left eye is on the left, you probably need more Base Out.
Please Let me know if you have any questions and if this works for you.