Prism Correction and Double Vision

Did you ever see the classic Mel Brooks comedy “Young Frankenstein”?
If not, watch it as soon as possible. It’s hilarious!
Go ahead, we’ll wait.
OK, you’ve watched the movie. Were we wrong? I mean, the “Puttin’ on the Ritz” dance? It doesn’t get any funnier than that.
Now that you’ve seen this comedy classic, let’s talk about one character in particular: Igor, played by Marty Feldman.
young frankenstein igor
There’s a reason his name is not pronounced “EE-gore” but “EYE-gore,” and it’s not just because Dr. Frankenstein’s name is pronounced “FRONK-en-steen”.
It’s that when Feldman is onscreen, you can’t take your eyes off his eyes, which do not point in the same direction. The actor had a condition called exotropia. Exotropia? What’s that? A space colony? No. Good try, though.
Exotropia is a form of strabismus.
“OK wise guy,” we hear you saying. “What are exotropia and strabismus?”
We thought you’d never ask.
Let’s start with strabismus.
Strabismus is a condition in which the eyes are not aligned with each other because not all of the eyes’ muscles are working together. Here’s how it works: There are six muscles in each eye that control the eye’s movements. These are called “extra-ocular muscles”.
extraocular muscles
They enable your eyes to rotate in all directions, which is necessary for you to be able to properly express sarcasm.
But sometimes, these six extra-ocular muscles are not perfectly coordinated with each other, meaning you’ve got a slacker in the bunch. If this is the case, your two eyes don’t look at the same thing at the same time. Just like Marty Feldman’s Igor.
Exotropia, as we said, is a form of strabismus. The prefix “Exo” means “to exit” or to “move out”. Try it when you want to get rid of (and befuddle) a bad roommate:
“It’s exotropia time, loser!”

Exotropia is the type of strabismus Marty Feldman had, as a result of a botched operation in his youth, causing his eyes to turn outward (exo). A more common way to refer to this is “walleyed”.
The other form of strabismus, in which one or both eyes turn inward, is called esotropia. This is more commonly referred to as being “cross-eyed”.

If this guy wasn’t just playing around to freak you out, and really is cross-eyed, he is one of about four percent of all people who have strabismus. Strabismus hampers the ability of the brain to fuse the two images that the eyes see into one 3-D image, which creates depth perception. Along with a lack of depth perception, a person with strabismus may also experience double vision.
In the optical world, double vision is known as diplopia. If you have diplopia, you may see two images, or one blurry one.

Fortunately, eyeglasses with a prism embedded in the lenses can correct this. However, diplopia can also be caused by having one blind eye or an eye with minimal vision. Unfortunately, this can’t be corrected by a prism alone; eye surgery on the extra-ocular muscles may be necessary, too.
But 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 lenses would be all you would need to correct your diplopia.
Here is a prism and its effect:
You may find that, after looking at this picture, you have an uncontrollable urge to listen to Pink Floyd’s “Dark Side of the Moon.”
This is an urge that should never be ignored.
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?

16 Responses

  1. Amanda says:

    Man! Not only is this article informative, but also completely hilarious! For those of us with Prisms (or unwittingly have crooked pictures all over our homes), it is wonderful to see a well written article that’s written for with the Average Joe in mind. Mel Brooks + Animal House + Science = AWESOME!

    Keep up the great work.

  2. Bryan says:

    My optometrist recommended prism for me but I don’t see the capability for ordering it on your site. Is it available?

    • Thanks for your comment, Bryan. We do offer lenses with prism corrections, for single vision (not bifocal or progressive) prescriptions. On the ORDER YOUR GLASSES page, you will see the opportunity to enter your prism number (ranging from zero to 5) and base direction (in, out, up, or down). A prism correction adds $9.95 to the order. If you have any questions, please telephone Customer Service, at 800-211-2105, and one of our knowledgeable customer service representatives will be happy to assist you. Good luck!

    • Julius says:

      yse up to +6 and -12 in single vision only

  3. Richard Ford says:

    Couldn’t agree more, Amanda. Entertaining and educational. I’m going to send the link to my colleague next door who just happens to run our Optical Sciences program. This will make her day, I hope.

  4. I’m so glad you liked the blog post, Richard!

  5. Catherine says:

    Thank you SO much for this article — it’s so well-written and accessible! And since I know I’ll never remember it all on my own, I’m going to copy this & put it in my medical file.

  6. says:

    Hello There. I found your blog the use of msn. That is an extremely neatly written article.
    I’ll make sure to bookmark it and come back to learn extra
    of your helpful info. Thanks for the post. I’ll certainly return.

  7. Thanks, Joan! I’m glad you liked it.

  8. Roy says:

    My prescription shows two (small) prism corrections for the same eye – can this be ordered on your site?
    Also, I noticed that my prescription shows a positive “Up” value for both eyes – does this make sense?

    • Thanks for your question, Roy! Each eye’s prism correction will have one number, between 0 and 5, and one direction, either up, down, in, or out. You will not have two different numbers for one eye. In addition, while it’s more common to have “up” in one eye and “down” in another, it’s not unheard of to have the same direction in each eye. However, if you have any questions about your prescription, you may clarify those by calling your eye doctor’s office. You may also telephone our customer service department, at 800-211-2105, and the customer service representative who answers will be happy to assist you. You may also fax a copy of your prescription to our customer service department, at 415-883-7020, and then call customer service when you have confirmation the fax was received. The representative who answers will be happy to retrieve your fax and discuss your prescription with you over the phone. Good luck!

  9. Tom Giangreco says:

    I had prisms prescribed previously several years ago but when I wore them, there was this rainbow effect. I stopped wearing them because I couldn’t tolerate it at all. I recently went to an optometrist who recommended what I think was a high level of prism. I’m just really concerned that I’m going to get them and they’ll have that rainbow effect which will cause me not to wear them. Is there is a specific level of adjustment where the rainbow effect kicks in?

    I hope that makes sense.

  10. Bill says:

    Matthew, loved your article sir. I have two issues. A senior with macular and a catarac in weak right eye I have 5 out in prisms of both lenses.I have a slight problem that looking thru the winshield seems to make my glasses stronger opposed to looking thru the side windows which vision is fine; The other thing is the doublle vision issue. I feel that 5 out is too much because my old glasses have no prism in left lense and 3 out in the right lense. The prescription is out dated of course but the less prism factor seems to make the old glasses more functional. Question; Can my need for prism be less after 3 yrs. Is it correctable with time? THANK YOU !

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