Skip to main content

See also:

What Is a prism correction on my eyeglasses, part 2

In part one we looked at exotropia. let’s say you have vision in both eyes, and you need only a mild nearsighted or farsighted prescription (and possibly an astigmatism correction). In this instance, if you have diplopia, it would most likely occur toward the end of the day, when your eyes are tired and you start noticing a little double vision.

This could be happening if you’re working on a computer all day, or focusing on printed text (especially small print) all day, and you find you’re experiencing eyestrain. In this case, a prism in your eyeglasses would be all you would need to correct your diplopia.

Here is a prism and its effect:

But first, let’s talk a little bit about prisms. As we know, and as the picture indicates, a prism is a clear glass or plastic pyramid, which causes the white light that enters it to exit in the rainbow color spectrum. We learned about this as kids with the mnemonic (memory device) “Roy G. Biv” (Red, Orange, Yellow, Green, Blue, Indigo, Violet). However, people who wear glasses with prism corrections don’t see the world in rainbow colors. Wouldn’t it be nice if we did.

In addition to dividing white light into its constituent parts, a prism also changes the direction of the light that passes through it. Therefore, when you put a prism correction in a pair of ophthalmic lenses, it displaces the image that the eyes’ double vision puts in the wrong place, restoring it to where it should be to provide one 3-D image.

But what about the rainbow effect?

Here’s what happens with that. Prism corrections in eyeglass lenses have two prisms per lens, each one meeting the other at the base (the bottom of the triangle) or the apex (the peak or point of the triangle).

When light passes from one prism to another, it is restored to the white light that entered the first prism.
Basically, light enters the first prism as white light and exits as a rainbow; it enters the second prism as a rainbow and leaves as white light. But when light leaves a prism, the direction of the light changes.

Just as with nearsighted or farsighted corrections, prism corrections are either plus or minus. When they are plus, the two prisms meet base to base. When they are minus, they meet apex to apex.

If your eye doctor put a prism correction on your prescription, you will see a little triangle indicating a prism. This is written as the Greek letter delta, which we all know from “Animal House.”

You will also see indications for the direction of the base. It will be either Base Up (BU), Base Down (BD), Base In (BI) or Base Out (BO). There will also be a number, somewhere between 0.50 and 5.00, indicating the amount of image displacement needed on one lens or both lenes to correct for double vision. This is written in diopters, which is the unit of measurement of the optical power of a lens.

A corrective prism on your lenses will be so thin you won’t even be able to tell it’s there. But your brain will, unconsciously. With a prism correction, the image the eye takes in is shifted for either the problematic eye or for both eyes. Your brain processes this information and fuses the images together.

Let’s say that only the right eye is the problem. For good binocular vision (which means both eyes acting together), let’s also say the prism correction needed is Base Up, 3.0 diopters. But instead of putting the whole prism correction in the right lens, an eye doctor will often split the difference, and make the right lens Base Up 1.5 and the left lens Base Down 1.5, equalizing the correction so the two lenses maintain the same thinness.

With this prism correction, it makes it appear to the brain that your eyes are working in concert. This directs the brain to fuse into one the two images the eyes see, pulling the disparate images together and creating the
3-D image that allows for depth perception.

Deep, Bluto. Now pass the keg. Hmm … kegs and toga parties … could that be the cause of the double vision?